8.PS.2-6-Lesson 3: Applying Newton’s Third Law – Motion and Reac

​8.PS.2-6-Lesson 3: Applying Newton’s Third Law – Motion and Reactions

By Jessica Freeman

Momentum and Collisions: What is Momentum?

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What is Momentum?

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Key Vocabulary

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Momentum

A measure of an object's motion, combining its mass and velocity. It indicates how hard it is to stop.

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Conservation of Momentum

The total momentum of objects before a collision is the same as the total momentum after the collision.

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Impulse

A force applied over a specific time that results in a change in an object's momentum.

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Elastic Collision

A collision where objects bounce off each other, and both momentum and kinetic energy are conserved.

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Inelastic Collision

A collision where objects stick together, and only momentum is conserved, while kinetic energy is not.

Multiple Choice

What two factors are multiplied to determine the momentum of a moving object?

• Mass and velocity

• Force and time

• Mass and acceleration

• Force and velocity

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Conservation of Momentum & Collisions

Elastic Collisions

Inelastic Collisions

Multiple Choice

What is the key difference between a perfectly elastic and a perfectly inelastic collision?

• Momentum is conserved in elastic collisions but not in inelastic ones.

• Objects stick together in elastic collisions and bounce apart in inelastic ones.

• Kinetic energy is conserved in elastic collisions, but not in inelastic collisions.

• Inelastic collisions only happen with objects of equal mass.

Multiple Choice

What is the momentum of a 3 kg ball moving at the velocity of 5 m/s east?

• 3 kg m/s east

• 5 kg m/s west

• 15 kg m/s west

• 15 kg m/s east

• 0 kg m/s

Multiple Choice

What do we call it when two or more moving objects exert forces on each other for a short period of time?

• Momentum

• Work

• Collisions

• Friction

• Energy

Multiple Choice

If a ball is moving at the velocity of 10 m/s east with a momentum of 50 kg m/s east, what is its mass?1 kg

• 1 kg

• 2.5 kg

• 5 kg

• 25 kg

• 500 kg

The Law of Conservation of Momentum

• This law is true for any collision if no forces act on the colliding objects.

• It applies whether the objects stick together or bounce off each other after they collide.

Objects Sticking Together

​Sometimes, objects stick together after a collision. After two objects stick together, they move as one object. The mass of the combined objects is equal to the masses of the two objects added together. In a head-on collision, the combined objects move in the direction of the object that had the greater momentum prior to the collision. Together, the objects have a velocity that differs from the velocity of either object before the collision because momentum is conserved and depends on mass and velocity. When mass changes, velocity must change too.

​Objects Sticking Together

​​Sometimes, objects stick together after a collision. After two objects stick together, they move as one object. The mass of the combined objects is equal to the masses of the two objects added together. In a head-on collision, the combined objects move in the direction of the object that had the greater momentum prior to the collision. Together, the objects have a velocity that differs from the velocity of either object before the collision because momentum is conserved and depends on mass and velocity. When mass changes, velocity must change too.

​Objects Bouncing Off Each Other

In some collisions, the objects bounce off each other. During such collisions, momentum is usually transferred from one object to another object. The transfer of momentum causes the objects to move in different directions at different speeds. However, the total momentum of all the objects will remain the same before and after the collision.

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​Examples of such collisions:

• ​bowling ball and bowling pins

• ​billiard balls

• ​bumper cars

​EXAMPLES OF CONSERVATION OF MOMENTUM

Conservation of Momentum and Newton's Third Law

• Newton's third law of motion can explain conservation of momentum.

• ​In the billiard ball example, the cue ball hit the billiard ball with a certain amount of force. This was the action force. The reaction force was the equal but opposite force exerted by the billiard ball on the cue ball.

• The action force made the billiard balls start moving, and the reaction force made the cue ball stop moving.

• Because the action and reaction forces are equal and opposite, momentum is neither gained nor lost.

Conservation of Momentum and Newton's Third Law

• Newton's third law of motion can explain conservation of momentum.

• ​In the billiard ball example, the cue ball hit the billiard ball with a certain amount of force. This was the action force. The reaction force was the equal but opposite force exerted by the billiard ball on the cue ball.

• The action force made the billiard balls start moving, and the reaction force made the cue ball stop moving.

• Because the action and reaction forces are equal and opposite, momentum is neither gained nor lost.

Multiple Choice

Which of the following has the smallest amount of momentum?

• a loaded truck driven at highway speeds

• a track athlete running a race

• a baby crawling on the floor

• a jet airplane being towed toward an airport

Multiple Choice

What is momentum?

