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8th Grade
Forces
2015-08-19
www.njctl.org
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Forces and Motion
Click on the topic to go to that section
· Motion
· Graphs of Motion
· Forces
· Newton's Laws of Motion
· Newton's 3rd Law & Momentum
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Motion
Return to Table
of Contents
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What does it mean to be in motion?
With a partner, come up
with a scientific
explanation of what it
means if an object is in
motion. Don't just say an
object is moving!
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What does it mean to be in motion?
An object is in motion if it
changes position in
relation to a certain place,
called a reference point.
Reference points are
places or objects used to
determine the motion of an
object.
It is extremely important to choose reference points
carefully.
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Relative Motion
Motion is relative as
it is dependent upon
the reference point.
Are you in motion
right now? Think
about it...
Are you in motion compared to your desk?
Are you in motion compared to the sun?
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Measuring Motion
Speed is one of the many ways we measure motion.
Speed is a measure of the distance traveled per unit of
time.
That means you can find the speed of any object that is
in motion!
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Finding Speed
Speed = Distance ⁄ Time
and
We can rearrange the letters in the speed equation and solve
for any of the other 2 pieces of information.
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Finding Speed
Discussion time!
· What are some possible units to measure speed? Remember
that speed= distance/time!
· How do you think speed is measured when driving in a car?
· How would you measure speed of an engine? For reference,
an animation of an internal combustion engine is shown below.
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Units for Speed
The SI unit for distance is meters (m) and for time is seconds (s).
Given these units, what will be the SI unit for speed? (Hint: recall
the speed equation
speed= distance/time)
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1 A snail travels a distance of 10 m in 6000
seconds. What is the snail's average speed?
A 60000 m/s
B
0.02 m/s
C
600 m/s
D
0.002 m/s
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2 A blimp travels at 3 m/s for 1500 s. What
distance does the blimp cover in that time?
A 500 m/s
B
4500 m/s
C
4500 m
D
500 m
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Manipulate the speed of each car, solve for time, and predict which
car will win!
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Speed
When we talk about speed, it is important to know that there is
a difference between instantaneous speed and average speed.
Average speed can be calculated by dividing the total distance
by the total time. This is what is usually calculated by runners
in a race.
Instantaneous speed is
the speed of an object at
any moment in time (an
instant of time). Can you
think of an example
where you would use
instantaneous time?
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Average vs Instantaneous Speed
It might take 3 hours to travel 300 km in a car. What would the
average speed of the car be? Calculate it now.
Does that mean the car was going
that speed for the whole 3 hours
of the trip?
When you go on a trip in the car,
do you go the same speed the
whole time? Talk to a partner
about this.
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Average vs Instantaneous Speed
Have you ever run a mile? Do you think you ran at the exact same
speed the entire mile?
Think about
what runners
do at the
very end of a
race.
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Average vs Instantaneous Speed
When you ride on the bus
to school, does the bus
driver travel at the same
speed the entire trip?
School buses and other
vehicles have
speedometers that
measure the speed of the
vehicle at a specific
moment in time.
Do speedometers measure average speed or instantaneous speed?
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3
Your teacher times your mile run at 7.5 minutes,
that means your speed was 0.13 mile/min. Was this
your average speed or your instantaneous speed?
Average
A
Speed
Instantaneous
B
Speed
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4
A student has built a model car that is designed to
run at a constant speed. What should the student
measure to test whether the car runs at a constant
speed, average or instantaneous speed?
A Average Speed
B Instantaneous Speed
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5 A swordfish travels for two hours. The first
hour he covers 110 kilometers, and the
second hour he covers 84 kilometers. What
is the average speed of the swordfish?
A
110 km/h
B
84 km/h
C
97 km/h
D
194 km/h
Hint: Remember that
Average speed is total
distance travelled divided
by total time
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6 A dog walks outside to go the bathroom every day.
On a given day, she walks 75 meters before she finds
a spot to use the bathroom. She walks at an average
speed of 2.5 meters per second. How long does it take
her to find a place to use the bathroom?
A
30 seconds
B
2 minutes
C
188 seconds
D
90 seconds
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Velocity
Velocity is another way to measure motion. Simply put, velocity
is the speed of an object with a direction included.
