Motion
&
Forces
1.
2.
Force (F)- push or pull (cause of acceleration, or
change in object’s velocity)
A. Examples: pushing a box across the floor,
hitting a baseball, throwing a basketball, etc.
Forces influence motion
1. Changes the velocity- speeds up, slow down or
changes the direction
2. Forces do not cause motion; they cause
acceleration
3.
4.
Net force = the combination
of all forces acting on an
object.
Objects accelerate in direction
of the net force (if it is zero,
then there is no acceleration)
1.
2.
3.
Balanced forces – Net Force = 0
(They cancel each other out)= no
acceleration but could be at
constant velocity.
Unbalanced forces – Net force ≠ 0
(they do not cancel) =objects will
accelerate
Forces in same direction add,
opposite directions subtract
1.
Measured in Newtons (N)- amount of force it
takes to cause1kg of mass to accelerate at 1m/s2.
(1 N= 1 kg•m/s2).
2.
Representing Force-use an arrow
The arrow points in the direction of the force.
The length of the arrow represents the strength of
the force.
Examples: Give direction and which is strongest:



A
B
C
1.
2.

Arrows pointing in same direction add
together.
Arrows pointing in opposite directions
subtract from each other.
Example:
3N
3N
1N
1N
= 4N to right
= 2N to right

Look at the Force diagrams below: Figure
up the net Force and in which direction the
object will accelerate.
DO NOT COPY -
CHECKING YOUR UNDERSTANDING
If the forces acting upon an object are balanced, then
the object
a. must not be moving.
b. must be moving with a constant velocity.
c. must not be accelerating.
d. none of these
The answer could be A and it could be B .
An object having balanced forces definitely cannot be accelerating.
This means that it could be at rest and staying at rest (one option) or could be
in motion at constant velocity (a second option).
Either way, it definitely is not accelerating
1.
2.
3.
Friction- force that
acts in a direction
opposite the
moving object
Causes the object to
slow down and
eventually stop
To move an object, your
force must be larger
than the force of friction
1.
2.
3.
4.
Static Friction- force that surface acts on an
object to keep it from moving. Ex: box sitting on
ground
Sliding friction- force that opposes the direction
of motion of an object as it slides over a surface:
Ex: slide
Rolling friction- force that opposes motion of a
rolling object. Ex: tires
Fluid friction- friction opposing object moving
through a fluid such as gas or water. Ex:
airplane, submarine
1.
2.
3.
Gravity – attraction
between two objects due
to their mass
All falling objects
accelerate at the same rate
(9.8 m/s2)
It depends on the mass of
the two objects as well as
the distance between
them
Gravity depends on:
4.


5.
6.
the mass of the two objects
the distance between the objects
Greater mass = greater the
attraction of gravity
Greater distance b/t objects =
gravity decreases
1.
2.
Free fall – motion of an object
when gravity is the only force
acting on it(no air resistance)
All objects near earth
accelerate at 9.8 m/s2 in the
absence of air resistance
The 1000-kg baby elephant obviously has more mass
(or inertia). This increased mass has an inverse affect
upon the elephant's acceleration. And thus, the direct
affect of greater force on the 1000-kg elephant is offset
by the inverse affect of the greater mass of the 1000kg elephant; and so each object accelerates at the
same rate - approximately 10 m/s/s. The ratio of
force to mass (Fnet/m) is the same for the elephant
and the mouse under situations involving free fall

What would happen if air
resistance was present.

Weight – measure of the force of gravity on
an object ‘s mass (changes as gravity
changes)
Weight = mass x gravity(9.8 m/s2)
w = mg
Unit: Newton
g = 9.8 m/s2 near earth
1 N = 0.225 lb; 1 lb = 4.448 N

Mass = does not change EVER
Weight = changes as gravity
changes= Will change when
you go to another planet.
1.
A 10 kg mass would weigh ____
1.
A 50 kg mass would weigh ____
490 N
1.
A 100 kg mass would weigh ___
980 N
1.
An astronaut that weighs 600 N on
Earth is standing on an asteroid
with a gravitational force one
hundredth that of Earth. What is
his weight on the asteroid?
98 Newtons
6 Newtons
 Air
resistance
is the friction
caused by air.

