# What is Newton’s Third Law

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What is Newton’s Third Law
Describes what happens when one object exerts a force on another object. Forces always act in equal but opposite pairs.

Review: What is Newton’s First
Question: What is Newton’s First Law?

Review: What is Newton’s First
If the forces acting on the object are balanced, the object will remain at rest or stay in motion at a constant velocity. If the forces are unbalanced, the object will accelerate in the direction of the net force.

Review: What is Newton’s Second
Question: What is Newton’s Second Law of Motion?

Review: What is Newton’s Second
An object acted upon by a force will accelerate in the direction of the force. a = f/m f = ma

What is Newton’s Third Law
Question: If you push on a wall does the wall also push on you?

What is Newton’s Third Law
The wall actually also pushes on you. So when you push on a wall the wall pushes back on you with a force equal in strength to the force you exerted.

What is Newton’s Third Law
Forces always act in equal but opposite pairs. Another way to say this is: For every action, there is an equal but opposite reaction.

What is Newton’s Third Law
When one object exerts a force on another object, the second object exerts the same size force on the first object.

What are Action and Reaction Forces?
Forces exerted by two objects on each other are often called an action-reaction pair.

What are Action and Reaction Forces?
Question: Since the action-reaction forces are equal and opposite do they cancel?

What are Action and Reaction Forces?
Action-reaction forces don’t cancel because they act on different objects. Forces can only cancel if they act on the same object.

What are Action and Reaction Forces?
Here are 5 examples of action and reaction forces: 1.When you push on a wall the wall pushes back on you When you walk on the the ground, you push the ground and the ground pushes you. 3.When the bat hits the ball the ball hits the bat. 4.When your finger touches your nose your nose touches your finger. 5. When you push on a table the table pushes on you.

What are Action and Reaction Forces?
Newton’s 3rd law applies to driving because when you drive the action force is the pushing against the road and the reaction is the road pushing against the tires. Also when a car hits a person, the person hits the car. When a car hits telephone pole the pole hits the car. When a car hits a wall the wall hits the car.

What are Action and Reaction Forces?
Question: Explain using Newton’s Third Law why is driving on icy roads so difficult.

What are Action and Reaction Forces?
It is difficult to drive on a icy road because the tire can not grip the road because there is less fiction. So when the car tries to push on the road it can not because the road is icy. If the car can't push on the road, the road can't push on the car. As a result the car slides in the ice

What are Action and Reaction Forces?
You constantly use action-reaction forces as you move about. When you jump, you push on the ground. The ground pushes up on you. It is this upward force that pushes you into the air.

What are Action and Reaction Forces?
When a bird flies, its wings push in a downward and backward direction. This pushes air downward and backward. By Newton’s third, the air pushes back on the bird in the opposite direction; upward and forward.

What are Action and Reaction Forces?
Question: Explain if the photo shows an action-reaction.

What are Action and Reaction Forces?

What are Action and Reaction Forces?
Some action-reaction pairs are not noticeable because one of the objects is much more massive and appears to remain motionless when a force acts upon it. Question: Give an example of the above.

What are Action and Reaction Forces?
Earth has so much mass compared to you that it does not move noticeably when you push it. If you step on something with less mass than you do, you can see it being pushed back.

What are Action and Reaction Forces?
Question: How is a rocket launch an example of Newton’s Third Law?

What are Action and Reaction Forces?
Rocket engines supply the force, called thrust, that lifts the rocket. When the rocket fuel is ignited, a hot gas is produced.

What are Action and Reaction Forces?
As gas molecules collide with the inside walls, the walls exert a force that pushes them out of the bottom of the engine. This downward push is the action force.

What are Action and Reaction Forces?
The reaction force is the upward push on the rocket engine by the gas molecules. This is the thrust that propels the rocket upward.

How do You Measure Weight?
When you stand on a scale, your weight pushes down on the scale. This causes the scale pointer to point to your weight.

How do You Measure Weight?
At the same time, by Newton’s Third Law the scale pushes up on you with a force equal to your weight. This force balances the downward pull of gravity on you.

Free Fall and Weightlessness
Imagine standing on a scale in an elevator that is falling. Inside the elevator you and the scale are both in a free fall. The only force acting upon you is gravity, the scale is no longer pushing up on you.

Free Fall and Weightlessness
According to Newton’s Third Law, you no longer push down on the scale. So the scale pointer stays at zero and you seem to be weightless. Weightlessness is the condition that occurs in free fall when the weight of an object seems to be zero.

Weightlessness in Orbit
Imagine you were holding a ball in the free-falling elevator. If you let the ball go, the position of the ball relative to you and the elevator would not change, because everything is moving at the same speed.

Weightlessness in Orbit
Using the same ball, suppose you push the ball downward. This pushing force adds to the downward force of gravity. Now the acceleration of the ball is greater than the acceleration of you and the elevator. The ball will continue to move faster downward until it hits the elevator floor.

Weightlessness in Orbit
When the space shuttle orbits the Earth, the shuttle and all objects in it are in a free fall.

Weightlessness in Orbit
The shuttle and all object within are falling in a curved path around the Earth, instead of falling straight downward. As a result the objects in the shuttle appear to be weightless.

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