Work, Energy and Power

Energy:• Energy can neither be created nor

destroyed. • It comes in many forms:• Kinetic• Potential (gravitational, chemical, elastic)• Nuclear• Electric• Solar

Work and Energy:• If an object has energy, it can do

work.• Work produces energy.

Work:• Work is done when a force moves an object

through some distance.

• No work is done when the movement is at right angles to the force.(for instance no work is done walking with a heavy bag at a constant height because gravity is 90° to the distance the bag moves through)

Work:The work done when moving an object some distance with a force is equal to:

Kinetic Energy:• Kinetic energy is the type of energy

a moving object posesses.

More mass moving at the same speed has more energy.A faster moving mass has more energy.

Kinetic energy velocity

mass

Gravitational Potential:• When a mass is lifted we do work

against gravity, the force applied is the size of the weight (mg) and the distance is the change in height (h).

mass

Potential energy

gravity

height

Elastic Potential:• When we were working with springs we were applying a force

to extend/compress them. We were doing work.

This work is stored in the spring as elastic potential energy ()

Note: This is the same symbol as gravitational potential so you need to decide which one to use by identifying the context.

Also equal to the area under a F vs. x graph.

Elastic potential Spring

constant

extension

Work and Power:• Power is defined as the amount of work done,

divided by the time taken.

If two of the same mass are lifted to the same height, work done will be the same. But if one mass is lifted faster than the other, that one will have produced the greater power.

• Power is measured in Watts (W) (also Joules per second )

A 60 Watt lightbulb uses

60 Joules of energy per

second

CollisionsAs discussed, energy can be "lost" in many forms.Kinectic energy is not always conserved in a collision, one object might speed up, the other might slow down.

• In an elastic collision no kinetic energy is lost.• In an inelastic collision energy is lost (to sound, heat, deformation etc.)

Elastic vs. Inelastic