Work, Power, and
Machines
9.1

Work

A quantity that measures the effects of a force acting over a distance

Work = force x distance

W = Fd

Work

Work is measured in:

N  m

Joules (J)

Work Example

A crane uses an average force of 5200
N to lift a girder 25 m.
How much work does the crane do?

Work Example

Work = Fd

Work = (5200 N)(25m)

Work = 130000 N  m
= 130000 J

Power

A quantity that measures the rate at which work is done

Power = work/time

P = W/t

Power

Watts (W) is the SI unit for power

1 W = 1 J/s

Power Example

While rowing in a race,
John uses 19.8 N to travel 200.0 meters in
60.0 s. What is his power output in Watts?

Power Example

Work = Fd

Work = 19.8 N x 200.0 m= 3960 J

Power = W/t

Power = 3960 J/60.0 s

Power = 66.0 W

Machines

Help us do work by redistributing the force that we put into them

They do not change the amount of work

Machines

Change the direction of an input force (ex car jack)

Machines

Increase an output force by changing the distance over which the force is applied
(ex ramp)

Multiplying forces

Mechanical Advantage

A quantity that measures how much a machine multiples force or distance.

Mechanical Advantage
Mech. Adv =
Input distance
Output Distance
Mech. Adv. =
Output Force
Input Force

Mech. Adv. example

Calculate the mechanical advantage of a ramp that is 6.0 m long and 1.5 m high.

Mech. Adv. Example

Input = 6.0 m

Output = 1.5 m

Mech. Adv.=6.0m/1.5m

Mech. Adv. = 4.0

Simple Machines
9.2

Simple Machines

Most basic machines

Made up of two families

Levers

Inclined planes

The Lever Family

All levers have a rigid
arm that turns around a point called the
fulcrum.

The Lever Family

Levers are divided into three classes

Classes depend on the location of the fulcrum and the input/output forces.

First Class Levers

Have fulcrum in middle of arm.

The input/output forces act on opposite ends

Ex. Hammer, Pliers

First Class Levers
Output Force Input Force
Fulcrum

Second Class Levers

Fulcrum is at one end.

Input force is applied to the other end.

Ex. Wheel barrow, hinged doors, nutcracker

Second Class Levers
Output Force
Fulcrum
Input Force

Third Class Levers

Multiply distance rather than force.

Ex. Human forearm

Third Class Levers

The muscle contracts a short distance to move the hand a large distance

Third Class Levers
Output distance
Fulcrum
Input Force

Pulleys

Act like a modified member of the first-class lever family

Used to lift objects

Output
Force
Pulleys
Input force

The Inclined Plane

Incline planes multiply and redirect force by changing the distance

Ex loading ramp

The Inclined Plane

Turns a small input force into a large output force by spreading the work out over a large distance

A Wedge

Functions like two inclined planes back to back

A Wedge

Turns a single downward force into two forces directed out to the sides

Ex. An axe , nail

Or Wedge Antilles from Star Wars

Not to be mistaken with a wedgIEEEEE

A Screw

Inclined plane wrapped around a cylinder

A Screw

Tightening a screw requires less input force over a greater distance

Ex. Jar lids

Compound Machines

A machine that combines two or more simple machines

Ex. Scissors, bike gears, car jacks

Energy
9.3-9.4

Energy and Work

Energy is the ability to do work
 whenever work is done, energy is transformed or transferred to another system.


Energy is measured in:

Joules (J)

Energy can only be observed when work is being done on an object

Potential Energy PE
 the stored energy resulting from the relative positions of objects in a system

Potential Energy PE

PE of any stretched elastic material is called
Elastic PE
 ex. a rubber band, bungee cord, clock spring

Gravitational PE
 energy that could potentially do work on an object do to the forces of gravity.

Gravitational PE
 depends both on the mass of the object and the distance between them
(height)

Gravitational PE
Equation grav. PE= mass x gravity x height
PE = mgh or
PE = wh

PE Example

A 65 kg rock climber ascends a cliff. What is the climber’s gravitational PE at a point 35 m above the base of the cliff?

PE Example

PE = mgh

PE=(65kg)(9.8m/s 2 )(35m)

PE = 2.2 x 10 4 J

PE = 22000 J

Kinetic Energy
 the energy of a moving object due to its motion.
 depends on an objects mass and speed.

Kinetic Energy

What influences energy more: speed or mass?
ex. Car crashes

Speed does

Kinetic Energy
Equation
KE=1/2 x mass x speed squared
KE = ½ mv 2

KE Example

What is the kinetic energy of a 44 kg cheetah running at
31 m/s?

KE Example

KE = ½ mv 2

KE= ½(44kg)(31m/s) 2

KE=2.1 x 10 4 J

KE = 21000 J

Mechanical Energy
 the sum of the KE and the PE of large-scale objects in a system
 work being done

Nonmechanical
Energy

Energy that lies at the level of atoms and does not affect motion on a large scale.

Atoms

Atoms have KE, because they at constantly in motion.

KE  particles heat up

KE  particles cool down

Chemical Reactions
 during reactions stored energy (called chemical energy)is released

So PE is converted to
KE

Other Forms
 nuclear fusion
 nuclear fission

Electricity

Light

Energy
Transformations
9.4

Conservation of
Energy

Energy is neither created nor destroyed

Energy is transferred

Energy
Transformation

PE becomes KE
 car going down a hill on a roller coaster

Energy
Transformation

KE can become PE
 car going up a hill
KE starts converting to PE

Physics of roller coasters
 http://www.funderstanding.com/k12/coa ster/