Runner's speed: 10 km/hr
Runner's velocity: 10 km/hr to the East
What do you notice about the units for speed and velocity?
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Why is velocity important?
Have you ever been in an airplane? Would would be the consequence of
a pilot only knowing the speed of other nearby planes?
Tornadoes travel at about 170 km/h.
Why is knowing the velocity of a
tornado important?
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7 Carlos and Gina are riding on their horses to go
into town. They travel 70 meters in 7 seconds going
west. What is their velocity?
A
490 m/s west
B
10 m/s west
C
490 m/s
D
10 m/s
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8 A car travels 100 km/h west for 2 hours. The car
then travels 50 km/h east for one hour. What is the
car's final position relative to its starting position?
A
50 km west
B
150 km west
C
250 km west
D
50 km east
Hint: Find the distance
and direction traveled for
each leg of the trip first.
Draw these distances on
a number line using 0km
as the starting point.
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Graphing Motion
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of Contents
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Graphing Motion
Graphs can be used to show motion and can be used to determine
relationships!
When graphing data, position should be on the y-axis and time on the
x-axis. Drag and drop the variables onto the correct axis on the graph
below.
Position
Time
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Graphing Motion
Use the graph below to calculate
speed at 1, 2, and 3 seconds.
Do you notice a pattern?
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Graphing Motion
6 m/ 3s = 2 m/s
2 m/ 1s = 2 m/s
4 m/ 2s = 2 m/s
The speed is 2 m/s at each of the three seconds! In other
words, the speed is constant.
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Graphing Motion
When interpreting a graph, it is important to look for relationships
between variables. These relationships can be strong, weak, or not
present at all.
The graph below shows a strong relationship between position and
time. Can you describe what this relationship is?
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Graphing Motion
A weak relationship is when significant changes in one variable
cause minimal changes in the other variable. Is there any pattern in
the graph below? Circle the patterns.
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Graphing Motion
When there is no relationship between the variables, there will not
be a pattern present. Is there any pattern in the graph below?
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The Importance of Slope
Position versus time graphs can be used to find speed and
compare speeds.When we talk about the slope of a line, we
are talking about how steep a line is.
Look at the skier to
the right. He is on a
very difficult trail.
Would an easier trail
be steeper or more
flat?
How would the
slopes of these two
trails compare?
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The Importance of Slope
In a position versus time graph, the y-axis is the position and the
x-axis is the time. Recall that the relationship between position
and time is speed. So when we are looking at the slope, we are
looking at the speed .
The slope of the
black line gives us
the speed of the
bicycler.
The steeper the slope of the line on a position versus time graph,
the greater the speed!
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9
Which walker has a greater speed?
How can you tell in terms of the slopes of the
graphs?
A
B
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Graphing Motion Simulation Lab
Click on the image to the left to launch
the simulation. You will need to
download it to be able to use it.
Click on the CHARTS tab at the top.
Minimize the velocity graph and the
acceleration graph by clicking on the
RED dash on each graph. KEEP
POSITION!
CLICK PLAY
Drag the man to the RIGHT at a
constant speed, STOP, and finally drag
the man at a constant speed Left back to
the starting point (zero).
Relate the shape of the graph (slopes!) to the man's motion.
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Acceleration
Constant speed is when an object's speed does not change; however,
most objects do not travel at a constant speed. Acceleration is a way
to measure changing motion.
Do you walk at the exact same speed
from class to class? What if you are late
for class?
Are you ever accelerating when you go
to class? Can you define acceleration?
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Acceleration
Acceleration occurs when there is a change in velocity. Remember,
velocity is speed with direction.
So acceleration occurs any time there is an increase in speed, a
decrease in speed, or a change in direction.
speed
speed
change in
direction
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Acceleration
Acceleration is a measure of the change in velocity per unit of time.
Here is the equation for acceleration.
a = vf -vo
t
acceleration = (final velocity-initial velocity)
time
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10 A school bus driver sees an old man crossing the
street at an intersection. The bus driver hits the
brake pedal and turns the steering wheel to avoid
him. Describe the acceleration of the bus.
A It increased speed
B It decreased speed
C It changed direction
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11 A dragster launches from rest to 50 mi/hr at the
start of a race. How did the car accelerate?
