Work, Power, and
Machines
What would life be like without machines?
How would you get a heavy object up a hill without a machine?
Objectives
Define work and power.
 Calculate the work done on an object and
the rate at which work is done.
 Calculate power.
 Use the concept of mechanical advantage
to explain how machines make doing work
easier.
 Calculate the mechanical advantage and
efficiency of various machines.

Work & Power




Work = product of
force and the distance
over which it is
applied.
Work = force x
distance
W = Fd
Units: newton meter
or joule
n-m or j




Power = rate of doing
work
Power = Work/time
P= W/t
Units: nm/s or j/s or
Watt
746 w = 1 horsepower
(see page 416 in your
book)
So 3800w 1hp = 5 hp
746w
W
F
d
Add these two
formulas to your
circle sheet!
W
P
t
Work problem sample
A crane uses an average force of 5200 N
to lift a girder 25 m. How much work does
the crane do on the girder?
 W = Fd = 5200N (25m)
 W = 130000 or 1.3 x 105 J

Power problem sample
While rowing in a race, John does 396 0 J
or work on the oars in 6 0.0 s. What is his
power output in watts?
 P = Work / time = W / t

= 39 6 0 J / 6 0.0 s

= 66 . 0 Watts, or w

Your assignment
Is on page 415 /practice 1-3
 And, as usual,
 SHOW:






The Formula
Your work with units
Your answer with units
Because …
It makes Mrs. C. happy
Answers to 415/1-3
1. W

P
 2. W

P
 3. W

P

=
=
=
=
=
=
Fd = 200N(1.5m) = 300 J
W/T = 300J / 1.0 s = 300 w
Fd =15.0N(1.0m) = 15 J
W/T = 15 J / 2.0 s = 7.5 w
Fd = 10.0N)(0.5m) = 5 J
W/T = 5 J / 1.0 s = 5 w
Hydraulic crane

http://science.howstuffworks.com/hydraulic-crane.htm
Machines and Mechanical Advantage
Mechanical Advantage measures how
much a machine multiplies force or
distance (Fout / Fin)
 A machine can





Make work easier
Redistribute work
Change size or direction of input force
Increase output force by changing distance
over which force is applied
Inclined Plane and Mechanical
Advantage

Lift up a box

1m
Push a box up an
inclined plane
1m
Which is easier work? Lifting a heavy box 1 meter or pushing the
box up an inclined plane?
Lifting the box 1 m into the truck
W = Fd
F = weight of the box = 225N
 W = 225N(1.00m)
 W = 225N-m or 225 J

Sliding the box up the inclined plane
Let’s say the force to slide it up is only
75.0N
The distance you push the box up the
inclined plane is 3.00 m.
W = F x d
 W = 75.0N x 3.00M
 W = 225 N-m or 225 j
What do you notice about the work
being done?
Lifting work = 225 J
 Pushing work = 225 J
 But…
 It takes (less or more) force?
 Less!!!

This brings us to MECHANICAL
ADVANTAGE!!!
M. A. = output force = input distance
input force
output distance
(Actual)
(Ideal)
AMA compares forces
IMA compares distances
AMA is less than IMA.
For a machine to be helpful, its
mechanical advantage must be
greater than 1.
MA>1
Turn to page 425 in your book and let’s look at the
If a mechanic drives a car 1.8 m along a
ramp to raise a car 0.3 m, what is the ideal
mechanical advantage (IMA) of the ramp?
IMA =
Distance in
Distance out
= 1.8 m
0.3 m
IMA = 6
Your assignment

is 425 / Practice 1-3
Answers to 425/Practice 1-3
1. IMA = input distance = 3 m = 6
output distance
0.5 m
2. IMA = input distance
= 0.5 m = 10
output distance
0.05 m
3. Input distance = IMA(Output distance)
= 2.5 (1.0m) = 2.5 m
Efficiency = Wout / Win x 100%
Friction always works against machines
and makes a machine less efficient.
Reducing friction increases efficiency.
 Sample: If the efficiency of a machine is
75% and the machine requires 10.0 J of
work input, find the work output.
 Wout = (Eff x Win) = 75% (10.0 J)
100%
100%
 Wout = 7.5 J

Remaining assignments for Work,
Power, and Machines
On page 426 do math practice 8. and 9.
On page 441 do reviewing content 1.
through 7.
Put these formulae on your circle sheet:
IMA = Din
AMA = Fout
Dout
Fin
Eff = Wout x 100%
Win
Watts and horsepower

746 watt = 1 horsepower

A 750 W motor could be rated as ___HP.

750W

1 HP
746 W
= 1.0 HP