Unit 7
Forces and Motion
Pushing and Pulling
• Learning Goal:
I will understand what a force is and what
changes it can cause in an object.
Pushing and Pulling
• force – a push or pull that
causes motion.
– forces can:
• cause an object at rest to
move
• cause a moving object to
speed up, slow down, change
direction, or stop
• also change an object’s shape
• Forces are measured with
a spring scale in units
called newtons(N)
Forces
Start
up
Change
shape
Change
direction
Speed
up
Slow
down
Stop
Pushing and Pulling
• Brain Pop:
http://www.brainpop.com/science/motions
forcesandtime/force/
Pushing and Pulling
1. Describe the force that the road exerts on the cart on pg. 306.
2. What are some things that a force can do to an object?
3. What do all of these things have in common?
4. Is there any way that an object can move without being pushed or
pulled?
5. How can you use a push or a pull to do the same task?
6. Do all forces result in a change in motion of an object?
7. What forces are acting in the picture of the ball hitting the floor, on
page 307?
Two Common Forces
• Learning Goal:
I will be able to compare and contrast the
forces of gravity and friction.
Two Common Forces
• Gravity - is a force of attraction
between two objects. It acts even
when objects are not touching.
• Gravitational force – all objects in
the universe pull on each other.
– The more mass the object has the
stronger its gravitational force is
– The closer objects are to each other the
greater their gravitational pull.
• In this picture the spring scale
with weight attached is being used
to measure gravitational force.
Two Common Forces
• Brain Pop:
http://www.brainpop.com/science/motions
forcesandtime/gravity/
Two Common Forces
Two Common Forces
• Friction creates heat.
– It changes the energy of motion to
thermal energy (heat) as objects rub
together.
• Your body being dragged across the
carpet causes rug burns…Ouch!
• Resistance – friction that exist between
air and a moving object.
Two Common Forces
• Bill Nye :
Friction
Two Common Forces
1.
What two things determine the force of gravity between two objects?
2.
The distance between Earth and the sun is much greater than the
distance between Earth and the moon. Why does the sun have a stronger
gravitational pull on Earth than the moon does?
3.
Why do you put grease on the chain of your bicycle?
4.
How can you tell there is friction when you rub your hands together?
5.
Classify the role of friction in the following situations as “a helpful
force” or “a force to be overcome.”
– A person puts a rubber pad in front of a wet sink to prevent slipping.
– Furniture movers place rollers under a piano as the move it from room
to room.
– A person rubs her hands together to warm them up when they are
cold.
Balanced or Unbalanced
• Learning Goal:
I will understand how to explain the
causes and effects of balanced and
unbalanced forces on objects.
Balanced or Unbalanced
• balanced forces – are forces on an
object that are equal in size and
opposite in direction. They cancel each
other out.
• unbalanced forces – when one force is
stronger than another and motion
changes.
– object may start, speed up, slow down,
stop, or change its direction
A force must overcome the force of friction
before an object will move.
Balanced and Unbalanced Forces
• Baseball
– During a baseball game the batter hits the
ball.
– When the ball goes soaring away from the
batter the forces acting on the ball are
unbalanced
• Wrestling
– When two wrestlers exert force on each
other but they don’t move anywhere the
forces are balanced.
Balanced or Unbalanced
1. What kinds of kinds of forces must act on an
object in order for the object to remain at
rest?
2. A car pulls away from a stoplight. What kinds
of forces are acting on the car?
3. A car moves at a constant velocity down a
straight road. What kinds of forces are acting
on the car?
Balanced or Unbalanced
– Balanced forces MUST act on the same
object.
• Examples:
– the forces acting on you in your desk are an
upward force from the chair and a
downward force from gravity.
– You are between the two forces, both
forces are acting on you.
• BUT, at the same time, you are exerting a
force on the chair and the chair is exerting
a equal but opposite force on you.
– This is an action-reaction force, not
balanced forces because they don’t act on
the same object.
– There is nothing between you and the chair
so there are not two forces acting on either
you or the chair.
Balanced or Unbalanced
• The arrows represent the forces
applied to an object. A longer arrow
means a greater force. Which object
will move downward.
A
B
C
• net force – is the combination of all
forces acting on an object. It affects
an object’s motion
Balanced or Unbalanced?
• To determine Net Force You must look at the plus
force and the minus force to determine the net
force
– Example:
• 2 girls are pushing a box to the left.
• Each of these girls is exerting 50 N of force on the box
• A third girl is pushing the box to the right and is exerting 50N
of force.
• The net force on the box is 50 N and the box will move to the
left
– Example:
• 1 boy is pushing a box to the left and is exerting 100 N of
force.
• Another boy is pushing the same box to the right and is also
exerting 100 N of force.
• The net force on the box is 0 and the box will not move.
Pull (or Push) Harder!
• Large forces can cause objects to
change velocity, or accelerate, quickly.
(A large force will cause a large change
in motion. A small force will cause little
change.
