1. Physics 218: Mechanics Instructor: Dr. Tatiana Erukhimova Lecture 27

2. Center of Mass

3. Motion of the Center of Mass

4. The center of mass of a system moves as if all of the mass of the system were concentrated at that point and as if all of the forces were acting at that point There is only the external forces that affect the motion of the center of mass ( ) Only external forces affect the motion of the center of mass

5. Momentum is a vector! Vector equation!

6. If

7. Conservation of Momentum If there is no external force on a system, then the total momentum of the system is a constant

8. True in X and Y directions separately!

9. Problem Solving For Conservation of Momentum problems: 1.BEFORE and AFTER 2.Do X and Y Separately

10. Perfectly elastic collision A collision in which the total kinetic energy after the collision is the same than that before the collision is called an elastic collision. A B

11. A block of mass m is moving along x axis with a velocity of V 0. It collides with a block of mass M, initially at rest. 1) What is the change in kinetic energy of the system of two balls: a) if the collision is perfectly elastic; b) if the collision is perfectly inelastic (balls stick together after collision). 2) For m = M = m 0, find the velocity of each ball after a perfectly elastic collision.

12. Problem 2 from handout

13. The ballistic pendulum

14. Have a great day! Hw: Chapter 10 problems and exercises Reading: Chapter 10