Energy physics

Aya Sharaireh 9 A JUBILEE

Kinetic Energy If an object is moving, it has energy.

(Be careful, the converse of this statement is not always true!)

This energy is called kinetic energy - the energy of motion.

Kinetic EnergyAn object’s kinetic energy depends on: the object’s mass.

Kinetic energy is directly proportional to mass.

the object’s speed. Kinetic energy is directly proportional to

the square of the object’s speed.

Kinetic Energy In symbols:

KE = 12 mv2

Kinetic Energy

Kinetic energy is a scalar quantity.Common units of kinetic energy:

JoulesAn object with mass of 1 kg, moving

at 1 m/s, has a kinetic energy of 0.5 Joule.

Work

When the kinetic energy of an object changes, work has been done on the object.

Units of work: JoulesWork is a scalar quantity.

WorkWork depends on:

The amount of force applied to the object. The distance that the object moves while

the force is applied. The direction of the force with respect to

the direction the object moves.

Work If the force on the object is in the

direction the object moves, the work done is:

W = FxF

x

Work If the direction of the force is opposite

the direction the object moves, work is:

W = -Fx

F

x

Force is NOT Work If the force is perpendicular to the

direction the object moves, the work done is 0.

If the object doesn’t move, the work done is 0.

F

x

W = 0

Work and Kinetic EnergyThe work done on an object by the net

force equals the object’s change in kinetic energy.

Wnet = ∆KE

Potential EnergySometimes work is not converted

directly into kinetic energy. Instead it is “stored”, or “hidden”.

Potential energy is stored energy or stored work.

Potential EnergyPotential energy is energy that an

object (system) has due to its position or arrangement.

Calculating Potential Energy To calculate the potential energy of a

particular arrangement:

1. Pick a position or arrangement that you want to call the “potential energy = 0” situation.

Calculating Potential Energy

2. The potential energy of any other position or arrangement equals the negative of the work that the conservative force does in changing from the potential energy = 0 situation to that one.

PE = - WorkF

Conservative ForcesEnergy or work is stored when a force

does work “against” a force such as the gravitational force or a Hooke’s Law (spring) force.

Forces that store or hide energy are called conservative forces.

Gravitational PEThe gravitational potential energy of an

object at height h equals the negative of the work that gravity does when the object is lifted from the PE = 0 position.

GPE = mgh

Mechanical EnergyMechanical Energy = PE + KE

Conservation of Energy If no external forces act on a system,

the total energy of the system will remain constant.

PowerPower is the rate work is done.

Power = ∆Work

time

W

P t

PowerUnits of power: 1 Joule/sec = 1 Watt1000 Watts = 1 kilowattPower is a scalar quantity.

(Simple) MachinesA machine is a mechanical device used

to do work.Examples of simple machines:

Inclined plane Lever pulley

(Simple) MachinesA machine can never output more work

(energy) than is put into it.At best,

Workout = Workin

MachineWorkin

Workout

Mechanical AdvantageMachines can’t multiply work or energy,

but they can multiply force. Mechanical advantage measures how much a machine multiplies force.

MA =Force machine exerts

Force you exert

EfficiencyThe efficiency of a machine tells how

much of the energy (work) that goes into the machine actually does useful work.

It is usually expressed as a percent.

Efficiency =Useful work done

Energy inputx 100%

The End