ForcesChapter 2, Section 4Newton’s Third Law

Newton’s Third Law of Motion

•Newton’s Third Law of Motion – for every action there is an equal and opposite reaction.

•Action-Reaction pairs▫Jumping up in the air

Your feet are the action force, the ground pushing back is the reaction force

▫Rowing a boat in the water Your pushing of the paddle in the water is the

action force, the water pushing back on the paddle is the reaction force.

•Detecting Motion▫You cannot always detect motion.▫Ex. – Gravity

•Do Action-Reaction Forces Cancel?▫As learned earlier: if two forces are

balanced the forces cancel each other out.▫Does not apply to Action-Reaction Forces

because they are acting on different objects. Ex. – volleyball player hitting the ball.

p. 57 in textbook

Momentum•Momentum = a quantity of motion found

by taking Mass x Velocity. (see pg. 58 in book for example of the calculation)

•Described by its direction and quantity (momentum).

•The more the momentum, the harder it is to stop.

•Mass also affects momentum. ▫Ex. You can catch a baseball going 20 m/s but

you cannot stop a car going the same speed.

Conservation of Momentum•Conservation refers to the conditions before

and after an event.•The law of conservation of momentum states

that the momentum of an object is unchanged unless a force acts on it (like friction)

•Only an outside force can change the momentum of an object.

•Velocity may change after two objects collide, but the momentum remains the same.

Figure 18 in textbook, p. 60.