1. SECTION 3: Newton’s 3 rd Law and Momentum

2. LEARNING GOALS  State Newton’s 3 rd Law of Motion.  Identify action and reaction forces.  Calculate momentum.  Recognize when momentum is conserved.

3. NEWTON’S 3 RD LAW  Newton’s 3 rd Law of Motion: 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.

4. 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”.

5. ACTION AND REACTION FORCES DON’T CANCEL!  If the action and reaction forces occurred on the same object, they would result in zero motion.  The action and reaction forces must occur on different objects.

6. ACTION AND REACTION FORCES DON’T CANCEL! Example: A swimmer pushes the water. She “acts” on the water. The water “reacts” by moving and pushing her forward.

7. MOMENTUM  Momentum: property of a moving object that equals its mass times its velocity.  Momentum is related to how much force is needed to change an object’s motion.

8. MOMENTUM EQUATION p = momentum (kg x m/s) m = mass (kg) v = velocity (m/s)  Just like velocity, momentum has size and direction! p = mv

9. FORMULA SHEET p = mv m = p/v v = p/m

10. EXAMPLE  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?

11. 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!

12. CALCULATING THE FORCE USING MOMENTUM F = force in N mv f = final momentum in kg x m/s mv i = intital momentum in kg x m/s t = time F = (mv f – mv i ) / t

13. 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?

14. LAW OF CONSERVATION OF MOMENTUM  The momentum of an object does not change unless its mass, velocity, or both change.  Momentum can be transferred from one object to another.

15. LAW OF CONSERVATION OF MOMENTUM  Law of Conservation of Momentum: the total momentum of objects before a collision is equal to the total momentum of objects after a collision.  Total momentum is not lost or gained during a collision

16. LAW OF CONSERVATION OF MOMENTUM