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Work and Energy
Chapter 12
Section 1 – Work, Power, and Machines
Work is done when a force causes an object to move.

This meaning is different from the everyday meaning
of work.
Work is equal to force times distance. W = F x d

SI unit is joules. 1 J = 1 Nm = 1 kgm2/s2
Power is the rate at which work is done.

SI unit for power is the Watt (W)
𝑃=
𝑊
𝑡
Machines help people by redistributing the work put
into them.
 They change the size or the direction of the input
force.
Mechanical Advantage – describes how much the
machine multiplies force or increases distance.

𝑀𝐴 =
𝑜𝑢𝑡𝑝𝑢𝑡 𝑓𝑜𝑟𝑐𝑒
also
𝑖𝑛𝑝𝑢𝑡 𝑓𝑜𝑟𝑐𝑒
𝑀𝐴 =
𝑜𝑢𝑡𝑝𝑢𝑡 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒
𝑖𝑛𝑝𝑢𝑡 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒
If MA >1 then input force is multiplied
 If MA < 1 the speed or distance is multiplied

Section 2 – Simple Machines
The most basic machines
are called simple
machines.

Other machines are
modifications or
combinations of
simple machines.

There are six types of
machines in two
families.
The Lever Family
1. Levers have a rigid arm and a fulcrum. There are
three classes of levers (see Figure 5 pg. 386). Ex.
Claw hammer, wheelbarrow, forearm
The Lever Family con’t
2. Pulleys increase MA
(see Figure 6 pg. 387)
The Lever Family con’t
3. Wheel-and-axle is a lever or pulley connected to
a shaft – increases force Ex. Steering wheel,
screwdriver
Inclined Plane Family
4. Inclined Plane – increases force by spreading it
over a longer distance. Ex. Ramp
Inclined Plane Family con’t
5. Wedge – two inclined
planes together.
Redirects force. Ex. Axe
head, nail
Inclined Plane Family cont’d
6. Screw – inclined
plane wrapped
around cylinder. Ex.
Screw, jar lid, spiral
staircase

Compound machines
are made of two or
more simple
machines.

Many are very
complex

Mechanical Advantage of Simple Machines
𝑜𝑢𝑡𝑝𝑢𝑡 𝑓𝑜𝑟𝑐𝑒
or
𝑖𝑛𝑝𝑢𝑡 𝑓𝑜𝑟𝑐𝑒
𝑀𝐴 =
𝑖𝑛𝑝𝑢𝑡 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒
𝑜𝑢𝑡𝑝𝑢𝑡 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒

𝑀𝐴 =

Lever: Mechanical Advantage = length of effort arm/
length of resistance arm

Pulley: Count the number of rope segments that exert an
upward force on the object being moved.

Wheel and Axle: Mechanical Advantage = radius of
wheel/radius of axle

Inclined Plane: Mechanical Advantage = length of slope/
height of slope (Includes wedge height of slope and screw)
Section 3 – What is Energy?
Energy - the ability to do work.
 Like work, energy is measured in joules (J).
Mechanical Energy – the amount of work an object can do.
 The sum of the potential and kinetic energy.


Most systems also contain nonmechanical energy – energy
at the atomic level.
Nonmechanical energy does not usually affect systems on
a large scale.
Potential Energy - energy of position.

Elastic potential energy is stored in any stretched or
compressed elastic material.

The gravitational potential energy of an object is
determined by its mass, its height, and g, the free-fall
acceleration due to gravity. PE = mgh.
Kinetic Energy - energy of
motion.

Determined by its mass
and speed. KE = ½ mv2.
Section 4 – Conservation of Energy
Energy readily changes from one form to another.

In a mechanical system, potential energy can become
kinetic energy, and kinetic energy can become potential
energy.

Mechanical energy can change to nonmechanical energy as
a result of friction, air resistance, or other means.
Law of Conservation of Energy – energy cannot be created or
destroyed, although it may change form.
 A machine cannot do more work than the work required to
operate the machine.
 Because of friction, the work output of a machine is
always less than the work input.
Efficiency of a machine is the ratio of the useful work
performed by the machine to the work required to
operate the machine.
 No machine can 100% efficiency – friction.


☞All machines require energy input → Perpetual motion
machines are impossible in real life due to friction and air
resistance.