Newtons Third Law – Action and Reaction

Newton’s First and Second Laws – a review First – Law of Inertia: an object at rest or in motion will remain that way until acted upon by an unbalanced force. Second – Law of Mass & Acceleration: F = ma

Use the Cornell-Notetaking-Technique!

Newtons Third Law – Action and Reaction

Objetives: (please write into the cue column) I can list Newton’s third law of motion. I can define force. I can identify action-reaction. I can identify action-reaction on different masses. I can use Newton’s third law to explain how rockets and helicopters work. I can explain why action-reaction forces do not cancel.

For every force, there is an equal and opposite force.

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 3rd Law Whenever one object exerts a force on a second object, the second object exerts an equal but opposite force on the first object. For every ACTION force, there is an equal opposite REACTION force.

Newton’s 3rd 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

Identifying Action & Reaction

Identifying Action & Reaction Action: Tire pushes road Reaction: Road pushes tire

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

Identifying Action & Reaction

Identifying Action & Reaction The dog wags the tail and the tail wags the dog.

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!!

Action and Reaction on Different Masses The rocket recoils from the “molecular cannonballs” it fires and climbs upward.

Action and Reaction on Different Masses Using Newton’s third law, we can understand how a helicopter gets its lifting force. The whirling blades force air particles downward (action). The air forces the blades upward (reaction). This upward reaction force is called lift. When lift equals the weight of the craft, the helicopter hovers in midair. When lift is greater, the helicopter climbs upward.

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.

Action Equals Reaction