WORK AND ENERGY
SIMPLE MACHINES
WORK

It takes energy to move something.
– The energy required to move it is called work.

Work is force applied over a distance.

W=Fxd
He may expend energy when he pushes on
the wall, but if it doesn’t move, no work is
done on the wall.
WORK

W=Fxd
– Remember
 Force units – Newtons (N)
 Distance units– Meters (m)
Work units then = Nm = a Joule (J)
Ex. How much work is done on a 60N box that is
lifted 2.5m off the floor?
 Answer:
Joule, James


–W=Fxd
– W = 60N x 2.5m
– W = 150J
(1818-1889)
English physicist
WORKED ON HEAT TRANSFER
Work

Work depends on two factors:
– Distance over which the force is applied
– Amount of force applied
When a load is lifted two stories high, twice the work
is done because the distance is twice as much.
When two loads are lifted to the same height, twice as much
work is done because the force needed to lift them is twice as
much.
Concept check

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
How much work is needed to lift an object that
weighs 500 N to a height of 4 m?
W = F × d = 500 N × 4 m = 2000 J.
How much work is needed to lift it twice as
high?
Twice the height requires twice the work. That is, W = F × d =
500 N × 8 m = 4000 J.


How much work is needed to lift a 1000 N to a
height of 8 m?
Lifting twice the load twice as high requires four times the work.
That is, F × d = 1000 N × 8 m = 8000 J.
POWER
Lifting a load quickly is more difficult than
lifting the same load slowly. If equal loads
are lifted to the same height, the forces
and distances are equal, so the work is
the same. What’s different is the power.
 Power

– is the rate at which energy is changed from
one form to another.
– Also, power is the rate at which work is done.
POWER

POWER equals the amount of work done
divided by the time interval during which
the work occurs.
James Watt
1736-1819
Scottish Inventor
POWER = work / time

The unit of power is the joule
per second, called the watt (W).
STEAM ENGINE
Concept check




You do work when you do push-ups. If
you do the same number of push-ups in
half the time, how does your power
output compare?
Your power output is twice as much.
How many watts of power are needed
when a force of 1 N moves a book 2 m in
a time of 1 s?
P = W/t = (F x d) / t
= (1 x 2) / 1 = 2 Watts
Practice
Work on the problems on page 678 of the
text book.
p 678 calculating power 1-3
Show all work.
MECHANICAL ENERGY
When raised, the ram then has the
ability to do work on a piling beneath
it when it falls.
 When work is done by an archer in
drawing a bow, the bent bow has the
ability to do work on the arrow.
 When work is done to wind a spring
mechanism, the spring then has the
ability to do work on various gears to
run a clock, ring a bell, or sound an
alarm.

MECHANICAL ENERGY

This ability to do work is energy.
– Like work, energy is measured in joules.

Energy appears in many forms, such as heat,
light, sound, electricity, and radioactivity.
– It even takes the form of mass, as celebrated in
Einstein’s famous E = mc2 equation.

Potential and kinetic energy are both considered
to be kinds of mechanical energy.
POTENTIAL ENERGY
In the stored state, energy has the potential to
do work. Therefore, it is called potential energy
(PE).
 The potential energy due to elevated position is
called gravitational potential energy.
 The amount of gravitational potential energy
possessed by an elevated object is equal to the
work done against gravity in lifting it.

– Gravitational potential energy = weight × height
– PE = mgh
POTENTIAL ENERGY
The PE of the 10-N ball is the same (30 J) in all three cases.
Concept check
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
How much work is done in lifting the 200-N block of ice
shown in Figure a vertical distance of 2.5 m?
500 J. (We get this either by Fd or mgh.)
How much work is done in pushing the same block of ice
up the 5-m long ramp? The force needed is only 100 N
(which is why inclines are used).
500 J. (She pushes with half the force over twice the distance.)
What is the increase in the block’s potential energy in
each case?
Either way increases the block’s potential energy by 500 J. The ramp simply makes
this work easier to perform.
KINETIC ENERGY

A moving object is capable of doing work.
– It has energy of motion
– kinetic energy (KE).

