1. UNIT 4 Work, Energy, and Power

2. By what factor does the kinetic energy of a car change when its speed is tripled? 1) no change at all 2) factor of 3 3) factor of 6 4) factor of 9 5) factor of 12 ConcepTest 6.5aKinetic Energy I ConcepTest 6.5a Kinetic Energy I

3. By what factor does the kinetic energy of a car change when its speed is tripled? 1) no change at all 2) factor of 3 3) factor of 6 4) factor of 9 5) factor of 12 1/2 mv 2 speed increases by a factor of 3KE will increase by a factor of 9 Since the kinetic energy is 1/2 mv 2, if the speed increases by a factor of 3, then the KE will increase by a factor of 9. ConcepTest 6.5aKinetic Energy I ConcepTest 6.5a Kinetic Energy I Follow-up: How would you achieve a KE increase of a factor of 2?

4. Wednesday November 9 th 4 POTENTIAL ENERGY

5. TODAY’S AGENDA  Potential Energy  Hw: Practice C (All) p168 UPCOMING…  Thur: Conservation of Mechanical Energy  Fri: Bowling Ball Demo  Mon: Problem Quiz 1 Wednesday, November 9

6. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Energy Chapter 5 Potential Energy Potential Energy is the energy associated with an object because of the position, shape, or condition of the object. Gravitational potential energy is the potential energy stored in the gravitational fields of interacting bodies. Gravitational potential energy depends on height from a zero level. PE g = mgh gravitational PE = mass  free-fall acceleration  height

7. 7

8. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Energy Chapter 5 Potential Energy, continued Elastic potential energy is the energy available for use when a deformed elastic object returns to its original configuration. The symbol k is called the spring constant, a parameter that measures the spring’s resistance to being compressed or stretched.

9. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Chapter 5 Elastic Potential Energy Section 2 Energy

10. 10

11. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Energy Chapter 5 Sample Problem Potential Energy A 70.0 kg stuntman is attached to a bungee cord with an unstretched length of 15.0 m. He jumps off a bridge spanning a river from a height of 50.0 m. When he finally stops, the cord has a stretched length of 44.0 m. Treat the stuntman as a point mass, and disregard the weight of the bungee cord. Assuming the spring constant of the bungee cord is 71.8 N/m, what is the total potential energy relative to the water when the man stops falling?

12. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Energy Chapter 5 Sample Problem, continued Potential Energy 1. Define Given:m = 70.0 kg k = 71.8 N/m g = 9.81 m/s 2 h = 50.0 m – 44.0 m = 6.0 m x = 44.0 m – 15.0 m = 29.0 m PE = 0 J at river level Unknown:PE tot = ?

13. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Energy Chapter 5 Sample Problem, continued Potential Energy 3. Calculate Substitute the values into the equations and solve:

14. END 14