1. Newton’s 3 rd Law and Momentum

2. Newton’s 3 rd Law When one object exerts a force on a second object, the second object exerts a force on the first that is equal in strength and opposite in direction.

3. Action and Reaction Another way of stating Newton’s 3 rd Law is “to every action force there is an equal and opposite reaction force”. Example – You exert a downward force on a trampoline. The trampoline exerts an equal force upward on you.

4. Action and Reaction Forces Don’t Cancel! Action and reaction forces work on different objects, so they are not balanced. Example – A swimmer pushes the water. She “acts” on the water. The water “reacts” by moving and pushing her forward.

5. Momentum Momentum – how much force is needed to change an object’s motion. Depends on an object’s mass and velocity Momentum is given the symbol p.

6. Momentum Equation p = mv p = momentum in kg * m/s m = mass in kg v = velocity in m/s Just like velocity, momentum has size and direction!

7. Example Momentum Problem At the end of a race, a sprinter with a mass of 80 kg has a speed of 10 m/s. What is the sprinter’s momentum?

8. Force and Changing Momentum If you catch a baseball, your hand might sting (even with a glove). This is because the baseball exerted a force on your hand when it came to a stop and its momentum changed. This can also be calculated!

9. Calculating the Force Using Momentum F = (mv f – mv i ) / t F = force in N mv f = final momentum in kg * m/s mv i = intital momentum in kg* m/s t = time

10. Example of Calculating Force Using Momentum What is the force exerted by a catcher’s glove on a 0.15 kg baseball moving at 35 m/s that is stopped in 0.02 s?

11. Law of Conservation of Momentum The momentum of an object does not change unless it’s mass, velocity, or both change. Momentum can be transferred from one object to another. Example – A bowling ball traveling down the lane has momentum. That momentum is transferred to the pins as the ball strikes the pins.