A
increased speed
B
decreased speed
C
changed direction
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Practice Solving for Acceleration
The school bus picks you up at the bus stop and takes 60
seconds to accelerate to 120 km/h. What is the acceleration of
the school bus?
a = vf -vo
t
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12 A plane's speed increases from 25 m/s to 60 m/s in 5
seconds. What is the acceleration of the plane?
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13 After traveling for 10 seconds, a runner reaches a
speed of 12m/s. If the runner started from rest, what
is the runner's acceleration?
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14 A parachute opens and slows a skydiver from 65m/s
to 45m/s in a period of 5 seconds. What is the
acceleration of the skydiver?
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Acceleration and Graphs
If the speed and direction of an object are constant, then the
acceleration is zero. Look at the car's speed at 20s, 60s, and 100s.
Is the car accelerating?
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Acceleration and Graphs
Acceleration is
positive when speed
is increasing.
Negative acceleration
(deceleration) is when
speed is decreasing.
How could you describe the slopes of each of
these lines?
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15
A driver hits the brakes to slow down at an
intersection. As the car's speed is decreasing it has:
A positive acceleration
B negative acceleration
C no acceleration
D more information is needed
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Which of the following graphs shows a positive
acceleration?
Time (s)
C
x
Y
Time (s)
x
Time (s)
D
x
Y
Speed (m/s)
B
Y
Speed (m/s)
Y
Speed (m/s)
A
Speed (m/s)
16
Time (s)
x
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Constant Speed Graphical Analysis Lab
We will us a constant velocity vehicle to collect data
and graph our results.
The graph will help us determine the average speed.
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Acceleration and Graphs
The slope of a position versus time graph can also show
acceleration.
If the slope curves and gets
steeper, then positive
acceleration is occurring.
If the slope curves and becomes
less steep, then negative
acceleration is occurring.
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Distance vs. Time Graphs of Accelerating
Objects
Click on the image to download the
simulation.
Click on the CHARTS tab.
Type in 1 m/s2 into the acceleration
value and hit play.
Sketch the
shape of the
distance vs.
time graph:
Sketch the
shape of the
velocity vs.
time graph:
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17 Which of the following speed vs time graphs (on the
left) correspond to the position vs time graph on the
right?
A
B
C
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Accelerated Motion on an Inclined Plane Lab
In this lab you will record the time it takes for a car to
go different distances down an inclined plane.
The data will be graphed and then you will analyze the
results.
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Forces
Return to Table
of Contents
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What Are Forces?
Forces are ________ or
______ in a given direction.
Forces affect how objects move.
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Forces Affect Motion
At your table, make a list of ways forces can affect objects.
There are many ways. Write a few of these ways below.
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The Big Idea...
Since forces can cause changes in the speed or direction of an
object, we can say that forces cause changes in velocity, so
forces cause acceleration!
Forces
cause
Acceleration
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Units of Force
Forces are measured in newtons (N).
You probably measure yourself on a scale in pounds.
One pound is equal to 4.448 newtons.
Just like velocity, force has direction. When forces are
demonstrated both magnitude and direction should be shown.
10 N
What is the magnitude of the force shown above? How about the
direction?
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Balanced Forces
If you and a friend both push with the same strength on each side of
a table, will it move?
Two or more opposite forces acting on an object are considered
balanced if their effects cancel each other out. Balanced forces do
not cause a change in an object's motion.
The box to the right is
at rest and will remain
at rest since the
opposite forces acting
on it are balanced.
8N
8N
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Unbalanced Forces
If the effects of the forces don't cancel each other out (one force is
stronger than others), the forces are unbalanced forces. Unbalanced
forces do cause a change in motion; speed and/or direction.
Think about Tug-Of-War.
How does one side win?
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Unbalanced Forces
8N
15 N
Two ways to interpret this diagram are to say:
there is a 15N force to the right and an 8N force to the left
OR
we can say there is a +15N force and a -8N force.
What does the negative on the -8N tell us?
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Unbalanced Forces
15 N
8N
The box was initially at rest. Since the forces acting on the box
are unbalanced, the box will start moving.
Does the box accelerate?
Which way do you think the box will move and why?
