1. The Work Energy Principle Part 4 By Heather Britton

2. The Work Energy Principle The work energy principle states that the net work done on a system it equal to the change of kinetic energy To solve problems work and energy can be set equal (W = FΔx KE = (1/2)mv 2 ) Therefore (FΔx = (1/2)mv 2 )

3. Mechanical Energy It is possible for an object to have both potential and kinetic energy at the same time We define the sum of the potential energy and kinetic energy as the mechanical energy (E) E = PE + KE

4. Mechanical Energy Think of a ball thrown straight up Relative to the person’s hand it leaves with all kinetic energy As it rises it gains potential energy, and loses an equal amount of kinetic energy The total amount of energy remains the same

5. Mechanical Energy At the top of all of the kinetic energy has been transformed into potential energy As the ball descends the potential energy is converted back into kinetic energy When it reaches the level where it was launched, all the energy is now kinetic

6. The Law of Conservation of Energy Energy cannot be created or destroyed, but it can change forms As we saw with the ball, anywhere in the flight the total mechanical energy is constant However, the ratio of potential energy to kinetic energy is constantly changing

7. Conservation of Energy The change in potential energy plus the change of kinetic energy in a closed system is equal to zero 0 = ΔPE + ΔKE Another way to state the above is... PE 0 + KE 0 = PE + KE

8. Example 8 A baseball is thrown upward with an initial velocity of 30 m/s. What is the maximum height it will reach? Answer 46 m

9. Example 9 The height of the upper falls at Yellowstone Falls is 33.2 m. When the water reaches the bottom of the falls, its speed is 25.8 m/s. Neglecting air resistance, what is the speed of the water at the top of the falls? Answer 3.9 m/s

10. Practice www.physicsclassroom.com