Essential Question

• What is Newton’s Third Law & how does it apply to motion?

Newton’s First Law

“An object at rest tends to stay at rest and an object in motion tends to stay

in motion unless acted upon by an unbalanced force.”

Newton’s Second Law

“Force equals mass times acceleration.”

F = ma

Acceleration: a measurement of how quickly an object is changing speed.

More Laws of Motion• Newton’s First Law of Motion states that if

you kick a ball, the ball will move.

• Newton’s Second Law of Motion says that when a force acts on an object, the object accelerates. – If you kick the ball harder, it will move faster.– It also tells you that a heavy ball is harder to

move than a lighter ball.

Newton’s Third Law

“For every action there is an equal and opposite reaction.”

OUCH!• Let’s say you were running and weren’t

paying attention where you were going and ran into a wall. Are you hurt because you hit the wall?

• No. You can’t feel the forces on the wall. You DO feel the force on your body! You’re hurt because the wall hit you!

Forces occur in pairs

As the runner’s footpushes on the starting block (action force). The starting block is exerting a force on the runner’s foot (reaction force).

A Horse of a Good Time• A horse pulls on a cart. Can you name the

opposing force?

• Think of it this way…the horse pulls on the cart. So Newton’s Third Law tells you that the cart pulls the horse.

The horse pulls the cartThe cart pulls the horse

What does this mean?For every force acting on an object, there is an

equal force acting in the opposite direction. Right now, gravity is pulling you down in your seat, but Newton’s Third Law says your seat is pushing up

against you with equal force.

2 Forces at Work

• The forces that 2 objects put on each other are called an action-reaction force pair.

• The forces in the force pair are equal in strength, but opposite in direction.

• One is called the action force.

• The other is called the reaction force.

Newton’s 3rd Law at work…

When a rocket blows out gas at high speeds in one direction (action force),The rocket is pushed in the opposite direction (reaction force).

The gas pushes against the rocket and the rocket pushes back just as hard against the gas.

This is known as thrust!

More examples…Discuss with your lab partner which forces are action and reaction forces.

Think about it . . .

What happens if you are standing on a skateboard or a slippery floor and push against a wall? You slide in the opposite direction (away from the wall), because you pushed on the wall but the wall pushed back on you with equal and opposite force.

Why does it hurt so much when you stub your toe? When your toe exerts a force on a rock, the rock exerts an equal force back on

your toe. The harder you hit your toe against it, the more force the rock exerts back on your toe (and the more your toe

hurts).

ReviewNewton’s First Law:

Objects in motion tend to stay in motion and objects at rest tend to stay at rest unless acted upon by an unbalanced force.

Newton’s Second Law:

Force equals mass times acceleration (F = ma).

Newton’s Third Law:

For every action there is an equal and opposite reaction.

National Science Foundation

• http://www.nsf.gov/news/special_reports/football/newtonthirdlaw.jsp

• Newton's third law of motion states that __________.

A) an object will remain at rest or keep moving in a straight line with constant speed unless a force acts on it

B) when a force is applied on an object, there is an equal force applied by the object in the opposite direction

C) acceleration is calculated by dividing the force exerted on an object by the mass of the object, and that when a force acts on an object, its acceleration is in the same direction as the force

D) the force on an object can be found by dividing its mass by its acceleration

Let’s Review

• Whenever one object exerts a force on another object, the second object ALWAYS exerts an equal force back on the first object.

– A.) True

– B.) False

Let’s Review

• Forces always occur in ______________.– A.) Triplets

– B.) Pairs

– C.) by themselves

– D.) either singly or in pairs

Let’s Review

• The two forces of a force pair are called the __________ force and the __________ force.

• A) mass, acceleration

• B) gravitational, accelerator

• C) increasing, decreasing

• D) action, reaction

Let’s Review

Common Misconception…

• http://www.youtube.com/watch?v=8bTdMmNZm2M