The kinetic energy of an object depends
on its mass and speed.
More massive = more energy
Higher speed = more energy
Kinetic energy = 1/2 mass × speed2
 KE = 1/2 mv2

Concept Check

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
A car travels at 30 km/h and has kinetic energy
of 1 MJ. If it travels twice as fast, 60 km/h, how
much kinetic energy will it have?
Twice as fast means (22) four times the kinetic energy, or 4 MJ.
If it travels three times as fast, at 90 km/h, what
will be its kinetic energy?
Three times as fast means (32) nine times the kinetic energy, or 9 MJ.
If it travels four times as fast, at 120 km/h, what
will be its kinetic energy?
Four times as fast means (42) sixteen times the kinetic energy, or 16 MJ.
WORK-ENERGY THEOREM

Work-energy theorem
– The change in kinetic energy is equal to the
work done
– Work = DKE
Applies to potential energy as well
 No change in energy means no work done
 Changing the energy means there has to
be work done.

WORK-ENERGY THEOREM
The theorem also applies to decreasing speed.
 Decreasing KE requires work
 This work is the friction supplied by the brakes,
multiplied by the distance over which the force
acts.

– Since friction is the same for a given set of
brakes/tires/road, it is the distance that is affected
when trying to stop at different speeds.
WORK-ENERGY THEOREM
Remember that a car traveling twice as
fast would have 4 times the energy and
would require 4 times the work to stop it.
 This means 4 times the stopping distance.

Concept Check

When the brakes of a car are locked, the
car skids to a stop. How much farther will
the car skid if it’s moving 3 times as fast?

Nine times farther. The car has nine times as much
energy when it travels three times as fast: 1/2 m(3v)2 =
1/2 m9v2 = 9(1/2 mv2). The friction force will ordinarily
be the same in either case. Therefore, to do nine times
the work requires nine times as much sliding distance.
Conservation of Energy

The Law of Conservation
of Energy
– Energy cannot be
created or destroyed; it
may be transformed
from one form into
another or transferred
from one object to
another, but the total
amount of energy never
changes.
Concept Check

The values of kinetic energy and potential
energy for the block freely sliding down a
ramp are shown only at the bottom of the
ramp. Fill in the missing values.
Simple Machines

Simple Machines
– Multiply forces
– Change the direction of forces

They may decrease the effort required but they
do not reduce the amount of work.
– Actually more work is needed due to the friction in
the machine.

Mechanical Advantage
– How much the machine multiplies the effort force.
 Resistance Force / Effort Force – actual MA
 Effort distance / Resistance distance – Ideal MA
Simple Machines
Any machine that multiplies force does so at the
expense of distance.
 Likewise, any machine that multiplies distance
does so at the expense of force.
 No machine or device can put out more energy
than is put into it.
 No machine can create energy; it can only
transfer it or transform it from one form to
another.

Simple Machines

There are a few basic simple machines
– Levers
– Pulleys
– Inclined planes
– Wedges
– Wheel and axle
– Screws
– Hydraulic presses
Levers
Lever – A bar or rod that pivots
on a fulcrum.
 There are three classes of levers

Pulleys
Pulley a rope that turns around a wheel.
 There a re three basic types of pulleys.

Single fixed-This pulley acts like a lever.
It changes only the direction of the
input force.
Single movable-In this
arrangement, a load can
be lifted with half the
input force.
CombinationNote the load
is supported
by 7 strands
of rope. Each
strand
supports 1/7
the load. The
tension in the
rope pulled
by the man is
likewise 1/7
the load.
Wheel and Axle

Wheel and Axle
– A wheel and axle has a larger wheel (or
wheels) connected by a smaller cylinder (axle)
and is fastened to the wheel so that they turn
together.
– A lever in the round
– Doorknobs
– Wrenches
– Steering wheels
Inclined Planes

Inclined Plane- is a slope or a ramp.
– It can be any slanted surface used to raise a
load from a lower level to a higher level.
Wedges

Wedge- a moving inclined plane
– Wedges move into the resistance
– Usually used for cutting purposes
– They change the direction of the force
applied.
Screws

Screw- an inclined plane wrapped around
a cylinder.
– Space saving
Hydraulic Press

A combination of a large and
a small cylinder connected by
a pipe and filled with a fluid
so that the pressure created
in the fluid by a small force
acting on the piston in the
small cylinder will result in a
large force on the large
piston. The operation
depends upon Pascal's
principle, which states that
when a liquid is at rest the
addition of a pressure (force
per unit area) at one point
results in an identical
increase in pressure at all
points.
Efficiency

Efficiency is defined as the ratio of work
output to work input
Efficiency = work output
work input
 Nothing is 100% efficient

– Friction, friction, friction
– Energy converts to heat
 (molecular level Kinetic Energy)
Sources of Energy

Nuclear Energy
– Most forms can be traced back to the sun
 Coal, solar, petroleum, natural gas, wood, wind,
flowing water.
– Nuclear reactions in Earth’s interior
 Volcanic, geothermal