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18 Forces are all around us. Which of the following
do you think are examples of forces? (choose all
that apply)
A
Gravity
B
Friction
C
Muscles
D
Wind
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19 What is the SI unit for force?
A
Pounds
B
Kilograms
C
Newtons
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20 A +10 N force acts on a car and at the same time,
a -20 N force acts on the car. What is the net
force acting on the car and is it balanced?
A -30 N unbalanced
B -10 N unbalanced
C -10 N balanced
D + 30 balanced
Hint: Net force is the total
(or sum) of all the forces
acting on an object.
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Friction
Forces are present all around us, but can not usually be seen.
Friction is a force that resists motion and we experience daily.
When you run, walk, sit on the couch,
brush your hair, and write you
experience friction.
You are experiencing friction right
now!
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Friction
The force of friction is caused by microscopic particles touching each
other. These microscopic pieces on both surfaces cause friction.
Friction is affected by how hard the surfaces push together and the
types of surfaces involved.
There are many types of
friction.
Click here to see the force of friction clip
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Static Friction
Static friction acts on objects that are not moving. Have you ever
wondered why it is so hard to start moving a heavy object like a
dresser or couch, but then once it starts moving it is easier?
That is because you have to overcome the force of static friction!
Static friction holds the couch in place and keeps it from moving.
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Sliding Friction
Sliding friction occurs when objects slide over each other. Sliding
friction is easier to overcome than static friction. That is why the
couch is easier to move once it starts sliding!
Sliding friction is also known as kinetic friction.
Click here to see sliding friction in action.
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Fluid Friction
Fluid friction occurs when objects move through a fluid.
Remember, air is a fluid, so you continuously experience fluid friction!
Click here to see the effects of fluid
friction.
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Rolling Friction
Rolling friction exists
when objects roll
across surfaces.
With a partner, make a
list of other examples
of rolling friction. Hint:
There are many
examples in sports.
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Friction
The force of friction is measured in Newtons like all other forces.
When you are trying to determine where to put the friction force,
just remember that friction acts opposite to motion!
force applied by
person pushing box
stationary box
static friction
force
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21 Friction acts more on objects in motion than it
does on objects at rest.
True
False
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22 Friction always opposes the motion of an object.
True
False
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Gravity
Forces are present all around us and always act in pairs, so we usually
experience more than one force. Gravity is a force that pulls objects
towards each other.
How does gravity act on objects
here on Earth? Use the picture to
the right to help you with your
answer.
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Law of Gravitation
The Law of Universal Gravitation tells us that gravity acts between
all objects in the universe. This means that without exception, any
two objects in the universe attract each other!
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Factors Affecting Gravity
Gravity is affected by both mass and distance. The greater the distance
between two objects, the less the gravitational force between them. The
greater the mass of the object, the greater the object's gravitational
force.
You have learned about mass in the past. Can you define mass?
Which of these
examples has
more gravity?
Why?
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23 The force of gravitation between an object
and a planet is increased as they move
away from each other.
True
False
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24 The force of gravitation between an object
and a planet is decreased if the object
increases mass.
True
False
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Mass and Weight
What is weight?
You have also learned the difference between mass and weight in the
past. What can you recall about mass and weight?
Think about
astronauts on the
International
Space Station.
What do you
know about their
masses and
weights?
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Wait, so mass affects
gravity?
Weight is a measure of the gravitational force exerted on an object.
Weight varies depending on gravitational force, but mass does not.
Weight = mass x gravity
Our moon has a gravity that is
one sixth of that on Earth.
What would that do to your
weight on the moon?
Would your mass change on
the moon?
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What is "g"?
"g" is the acceleration due to gravity.
On Earth, our "g" is approximately 9.8 m/s2.
On other planets, acceleration due to gravity will vary depending
on the mass of the planet. In general, the more massive the
object, the larger the value of g!
Jupiter
g = 26.1 m/s2
Earth's Moon
g = 1.67 m/s2
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25 A 50 kg kid is on planet Earth, where g = 9.8 m/s2.
What is the boy's weight?
A
5.1 m/s
B
490 N
C
490 kg
D
5.1 N
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26 A 50 kg kid is on planet Earth, where g = 9.8 m/s2.
What is the boy's mass?
