The Work Energy Principle Part 4

By Heather Britton

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)mv2)

•Therefore (FΔx = (1/2)mv2)

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

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

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

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

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...

•PE0 + KE0 = PE + KE

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

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

Practice

•www.physicsclassroom.com