SACE Stage 1 Conceptual Physics Newtons Third Law

SACE Stage 1 Conceptual Physics Newtons Third Law – Action and Reaction

Interactions Produce Forces • A force is a push or pull. • On closer inspection, it is an interaction between one thing and another.

Interactions Produce Forces • When hitting a nail with a hammer, the nail is driven into the wood. • The nail must also exert a force on the hammer to stop it from moving.

Interactions Produce Forces • This leads us to Newton’s third law, the law of ACTION and REACTION

Newton’s rd 3 Law • Whenever one object exerts a force on a second object, the second object exerts an equal but opposite force on the first object.

Newton’s rd 3 Law • One force is called the action force and the other force is called the reaction force. • Neither force can exist with out the other.

Identifying Action & Reaction • Sometimes not easy to identify the Action Reaction pair. • Use this rule Object A exerts a force on Object B then in reverse Object B exerts a force on object A

Identifying Action & Reaction

Identifying Action & Reaction • Action: Tyre pushes road • Reaction: Road pushes tyre

Identifying Action & Reaction

Identifying Action & Reaction • Action: Rocket pushes gas • Reaction: Gas pushes rocket

Identifying Action & Reaction

Identifying Action & Reaction • Action: Earth pulls ball • Reaction: Ball pulls Earth

Action and Reaction on Different Masses • In the example of the ball falling to the Earth, can we say the Earth falls towards the ball?

Action and Reaction on Different Masses • YES!!! But not as much. • Why? • Although the size of the force is the same, the masses are different.

Action and Reaction on Different Masses • Small mass on bullet, LARGE acceleration.

Action and Reaction on Different Masses • By considering the recoil of a gun we can understand rocket propulsion. • Instead of bullets coming out of the rocket, it’s a gas!!

Why Action and Reaction Forces Don’t Cancel • In every interaction, one thing interacts with the other and vice versa. A on B & B on A. • Why don’t they cancel out?

Why Action and Reaction Forces Don’t Cancel • The answer is that the pair of forces act on different objects. Although A acts on B and B acts on A, the individual forces themselves act on different objects. The ball accelerates away and your foot slows down when it hits the ball.