A
50 kg
B
490 N
C
490 kg
D
5.1 N
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27 A 50 kg kid is on the moon, where g = 1.67 m/s2.
What is the boy's weight?
A
83.5 kg
B
29.9 kg
C
83.5 N
D
29.9 N
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28 A 50 kg kid is on the moon, where g = 1.67 m/s2.
What is the boy's mass?
A
83.5 kg
B
50 kg
C
83.5 N
D
50 N
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29 The larger the planet, the larger the __________.
A mass
B weight
C gravity
D all could be correct
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Gravity and Motion
Galileo showed that falling objects accelerate equally no matter
what their mass is. This is strictly true when gravity is the only
force acting on a falling object and is known as free fall.
When an object is in free fall, it accelerates at 9.8 m/s
2
If these two stones are dropped
from the same height at the same
time, which hits the ground first?
10kg
Click here to see a bowling
ball and golf ball being
dropped
1kg
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Gravity and Motion
The following video shows a feather and a ball bearing being
dropped from a small height.
Click here to see a feather and a ball bearing
being dropped.
Which simulation showed the objects in free fall?
Critical thinking: What was happening in the simulation that did not
illustrate free fall?
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Air Resistance
Objects are not always truly in free fall because they
experience air resistance. Air resistance is a fluid friction
experienced by falling objects. When objects experience air
resistance, they don't fall at a rate of 9.8 m/s2.
Try this: Crumple up two pieces of paper individually and drop
them both from the same height at the same time. Which hits
the ground first? Now drop a crumpled piece of a paper and a
non-crumpled piece of paper in the same way. What's the
difference?
Click here to see a feather
and a hammer being dropped
on the moon. What happens
with no air resistance?
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Air Resistance
The flat piece of paper fell at a slower rate because it had more
surface area. This greater surface area resulted in the paper
experiencing air resistance. For the crumpled paper, air
resistance was probably very tiny and thus could be ignored.
The crumpled paper was essentially in free fall.
People use parachutes when they
jump out of planes. Why?
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Net Force
As you know, many forces are acting on us and other objects.
To determine the total force acting on an object, the forces are
added and subtracted as appropriate to find the net force.
When several forces are acting on the same object, the net
force might be zero...
5N
-5 N
Net Force = 0
If the net force on an object is zero, then it is in equilibrium.
When an object is at rest, the net force is zero.
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Unbalanced Forces
If the net force is not equal to zero, then there is a change in the motion
of the object. The object is not in equilibrium. What is the net force in the
example below?
5N
-12 N
Net Force =
What direction is this box going to
move?
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Unbalanced Forces
5N
-12 N
Net Force = -7 N
In this case, the object will accelerate towards the left because
the NET FORCE is toward the left.
On Earth, gravity and friction are two of the unbalanced forces
that
frequently change an object's motion.
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30 What is the net force acting on the object below?
Is the object in equilibrium?
-10 N
7N
-8 N
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31 What is the net force acting on the object below?
Is the object in equilibrium?
-10 N
25 N
-15N
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32 Acceleration due to gravity on Jupiter is 26.1 m/s2.
How much would a 60 kg person weigh on Jupiter?
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33 Acceleration due to gravity on Earth's Moon
is 1.67 m/s2. How much would a 60 kg person weigh
on Earth's Moon?
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Sticky Sneakers Lab
How does the amount of friction between a shoe and a surface
compare for different shoes?
Which shoes would be best for playing basketball?
Which would be best for bowling?
These questions will be answered in this lab.
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Newton's Laws of Motion
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of Contents
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The History of the Laws of Motion
Aristotle, a Greek philosopher, and Galileo Galilei, an Italian
astronomer, may have been two of the first scientists to try to explain
gravity and motion.
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The History of the Laws of Motion
In the late 1600s, Sir Isaac Newton
used Galileo's ideas to create three
basic laws of motion.
Sir Isaac Newton contributed to
advances in physics, mathematics,
and astronomy.
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Laws of Motion
Newton may be one of the
greatest scientists in
history. The three laws of
motion he created are
three of the most used
natural laws in science.
These laws help us to
make sense of the world
around us.
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Laws of Motion
Newton was inspired by the apple falling from the tree and
asked himself if gravity might also be the force holding the
moon in orbit.
