1. Friday phys 1-25 Momentum Collisions Energy

2. The Correct answer to #79 I was wrong! 2.8 m/s [SW] and 2.8 m/s [NE] Before =.64 kg m/s [e] After = HORIZONTAL COMPONENTS of the momentum of the moving objects

3. Final Homework on Conservation of momentum Pages 870-871 Questions 8-12

4. Collisions All collisions between objects that conserve momentum are divided into 3 categories Completely Inelastic: object stick together after collision Inelastic: object bounce off each other Elastic: objects bounce off each other AND Kinetic energy of the system is conserved

5. Most common type of collision is inelastic in nature

6. Example of Elastic Energy There are no examples of large scale perfectly elastic collisions Some of the kinetic energy of a moving object is converted into internal (ex. Thermal) energy of the other object involved in the collision

7. Homework Question An ice skater with a mass of 75.0 kg pushes off against a second skater with a mass of 42.0 kg. Both skaters are initially at rest. After the push, the larger skater moves off with a speed of 0.75 m/s eastward. What is the velocity (magnitude and direction) of the smaller skater after the push?

8. Answer? 1.3 m/s, west

9. Energy, work and simple machines Objectives for the chapter Recognize that work and power describe how the external world changes the energy of a system Relate force to work and explain how machine ease the difficulty in performing a task

10. When does one do work?

11. Work must be done to something What does it mean to do physical work?

12. Definition of work Work done on an object is equal to the amount of force applied times the displacement of the object (as a result of the applied force) W = F x D

13. Important No displacement means no work done on object Direction of force must be in direction of motion for it to do work Units are Nm or Joules

14. Is the waiter doing work on the tray?

15. If force is applied at an angle to the motion Then only the component of the force in the direction of motion is doing work on object

16. Only the horizontal part of force is doing work