Forces

1. Forces Chapter 2, Section 4 Newton’s Third Law

2. 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.

3. 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

4. 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.

5. 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.

6. Figure 18 in textbook, p. 60.