Newton found that gravity plays a role in other orbital motions
as well!
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Newton's First Law of Motion
The first law of motion tells us that an object at rest stays at rest,
and an object moving at a constant velocity will continue moving at
a constant velocity, unless acted on by an unbalanced force.
On Earth, gravity and
friction are two of the
unbalanced forces that
frequently change an
object's motion.
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Newton's First Law of Motion
The first law of motion is sometimes referred to as the law of inertia.
Inertia is the tendency of an object to resist a change in motion.
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The First Law of Motion
The first law basically tells us that motion will not change without
a net force. So, if an object stops moving or starts moving, you
know there is a net force.
If there is a net force, then the forces are
unbalanced. As you know, unbalanced
forces cause changes in motion!
Apply Newton's First Law of Motion to
the baseball player sliding into second
base. What are some forces acting on
him?
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Inertia Simulation
Click on the image to the left to
launch simulation.
Click on the motion tab, check
speed to add speedometer, and
place a person on the skateboard.
Apply a force to the object and look
at the speedometer. What
happened?
Stop applying the force. What
happens?
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Application of The First Law of Motion
Have you ever wished that you could just tell
your clothing to move itself to the closet?
Unfortunately, we know that objects don't move on their own. An
unbalanced force is required to make an object change its state
of motion.
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Application of The First Law of Motion
Imagine you need to move a few pieces of furniture in your
room. Would you rather move your dresser with everything in
it or your dresser when its empty?
Be sure to use the term
inertia in your answer.
Based on your answer, how are inertia and mass of an
object related?
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34 Inertia is the resistance of an object to change in
its state of motion.
True
False
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35 The law of inertia applies to___________.
A
moving objects
B
nonmoving objects
C
both moving and nonmoving objects
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Which has more Inertia?
A Tennis Ball or a Bowling
Ball?
Why?
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36 Which object has the greatest inertia?
A
Car
B
Tennis Ball
C
Moving Freight Train
Slide 117 / 159
37 Which object has the greatest inertia?
A
B
C
Car
Tennis Ball
Freight Train at rest
Slide 118 / 159
38 Which object has the greatest inertia?
A
Moving Tennis Ball
B
Tennis Ball at Rest
C
Both have the same Inertia
Slide 119 / 159
39 A ball will accelerate when it is acted on by 2
equal forces pointing in opposite directions.
True
False
Slide 120 / 159
Newton's Second Law
Newton's second law states that acceleration depends on both force
and mass. It supports the idea that unbalanced forces cause
acceleration.
Remember, acceleration is an increase in speed, a decrease in speed,
or a change in direction.
Slide 121 / 159
Newton's Second Law
Unbalanced
forces cause
acceleration.
Forces that
cause a net
force on an
object are
unbalanced.
There is a direct
relationship
between force
and acceleration.
As force
increases,
acceleration
increases.
Mass and
acceleration are
inversely
proportional.
As mass
increases,
acceleration
decreases.
Slide 122 / 159
Newton's Second Law
Unbalanced forces cause acceleration, so unbalanced forces
cause an increase in speed, a decrease in speed, and/or a
change in direction.
The second law relates force, mass, and acceleration.
Force = mass x acceleration
Slide 123 / 159
Newton's Second Law
We can rearrange this to solve for the other variables. You do
not need to know how to rearrange it, but you do have to be able
to select the right formula for solving your problem.
F = ma
F
a=
m
m= F
a
Slide 124 / 159
Application of Newton's Second Law
The second law states that if force is increased, acceleration will also
increase.
If you want to
shoot a very fast
penalty kick into
the soccer net,
how do you kick
it?
Slide 125 / 159
Application of Newton's Second Law
The second law also tells us that the greater the mass, the less the
acceleration (if the force is constant).
Have you ever pulled your friends in a wagon?
How hard is it to pull one
person in the wagon?
What happens when you add
another person to the wagon?
Slide 126 / 159
40 When the force acting on an object
increases, the resulting acceleration will:
A
remain constant
B
increase
C
decrease
Slide 127 / 159
41 When an object's mass increases but the applied
force stays the same, the resulting acceleration
will:
A
remain constant
B
increase
C
decrease
Slide 128 / 159
42 You are riding your bicycle to the park to meet a
few friends. As you ride, you apply a force of 30 N
and accelerate at a rate of 0.4 m/s2. What is the
total mass of the bicycle and you?
