Newton’s Third Law of Motion

Action And

Reaction

Introduction• If you lean over – you

fall• If you lean over with

your hands against the wall – you don’t fall• Why?• Because the wall is

pushing on you and holds you in place

Forces and Interactions• Force – a push or pull• Newton saw an

interaction• Mutual action between

forces• Hammer and nail

• Hammer moves nail• Nail halts the hammer

Newton’s Third Law• Whenever one object exerts a

force on a second object, the second object exerts an equal and opposite force on the first object

Third Law Forces• Action force – • Reaction force –

• It doesn’t matter which is which

• Partners in the interaction

• Equal and opposite• Neither can exist

without the other

Examples• You push on floor, the floor

pushes on you• Car pushes on road, the

road pushes on the car• Swimming – you push water

backward, the water pushes you forwards

• More friction – more force

Questions• Does a stick of dynamite

contain force?• No, force is an

interaction between two objects. An object may be capable of creating a force, but it can’t possess it. Dynamite possesses ENERGY

Questions• A car accelerates along the road.

Strictly speaking, what is the force that moves the car?

• The road pushes the car. It provides a horizontal force.

Identifying Action and Reaction

• Sometimes it is hard to find the action/reaction pairs• What are the forces on a falling

boulder?• Gravity?• Weight?• Ground?

• No – none of these

Recipe for finding forces1. Identify the interaction2. Action: Object A exerts force on object

B3. Reaction: Object B exerts force on

object A

• Falling boulder – Earth exerts force on boulder, Boulder exerts force on Earth

Action and Reaction on Different Masses• Boulder-Earth interaction –

the forces are EQUAL• Does the earth fall into the

boulder?• Yes, but not as far

• The reactions are equal and opposite

• Because Earth is so large, we can’t sense its very small acceleration

Question• We know the Earth

pulls on the moon. Does the moon also pull on the earth? If so, which pull is stronger?

• Yes, the Earth and moon both pull on each other. Both pulls are equal and opposite.

Cannon/Cannon ball• When fired, the cannon ball

exerts a force on the cannon• The cannon recoils or “kicks”• Why doesn’t the cannon move

as fast as the cannon ball?• A given force exerted on a

small mass will result in a greater acceleration than on a larger mass

Rocket propulsion• Air escaping from a balloon• The balloon accelerates the

opposite direction as the escaping air.

• Rocket – each molecule of escaping gas acts like the cannon ball

Rocket in air and space• Once though the rocket needed air

to “push” against.• Actually, rockets work better

without air drag (in space)

Momentum• How can a karate

expert break a stack of bricks?

• Why does it hurt more to fall on concrete than a wooden floor?

• Why do you follow through in golf, baseball or bowling?

Momentum• Momentum = mass x velocity• p = m x v• How much momentum does a 400

kg bike moving 40 m/s have?• p= mv = (400 kg) ( 40 m/s) =

16,000 kgm/s

Helicopters• Helicopters – blades

push air down. Air pushes chopper up.

• Lift – upward reaction force

• When lift equals weight of chopper, it can hover

• When lift is greater, the chopper rises

Birds and Airplanes• Fly due to action/reaction forces• Wings of bird reflect air downward.

The air then pushes upward• Airplanes must have a continuous

flow of air in order to have lift.• The engines push air back. The air

pushes the jet forward

Do Action and Reaction Forces Cancel ?

• Why do the equal forces not cancel to zero • How can there be

acceleration?• There are MANY equal

and opposite forces in a system.

• If two people kick the ball at the same time – together they cancel out

Question• Suppose a friend who hears about

Newton’s law say that you can’t move a football by kicking it because the reaction force by the kicked ball would be equal and opposite to your kicking force. The net force would be zero, so no matter how hard you kick, the ball won’t move! What do you say to your friend?

Answer• Obviously kicking the ball will

accelerate it. Your kick acts on the ball. It accelerates the ball. The reaction force acts on your foot – it decelerates your foot.

The horse-cart problem

• If a horse pulls on a cart, doesn’t the cart pull back equally?

• Three views• Farmer – get the cart to

market• Horse • Horse-cart system

Farmer• Only concerned about force

exerted on the cart• Force on cart/mass of cart =

acceleration• Doesn’t care about the reaction on

the horse

Horse system• Opposite reaction force by the cart

restrains the horse• So how can the horse move

forward?• The horse must push on the

GROUND – which pushes back.

Horse-Cart system• The pull of the horse on the cart

and the reaction of the cart on the horse are internal forces.• They contribute nothing to the

acceleration• The acceleration comes from the

horse-cart system and the ground

Stalled car• You can’t move the car by sitting in your seat

and pushing against the dashboard• You must get outside and push against the

ground.• Don’t worry if you get confused, Newton

struggled with his law too.

Action Equal Reaction• What if you hit a wall?

• It hits back • You can’t hit it harder than

it can hit you back• What if you hit paper?

• It can not hit back as hard

Conclusion• Push the world hard – it

pushes back hard• Push the world gentle – it

pushes back gentle• You can not touch

without being touched!!!