1. Objectives: The student will be able to: 1. Define each type of mechanical energy and give examples of types of energy that are not mechanical. 2. State the work energy theorem and apply the theorem to solve problems.

2. 6-3 Kinetic Energy, and the Work-Energy Principle Energy was traditionally defined as the ability to do work. We now know that not all forces are able to do work; however, we are dealing in these chapters with mechanical energy, which does follow this definition.

3. ENERGY Kinetic Energy: * Energy associated with an object in motion * Depends on speed and mass * Scalar quantity * SI unit for all forms of energy = Joule (J) KE = ½ mv 2 KE = ½ x mass x (velocity) 2

4. Kinetic Energy If a bowling ball and a soccer ball are traveling at the same speed, which do you think has more kinetic energy? KE = ½ mv 2 * Both are moving with identical speeds * Both are moving with identical speeds * Bowling ball has more mass than the soccer ball  Bowling ball has more kinetic energy

5. Kinetic Energy Practice Problem 1 A 7 kg bowling ball moves at 3 m/s. How fast must a 2.45 g tennis ball move in order to have the same kinetic energy as the bowling ball? A 7 kg bowling ball moves at 3 m/s. How fast must a 2.45 g tennis ball move in order to have the same kinetic energy as the bowling ball? Velocity of tennis ball = 160 m/s

6. 6-3 Kinetic Energy, and the Work-Energy Principle If we write the acceleration in terms of the velocity and the distance, we find that the work done here is (show how it is derived) We define the kinetic energy: (6-2) (6-3)

7. 6-3 Kinetic Energy, and the Work-Energy Principle This means that the work done is equal to the change in the kinetic energy: If the net work is positive, the kinetic energy increases. If the net work is negative, the kinetic energy decreases. (6-4)

8. 6-3 Kinetic Energy, and the Work-Energy Principle Because work and kinetic energy can be equated, they must have the same units: kinetic energy is measured in joules.

9. Practice Problem 2 (#16) A. If the KE of an arrow is doubled, by what factor has its speed increased? A. If the KE of an arrow is doubled, by what factor has its speed increased? B. If its speed is doubled, by what factor does its KE increase? B. If its speed is doubled, by what factor does its KE increase?

10. Practice Problem 3 (#17) How much work is required to stop an electron (m = 9.11 x 10 -31 kg) which is moving with a speed of 1.90 x 10 6 m/s? How much work is required to stop an electron (m = 9.11 x 10 -31 kg) which is moving with a speed of 1.90 x 10 6 m/s?

11. Practice Problem 4 (#22) At an accident scene on a level road, investigators measure a car's skid mark to be 88 m long. It was a rainy day and the coefficient of friction was estimated to be 0.42. Use these data to determine the speed of the car when the driver slammed on (and locked) the brakes. (Why does the car/s mass not matter?) At an accident scene on a level road, investigators measure a car's skid mark to be 88 m long. It was a rainy day and the coefficient of friction was estimated to be 0.42. Use these data to determine the speed of the car when the driver slammed on (and locked) the brakes. (Why does the car/s mass not matter?)

12. Closure What is the relationship between kinetic energy and work? What is the relationship between kinetic energy and work?

13. Homework Chapter 6 Problems #15, 19, 23, 25 Chapter 6 Problems #15, 19, 23, 25