Terminal Velocity – when air resistance
balances weight and the object stops
accelerating and reaches its maximum
constant velocity
 320 km/h (200 mi/h)
Fair
 no
net force
 no acceleration
 constant velocity
Fgrav
DO NOT COPY CHECKING YOUR UNDERSTANDING
Which of the following statements are true of the quantity mass?
List all that apply.
a) The mass of an object is dependent upon the value of the
acceleration of gravity.
b) The standard metric unit of mass is the kilogram.
c) Mass depends on how much stuff is present in an object.
d) The mass of an object is variable and dependent upon its location.
e) An object would have more mass on Mount Everest than the same
object in the middle of Lake Michigan.
f) People in Weight Watcher's are really concerned about their mass
(they're mass watchers).
g) The mass of an object can be measured in pounds.
h) If all other variables are equal, then an object with a greater mass
would have a more difficult time accelerating.
i) The mass of an object is mathematically related to the weight of
the object.
ANSWER: B, C, F, H, I

Newton’s First Law- states an object
in rest will remain at rest, or an
object in motion will remain in
motion unless a force(unbalanced
force) acts upon it.
 Also known as the Law of Inertia
What will happen
when the car
hits the wall?
1.
2.
While riding a skateboard you fly forward off
the board when hitting a curb or rock or other
object which abruptly halts the motion of the
skateboard.
Headrests are placed in cars to prevent
whiplash injuries during rear-end collisions
Do Not Copy
Other Real life examples:
1. Blood rushes from your head to your feet while quickly stopping
when riding on a descending elevator.
2. The head of a hammer can be tightened onto the wooden handle by
banging the bottom of the handle against a hard surface.
3. A brick is painlessly broken over the hand of a physics teacher by
slamming it with a hammer. (CAUTION: do not attempt this at home!)
4. To dislodge ketchup from the bottom of a ketchup bottle, it is often
turned upside down and thrusted downward at high speeds and then
abruptly halted.
5. Headrests are placed in cars to prevent whiplash injuries during rearend collisions.
1.
2.


Inertia - the tendency of an object to
resist a change in its motion
Inertia depends Only on the mass of an
object
 More mass = more inertia
Examples:
 In car crashes, you tend to remain in
motion until you are acted on by a
force (until you hit something that
resists you)
Seatbelts
DO NOT COPYCHECKING YOUR UNDERSTANDING
1. A 2-kg object is moving horizontally with a speed of 4 m/s. How
much net force is required to keep the object moving at this speed
and in this direction?
Answer: 0 N
An object in motion will maintain its state of motion. The presence of
an unbalanced force changes the velocity of the object.
2. Mac and Tosh are arguing in the cafeteria. Mac says that if he
flings the Jell-O with a greater speed it will have a greater inertia.
Tosh argues that inertia does not depend upon speed, but rather
upon mass. Who do you agree with? Explain why.
Tosh is correct. Inertia is that quantity which depends solely upon
mass. The more mass, the more inertia.
3. Supposing you were in space in a weightless environment, would it
require a force to set an object in motion?
Absolutely yes!
Even in space objects have mass. And if they have mass, they have
inertia. That is, an object in space resists changes in its state of
motion. A force must be applied to set a stationary object in motion.
Newton's laws rule - everywhere!
1.
The unbalanced force acting
on an object equals the
object’s mass times its
acceleration
F
m a
F = ma
F
m a
a=acceleration (m/s2)
 m-mass (kg)
 F= force (N) newtons


1N = 1kg x m/s2
1.
Determine the accelerations which result when
a 12-N net force is applied to a 3-kg object and
then to a 6-kg object
A 3-kg object experiences an acceleration of 4
m/s/s. A 6-kg object experiences an
acceleration of 2 m/s/s.
2.
An automobile with a mass of 1000
kilograms accelerates when the
traffic light turns green. If the net
force on the car is 4000 newtons,
what is the car’s acceleration?
4000N/1000kg=4m/s2
1.
2.
For every action
force, there is an
equal and
opposite
reaction force.
Forces occur in
pairs
4.
The action force and
the reaction force
occur to different
objects so the force is
not balanced.
(movement occurs)
DO NOT
COPY
1.
2.
You push on wall & wall pushes on you
Swimming = you push on water and
water pushes on you
3.
The hammer exerts a force on the nail
to the right. The nail exerts an equal but
opposite force on the hammer to the
left.
4.
The rocket exerts a downward force on
the exhaust gases.
5.
The gases exert an equal but opposite
upward force on the rocket.
Quick Video
 If
you dropped
a 1.5 kg book
and a 15 kg
rock from the
same height,
which would
hit the ground
first?
ANSWER:
They would
hit at about
the same
time

The extra
mass of the
heavy object
exactly
compensates
for the
additional
gravitational
force.
A heavier object experiences a
greater gravitational force than
a lighter object
But –
A heavier object is also harder to
accelerate because it has more
mass (greater inertia)
Must be connected to the internet to Play Video