ENERGYENERGY

What is Energy?

Ability to do WORK

Measured in Joules (calories or kilocalories)

What is Work?What is Work?

Work 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:

Work is done on an object whenever a force is exerted on an object through some distance.

W = 0, if F = 0

W = F d

If the force is constant and parallel to the direction of movement then:

W = 0, if d = 0No work done if

object doesn’t move.

No work done, whenever there is no force.

If the force on the object is in the direction the object moves, the work done is:

W = FxF

x

If the direction of the force is opposite the direction the object moves, work is:

W = -FxF

x

Force is Force is NOTNOT Work 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

xW = 0

How is all energy divided?How is all energy divided?

PotentialEnergy

Kinetic

Energy

All Energy

Gravitation

Potential

Energy

ElasticPotenti

alEnergy

ChemicalPotentia

lEnergy

What is Potential Energy?What is Potential Energy?

oEnergy that is stored and waiting to be used later

What is Gravitational Potential What is Gravitational Potential Energy?Energy?

o Potential energy due to an object’s position

o P.E. (gravitational) = mass x

height x gravityDon’t look

down, Rover!Good boy!

What is Elastic Potential What is Elastic Potential Energy?Energy?

o Potential energy due compression or expansion of an elastic object.

Notice the ball compressingand expanding

Work done by a spring =Work done by a spring = ½ Kx ½ Kx22

What is Chemical Potential What is Chemical Potential Energy?Energy?o Potential energy

stored within the chemical bonds of an object

What is Kinetic Energy?What is Kinetic Energy?

An object’s kinetic energy depends on:the object’s mass.

◦Kinetic energy is directly proportional to mass.

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

to the square of the object’s speed.o Energy an object has due to its motion

K.E. = ½ mv2

Kinetic energy is a scalar quantity.When the kinetic energy of an object

changes, work has been done on the object.Work is a scalar quantity.

What is the kinetic energy of an object with mass of 1 kg, moving at 1 m/s?

0.5 Joule.

Work and Kinetic EnergyWork and Kinetic Energy

The work done on an object by the net force equals the object’s change in kinetic energy.

Wnet = KE

Law of Conservation of Law of Conservation of EnergyEnergy

If no external forces act on a system, the total energy of the system will remain constant.

Mechanical Energy = PE + KE

What are different Forms of What are different Forms of Energy/Energy Transformations?Energy/Energy Transformations?

Work done by FrictionWork done by Friction

Suppose you have to exert a force of 10 Newtons to push a book 1 meter across a horizontal table at constant velocity (as shown above). Since your force is in the direction the book moves, the work you do on the book is given by:Your work = Fx = (10 N)(1 m) = 10 Joules

•The work-energy equation would lead you to think that doing 10 Joules of work on the book should increase the kinetic energy of the book by 10 Joules - but that doesn't happen (the speed of the book is constant). What's going on?•A friction force opposes the motion of the book. This force must also be 10 Newtons (Since the book moves at constant velocity, the net force on it must be zero.). •The friction force pulls in the opposite direction from the direction the book moves, the work done by friction is given by:

Work done by friction = -Fx = -(10 N)(1 m) = -10 Joules

•This means that the friction force removes 10 Joules of energy from the book. So, while you were adding 10 Joules of energy to the book, friction was busy taking the 10 Joules of energy away from the book. This is why the kinetic energy of the book does not change!

Power is the rate work is done. Units of power: 1 Joule/sec = 1 Watt1000 Watts = 1 kilowattPower is a scalar quantity.

Power =

Worktime

W

P t

What are (Simple) Machines?What are (Simple) Machines?

A machine is a mechanical device used to do work.

Examples of simple machines:◦Inclined plane◦Lever◦pulley

(Simple) Machines(Simple) Machines

A machine can never output more work (energy) than is put into it.

At best,

Workout = Workin

MachineWorki

n

Workout

What is Mechanical Advantage?What is Mechanical Advantage?

Machines 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

How do we Measure Efficiency?How do we Measure Efficiency?

The 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 doneEnergy input

x 100%