• product of mass and acceleration

• product of mass and velocity

• product of time and force

• product of imagination

downward

upward

opposite

opposite

equal

motion

Multiple Choice

Newton’s law states that for every action, there is an ________ reaction.

• unequal and opposite

• equal and similar

• stronger and opposite

• equal and opposite

Multiple Choice

When a cannon fires a cannonball we observe Newton's third law. Which is the reaction force?

• The cannon moving forward

• The cannon moving backwards

• The cannonball moving backwards

• The cannonball moving forward

Multiple Choice

Action reaction forces never cancel each other out because

• They act on different objects.

• They act on the same objects

• They do not act on objects.

Multiple Choice

How is this an example of Newton’s Third law?

• Only the ball is using the head

• They are both exerting an equal force on each other

• Only the head is pushing the ball

Multiple Choice

If two people with the same mass collide, they experience the same amount of force.

• True

• False

Multiple Choice

Force always come in________

• waves

• triplets

• pairs

Multiple Choice

The forces in a force pair are in _______ direction

• opposite

• same

Multiple Choice

A person steps out of the boat and onto the dock.  As the person moves forward onto the dock, the boat moves backward.  Which statement best describes the backward motion of the boat as the person steps forward?

• For every action there is an equal and opposite reaction.
• Every mass exerts a force on every other mass.
• An object at rest tends to stay at rest.
• Friction opposes the motion of an object.

Multiple Choice

This picture shows .............

• Newton 2nd law of motion

• Newton 3rd law of motion

• Newton 1st law of motion

• I don't know

Multiple Choice

What happens to the balloon if the air is released ?

• Balloon goes right

• Balloon goes up

• Balloon goes down

• Balloon goes left

Multiple Choice

The two paired forces in Newton's third law:

• act on different objects

• are equal in size

• are opposite in direction

• are the same type

• all of the above

Multiple Choice

A semi truck and a Volkswagen (small car) traveling at the same speed have a head on collision. Which statement is correct?

• The impact force is greater on the Volkswagen.

• The impact force is greater on the semi truck.

• The impact force is less on the semi truck.

• The impact force is equal for both.

Open Ended

What does Newton's third law state?

Open Ended

^{What does action-reaction mean?}

Open Ended

If an object srikes another object, the objects move in opposite directions, but why don't they move the same opposite distance?

Open Ended

Two metal spheres are suspended on wires so that they hang next to each other, just touching. Sphere X is raised to a certain height and released. Without friction or air resistance, describe what would happen to Sphere Y.

Open Ended

A 500 N girl standing on a dock exerts a force of 100 N on a sailboat as she pushes it away from the dock. Describe the amount of force that the sailboat exerts on the girl.

Open Ended

Identify the action and reaction forces of a rocket blasting off.

Open Ended

Engineers have to think about safety in the cars they design. Describe how a vehicle restraint, such as a seat belt, shows Newton’s third law of action, which is reaction, when it helps keep you safe in a car crash.

Multiple Choice

The action of the rocket is the expelling of gas from the engine. The reaction is the movement of the rocket in the opposite direction.

• 100 Newtons, same direction

• 
  100 Newtons, opposite direction

• 
  200 Newtons, same direction

• 
  200 Newtons, opposite direction

Multiple Choice

According to Newton’s Third Law, if someone jumps off a skateboard, the skateboard will roll backwards because -

• the mass of the skateboard is the same as the mass of the person jumping off.

• the pull of gravity on the person is the same as on the skateboard, regardless of size.

• 
  the air resistance on the person is great and the friction on the skateboard is small.

• 
  for the force that sent the person forward, there is an equal force pushing back on the skateboard.

Multiple Choice

The nozzle of a rocket is pointed downward so that as fuel is ignited, the exhaust pushes downward. Why is this arrangement necessary for a rocket to function properly?

• 
  As fuel is combusted, the rocket pushes the gases backward and the gases push the rocket forward.

• The rocket is protected from the heat of the exhaust.

• 
  Burned fuel can be collected on the ground.

• This allows the rocket to be steered properly.

Multiple Choice

Newtons third law describes action and reaction forces. A table of four scenarios is provided.

Which of the scenarios described in the table show equal and opposite reaction forces?

• 
  The train and girl

• The rocket and train

• The rocket and balloon

• 
  The balloon and girl

Multiple Choice

The head of a hammer is often made of steel. This makes the head heavy, which helps create a strong force for driving nails. Steel is also strong. The head of a hammer must be strong to resist what force?