A 75 kg
B 12 kg
C 120 kg
Slide 129 / 159
43 You are pushing a shopping cart so that it
accelerates at 0.2 m/s2 . You start filling the
shopping cart with food. Do you need to change
how you push on the cart for it to maintain the
same acceleration? If so, in what way?
A
No
B
Yes, push harder
C
Yes, push softer
Slide 130 / 159
Newton's 2nd Law Simulation
Click on the image to the left to download
the Simulation.
Click the Acceleration Lab Tab
Check show Forces, masses, acceleration,
and turn friction to none.
Place 1 crate onto the surface and apply a
500 N force, note the acceleration.
Stack 2 crates onto the surface and apply
a 500 N force,note the acceleration.
What was the effect of adding mass to the simulation on the resulting
acceleration produced?
Slide 131 / 159
Newton's 3rd Law of Motion
& Momentum
Return to Table
of Contents
Slide 132 / 159
Newton's Third Law of Motion
Newton's third law of motion,
unlike the first and second,
pertains to forces between two
objects.
When you kick a soccer ball,
do you feel the force of the
ball against your foot?
Does the ball "feel" the force
of your foot?
Newton's third law explains this occurrence.
Slide 133 / 159
Newton's Third Law of Motion
Have you ever jumped off a skateboard? What happens to the
board when you jump off? Draw an arrow below to show what
direction the board will go.
This is Newton's third law of motion at work! You applied a force to
the board and the board applied an equal (in magnitude) and
opposite (in direction) force on you.
We call these Action-Reaction Forces.
Slide 134 / 159
44 When you sit on a chair, the seat of the chair
pushes up on you with more force than your
weight.
True
False
Slide 135 / 159
45 Action-Reaction forces are always found
in pairs that are equal and opposite.
True
False
Slide 136 / 159
Newton's Third Law of Motion
Forces always exist in pairs! Newton's third law defines these
action and reaction forces.
Click here to see how Newton's Third Law
applies to the physics of a rocket.
Slide 137 / 159
Newton's Third Law of Motion
As you kick the soccer ball, you apply an action force to the ball, and
the ball applies a reaction force on your foot.
These forces are equal in strength and opposite in direction. Why
does the ball accelerate more quickly than you and your foot?
Slide 138 / 159
Newton's Third Law of Motion
n
ti o
ac
re
ac
ti o
n
The third law states that for every action force, there is an equal,
but opposite reaction force.
This means that if one object applies a force to another object, then the
other object exerts an equal and opposite force on the first object.
Slide 139 / 159
46 When you jump off a skateboard, which of the
following is true?
A You accelerate more than the skateboard.
You and the skateboard accelerate equal but
B opposite amounts.
C The skateboard accelerates more than you.
Slide 140 / 159
Newton's Third Law of Motion
According to Newton's 3rd law, the cart pulls on the man just as
hard as the man pulls on the cart. Do these forces cancel each
other out preventing the cart and man from moving?
Slide 141 / 159
Newton's Third Law of Motion
Action: The man applies force to the cart that moves the cart forward.
Reaction: The cart applies an equal and opposite force on the man.
Each force in the pair acts on a different object!
Slide 142 / 159
The Truth about Action Reaction Forces
If a force occurs, there are action reaction forces!
Action reaction forces:
can cause
changes in
motion
are equal in
strength but
opposite in
direction
act on different
objects
Slide 143 / 159
47 Which of the following statements pertains to the
third law of motion?
Action and reaction pairs always act on the
A same object.
Mass is indirectly proportional to
B acceleration.
An object at rest will remain at rest unless
C acted on by an unbalanced force.
If a force occurs, action reaction forces are
D present.
Slide 144 / 159
48 A textbook is resting on a table. Tommy pushes
the book rightwards. What is an action reaction
pair in this scenario?
The book exerts a downward force on the
A table. The table exerts an upward on Tommy.
Tommy exerts a rightward force on the book. The
B book exerts a downward force on the table.
The book exerts a leftward force on Tommy.
C Tommy exerts a rightward force on the book.