• Changes in velocity include changes in
speed and changes in direction.
Pull (or Push) Harder!
• speed – the distance an object travels in a
certain time
– Speed = distance / time
– If two cars of different masses both
travel 1 m/sec they have the SAME
speed!
– If one of those cars accelerates its
speed will be greater.
• velocity – the distance an object travels in
a certain direction in a certain time
– if you change speed OR direction your
velocity changes
– if two cars are traveling at the same
speed, in the same direction, then their
velocities are equal
Pull (or Push) Harder!
• The velocity of a
motorcycle rider continues
to change during a 1-minute
period.
– What can you infer about
the motorcycle and the
rider?
Pull (or Push) Harder
1. In track and field, one event is called shot put. Competitors throw
heavy balls as far as they can. What are two thing that would cause a
person to be able to throw a ball farther?
2. Which would cause a greater change in motion – a small fan blowing
on a stack of papers or a large fan?
3. What causes the piece of metal to ring the bell in the picture on this
page?
4. A man uses a force of 10N to cause the metal that rings the bell to
have an initial acceleration of 5 m/s². How much force would he need
to accelerate the metal at 2.5 m/s² ? 10m/s²? 20m/s²?
5. If gravity pulls down on the piece of metal that rings the bell with a
force of 10N, how much force is needed to cause the piece of metal
to rise?
I’m Not Moving!
• Learning Goal:
I will be able to summarize the
relationship between the force used to
move an object and the mass of that
object.
I’m Not Moving!
• Acceleration – the rate at which
an object’s velocity changes
– Acceleration = change in velocity /
time
– Includes speeding up AND slowing
down .
– You can use acceleration to
determine speed!
• Example: a car is stopped at a red light.
The light turns green, and the car
starts accelerating a 10km/hr every
second. Its speed at the end of 3
seconds = 30km/hr
• 30 km/hr = 10km/hr x 3 seconds
I’m Not Moving
• Brain Pop:
http://www.brainpop.com/science/motions
forcesandtime/acceleration/
I’m Not Moving!
• If the same force is
applied, objects with
greater mass accelerate
slower than objects
with less mass.
• How can the steel
ball be made to go
higher?
I’m Not Moving!
• Why does it take
the full truck
longer to stop than
the empty truck?
I’m Not Moving
• If an unbalanced force is applied to an
object, the object moves in the
direction of the force. Draw a graph
that shows how the same force will
affect objects that have different
masses?
Increasing
motion
Increasing mass
I’m Not Moving!
1. What is the relationship between the mass of an
object and the force it takes to change its motion?
2. Suppose vehicle A has twice the mass of vehicle B.
The drivers step on the brakes, and the brakes
apply the same force to each vehicle. What will
happen? Why?
3. When Janelle plays tennis, she often hits the ball
into the net. She decides that she needs to use a
heavier racket to apply more force to the ball so
that it will clear the net. Is Janelle’s idea correct?
Explain.
Let’s Go to Mars!
• Learning Goal:
I will be able to explain the steps it takes
to overcome forces and send a Rover to
Mars.
Let’s Go to Mars!
• Unbalanced forces can allow rockets to oppose Earth’s
gravity.
• Small booster rockets can change the direction of the
vehicle’s motion.
• Rockets reach an “escape velocity” at which Earth’s
gravity will no longer pull the vehicle back down.
• Balanced forces allow a spacecraft to move in one
direction at a constant velocity through space.
• Once the spacecraft reaches Mars, it will accelerate
toward the surface because of gravity.
Let’s Go to Mars!
1. In order to send a rocket to the moon or beyond, the engines
have to accelerate the rocket to “escape velocity.”
– When the rocket reaches this speed, Earth’s gravity still
acts on it, but the rocket is fast enough to leave Earth’s
orbit.
– What can you infer about the relationship between Earth’s
gravity and the velocity of a rocket?
2. How do you know that the forces acting on the rocket before
it leaves Earth are balanced?
3. Use forces to explain the purpose of the air bags around the
Mars Rover.
4. How can you tell whether the forces are balance or
unbalanced?
How Do Forces Affect Motion?
• Learning Goal:
I will understand how forces of various
sizes affect objects of various sizes.
How Do Forces Affect Motion?
• Look at the table on page 323, why are
there two sections to the table?
• What is the value of displaying data in a
table such as this?
• On the experiment what does pulling the
band back farther do?
How Do Forces Affect Motion?
• What happens to the distance traveled
by the truck as the force increases?
• What variable changes in the second
part of the investigation?
• What happens to the distance traveled
by the truck as its mass increases?
What Are Balanced and
Unbalanced Forces
• Learning Goal:
I will understand about the forces that
are required to make an object move.
What Are Balanced and
Unbalanced Forces
• Add a column to the data table to the right
of the steps in which you did a trail three
times. What do you think you should do with
the values you got in these trials? Why?
• Suppose that students did the same
investigation in several different class rooms.
Would their data turn out the same?
• What is the value of having many people
repeat the same experiment?