Work and Machines
Chapter 5 Section 1
What is work?
Work
is the transfer of
energy that occurs when a
force makes an object move
The amount of work is dependent
on
 a.
the force exerted
 b. the distance over which the force is
applied
Work = Force X distance
Two conditions that must be met for
work to occur

An object MUST MOVE!


Explain why this is true.
The motion of the object must be in the
same direction as the applied force on the
object
How are work and energy related?
1. Energy is always transferred. . .
a. from the object doing the work
b. to the object on which the work is
done.
 2. Both work & energy are measured in
Joules, the SI unit of work & energy
 3. The unit Joule was named after an
English physicist, James Prescott Joule.
4. One Joule is equal to
a Newton-meter (N●m)

Calculating Work

Work equals force (in Newtons) times
distance
 w= f x d
w
F
D
Practice Problems
The brakes on a bicycle apply 125 N of
frictional force to the wheels as the
bicycle travels 14.0 m. How much work
have the brakes done on the bicycle?
F = 125 N W = F x d
d = 14.0 m
W=?
W = (125 N)(14.0 m)
W = 1,750 J
Practice Problems
A hydraulic lift performs 12,500
J of work raising a car 0.5 m.
How much force has the lift
applied to the car?
W = 12,500 J F = W / d
d = 0.5 m
F=?
F = 12,500 J / 0.5 m F = 25,000 J
POWER
1. The amount of work done in a certain
amount of time (rate at which work is done).
2. Power is measured in Watts (W), named
after the Scottish inventor James Watts.
Calculating Power
 Power
equals work divided by
time
 P=w/T
 Power
is measure in watts (W)
Power Triangle
w
P
t
For example
The number of Watts marked on light
bulbs tells you how much work the bulb
does in a period of time - that is power.
 Do you see how work and power and
related?

Practice Problems
While rowing in a race, John does
3960 J of work on the oars in 60.0 s.
What is his power output in watts?
W = 3960 J
t = 60.0 s
P=?
P=W/t
P = 3960 J / 60.0 s
P = 66 W
Practice Problems
While carrying a box of books for
20.0 s, your power output is 720 W.
How much work have you done?
P = 720 W
t = 20.0 s
W=?
W = Pt
W = (720 J)(20.0 s)
W = 14,400 J
Chapter 5
Section 2
Machines

Transfer mechanical energy
from one object to another

Make work seem easier (cannot reduce
the amount of work done)

Can multiply force (wrench removing
bolt), change the direction of a force
(flagpole) or change the distance over
which a force is applied (loading ramp)
Two types of forces are
involved in machines:


INPUT FORCE –
force put in; also
called the effort force
OUTPUT FORCE – force out of
machine; also called the
resistance force
The same amount of work can be
done by:


1. Applying a small amount of force over a long
distance
 OR
2. By applying a large force over a short
distance
 Increasing
distance reduces the
amount of force needed to do the
work
 Machines
help move things
that resist being moved
 Ex: Dolly used to move a
heavy refrigerator
Mechanical Advantage



Measures how much a machine
multiplies force or distance
MA>1: multiplies the effort force
MA<1: increases distance or speed
MA = output force = input distance
input force output distance
Has no units! (they cancel out)
 An
ideal machine with no
friction would have the same
input work and output work,
however some energy
transferred is change to heat
due to friction
EFFICIENCY



How much useful work a
machine can do (expressed as %).
Some work always overcomes friction,
therefore some energy is always lost
as heat! This means %eff < 100% !!
% efficiency = work output x 100
work input
% = WO / WI x 100
The
efficiency of a machine
is always less than 100 %
Why?
Friction
plays a big role in the
output of a machine