1. Conservation of Energy (a.k.a Slacker Physics)

2. Now, Really…Conservation of Energy In a system, energy can not be created or destroyed. Energy can change form. The total amount of energy remains constant. Total Mechanical Energy = KE + PE

3. At the top the total energy is all potential (velocity = 0 m/s). Conservation of Energy In between the total energy is potential + kinetic. At the bottom the total energy is all kinetic (height = 0 m).

4. Gravitational Potential Energy The elephant is 4 meters above the ground and has a mass of 500 kg. What is his potential energy? What is his kinetic energy?

5. Conservation of Energy The elephant falls off the rock formation and is 2 meter above the ground What is his potential energy? What is his kinetic energy?

6. Conservation of Energy Right before he falls into the water, what is his potential energy? What is his kinetic energy? What is the magnitude of his velocity?

7. Energy of a Pendulum Energy Transfers from Potential to Kinetic to Potential as the pendulum swings. PE max (max height) KE = 0 (stop and turn) PE 0 (height = 0) KE = max PE max (max height) KE = 0 (stop and turn)

8. Work-energy Theorem W=∆KE = KE f - KE i = ½ mv f 2 - ½ mv i 2 Work = the change in kinetic energy Work = final kinetic energy – initial kinetic energy

9. Work – Energy Theorem How much work is done to stop the elephant when he hits the ground? W = KE f - KE i = 0 J – 19600 J = -19600 J

10. Negative and Positive Work Positive work The force is in the same direction as the motion. Negative work The force is in the opposite direction of motion. Often, the object being acted upon will be slowed or stopped.

11. KE = ½ mv 2 A 75 kg skater is moving at 20 m/s across the concrete. How much kinetic energy does he have? Kinetic Energy Example

12. A 75 kg skater is moving at 20 m/s across the concrete. The skater slows to 5 m/s to avoid wiping out. How much work is done by the skater? Work - Energy Example W=Ke f - Ke i = ½ mv f 2 - ½ mv i 2