• 
  The force of the nail pushing in the opposite direction

• The force of the air resistance on the moving hammer

• The force of the person using the hammer

• The force of gravity pulling down on the hammer

Multiple Choice

When a heavy football player and a light one run into each other, who exerts more force?

• The heavy one
• The light one
• They are equal
• there is no such thing as force

Match

Match the following

• This is always equal and opposite to the action force

• To strike or hit something with great force

• A change in position of an object relating to time

• For every action, there is an equal and opposite reaction

• A push or pull that changes an object's motion

Reaction

Collide

Motion

Newton's third law

Force

• Reaction

• Collide

• Motion

• Newton's third law

• Force

Drag and Drop

For every

Placeholder

there is an equal and opposite ​reaction between ​

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. A ​

Placeholder

is the unit of measurement for force. When you are sitting in a chair, your body exerts a ​

Placeholder

on the chair and the chair exerts the ​

Placeholder

force back.

Drag these tiles and drop them in the correct blank above

action

interacting objects

newton

force

same

Open Ended

Explain what force pairs are acting in this example of Newton's 3rd Law.

Multiple Choice

A bug hits a windshield. The force of the bug on the windshield is _______ the force of the windshield on the bug.

• greater than

• less than

• the same as

Multiple Choice

Imagine you are playing pool and you hit the cue ball into the eight ball. According to Newton's third law of motion, the eight ball will exert a force back onto the cue ball with equal strength, but in which direction?

• In the same direction

• In the opposite direction

• In a random direction

• In no direction

Multiple Choice

What happens when a swimmer pushes against a pool wall?

• The swimmer moves towards the wall

• The wall moves towards the swimmer

• Both the swimmer and the wall move towards each other

• The wall exerts an equal and opposite force on the swimmer

Multiple Choice

When a car strikes a wall, what force does the wall exert on the car?

• A force in the same direction as the car's motion

• A force in the opposite direction of the car's motion

• A force perpendicular to the car's motion

• No force

Multiple Choice

When you push a pole downward on Earth to make a pole vault, Earth exerts an UNequal force on the pole.

• TRUE

• FALSE

Multiple Choice

If you push on a building with a force of 50 N, the force acting on you from the building is which of the following?

• 0 N

• 50 N

• -50 N

• Buildings cannot push back

Multiple Choice

The statement "for every action, there is an equal but opposite reaction" is a statement of ____.

• the law of conservation of momentum

• Newton's first law

• Newton's second law

• Newton's third law

Multiple Choice

When a tennis ball bounces off a racket, the force of the racket on the ball equals the force of the ball on the racket.

• True

• False

Multiple Choice

Pick the best example of Newton’s third law of motion in action.

• A parachute pushes down on the air and the air pushes up in the opposite direction.

• A soccer ball comes to a stop because friction is pulling it in the opposite direction.

• An planet moving around the Sun while being pulled toward the Sun.

• A dancer spinning because there are no forces acting on it.

Multiple Choice

Based on Newton's third law of motion, when a person on a wagon throws a heavy concrete block to the west, the person will be pushed to the ___________ .

• North

• South

• East

• West

Multiple Choice

If you throw a ball at the ground, the ground exerts a force on the ball. The ball exerts no force on the ground.

• True

• False

Multiple Choice

Emma sits on a bench in the park.

Which of the following correctly names force pairs?

• Emma pushes on the bench, and the bench pushes on Earth.

• Earth pulls on Emma, and Emma pulls on Earth

• The bench pushes on the Earth, and Earth pulls on the Emma.

Multiple Choice

What would happen if he released MORE water out of the jetpack?

• He would fly higher

• He would fly lower

• He would go under the water

• He would fly sideways

Multiple Choice

How are action and reaction force pairs and balanced forces related?

• All forces cancel out in both

• Action/Reaction forces are in the same direction and Balanced Forces are opposites.

• Action/Reaction forces are opposite directions and Balanced Forces are the same direction.

• Action/Reaction forces act on 2 objects while Balanced Forces act on one object

Multiple Choice

The airboat is a good example of

• Newton's 1st Law

• Newton's 2nd Law

• Newton's 3rd Law

Multiple Choice

A trampoline is a good example of

• Newton's 1st Law

• Newton's 2nd Law

• Newton's 3rd Law

Multiple Choice

The inertia apparatus is a good example of

• Newton's 1st Law

• Newton's 2nd Law

• Newton's 3rd Law

Multiple Choice

This diagram is a good example of

• Newton's 1st Law

• Newton's 2nd Law

• Newton's 3rd Law