Tommy exerts a downward force on the table.
D The table exerts a rightward force on Tommy.
Slide 145 / 159
49 Action reaction pairs cancel each other out since
they are equal and opposite to each other.
True
False
Slide 146 / 159
Momentum
Newton's third law tells us that action reaction forces are equal and
opposite, but that does not mean that the effects of those forces are
equal.
Click here to see how equal action
reaction pairs cause different motions.
After watching the video, discuss the following:
· what was the action reaction pair?
· did both carts accelerate the same? Why or why not?
· how is this related to Newton's 1st and 2nd law of motion?
Slide 147 / 159
Momentum
Both carts experience the same strength in force (Newton's 3rd
law). But the cart on the right experiences a greater change in
motion because it has less mass. Why?
Remember Newton's law of inertia. Objects with more mass (more
inertia) have more reluctance to change their motion and vice versa.
Also recall Newton's 2nd law. Objects with more mass accelerate less
than objects with less mass under the same force (and vice versa).
Slide 148 / 159
Momentum
If we understand Newton's third law and momentum, we can predict
how the motion of colliding objects will change.
Momentum is the result of the mass of the object times the object's
velocity.
momentum (kg-m/s) = mass (kg) x velocity (m/s)
Slide 149 / 159
Momentum
Find the momentum of a skateboarder with a mass of 50 kg traveling
at a velocity of 4 m/s west.
p = mv
Slide 150 / 159
50 How can a small insect have the same momentum
as a large car?
A
insect has large speed
B
both car and insect are at rest
C
insect has no mass
D
A&B
Slide 151 / 159
51 If a 2 kg toy truck is moving at 4 m/s, what is the
toy's momentum?
A
3 m/s
B
2 kg m/s
C
8 kg m/s
D
8 m/s
Slide 152 / 159
Law of Conservation of Momentum
Momentum is conserved (remains the same) during an
interaction as long as the objects are not affected by outside
forces.
This means that the total momentum of objects prior to hitting
each other will equal the total momentum of the objects after
the interaction.
Any momentum lost by one object is gained by the other!
Slide 153 / 159
Momentum Simulation
Click on the image to the left to
launch and play the simulation.
Make each object the same
mass by moving sliders (1 kg
each works best).
Click show values.
Compare the total momentum of
the balls added before and after
the collision.
What happens to the total amount of momentum before and after the
collision?
Slide 154 / 159
Law of Conservation of Momentum
Fill in the missing values. Remember that the law of conservation
tells us that the total momentum before a collision is equal to the
total momentum after a collision.
After Collision Data
Before Collision Data
Mass Velocity Momentum
Car
(kg m/s)
(kg) (m/s)
Car
Mass Velocity Momentum
(kg m/s)
(kg) (m/s)
1
1000
6
1
1000
2
2000
0
2
2000
Total Momentum: ___________
-12
Total Momentum: ___________
HINT: p=mv can be rearranged to v=p/m and a negative momentum
means the object moves left!
Slide 155 / 159
52 What two variables does momentum depend on?
A
mass and volume
B
mass and acceleration
C
mass and velocity
D
mass and force
Slide 156 / 159
53 The total amount of momentum before and after a
collision may vary.
True
False
Slide 157 / 159
Newton's Third Law and Momentum Lab
Click on the picture to go to the website for the lab.
Slide 158 / 159
Newton's Laws of Motion Lab
Rotate from station to station in the classroom.
Follow the directions that correspond with your station.
Make observations, record your results, and answer the questions
for each station.
Station 1: Inertia is Nuts!
Station 2: Balloon Blow Out
Station 3: Spinning Penny
Station 4: Water Whirl
Station 5: Dominoes
Station 6: Yay for Seatbelts and Airbags
Station 7: Free Fallin'
Station 8: Rolling Chair
Slide 159 / 159
Images Cited
A7N8X 2012, 4-Stroke Engine, gif, viewed 29 June 2015,
<https://commons.wikimedia.org/wiki/File:4-Stroke-Engine.gif>.
Luke Ma 2012, Pull Carts Kyoto, Japan, jpg, viewed 7 July 2015,
<https://commons.wikimedia.org/wiki/File:Pull_carts,_Kyoto,_Japan_(8587844087).jpg>