Work and Power
May 18, 2015
Think about it…..
You are sitting on your bed
doing your homework. Are
you doing work?
Think about it…..
You are not doing work
when you are sitting on
your bed doing your
homework.
Today, I can describe and
calculate work and power.
May 18, 2015
Work the transfer of energy to an
object by using a force that causes the
object to move in the direction of the
force.
Difference between Force & Work
• Applying a force doesn’t always result in
work being done.
–Whenever you apply a force to an
object and the object moves in the
direction of the force, you have done
work on the object.
Is it work or not? (p. 211)
Work is done on an
object if two things
happen:
(1) the object moves as
Force is applied
(2) the direction of the
object’s motion is
the same as the
direction of the
Force
How much work is done?
The amount of work (W) done in moving an object
force (F) applied to the object by the distance (d)
through which the force is applied
SI units:
Newton for Force … meter for length/distance …
work is the Newton-meter (N × m) … more simply
called a joule (J)
Work (p. 213)
Let’s work it out….
A horse applies a force of 120 N to pull a wagon
a distance of 50,000 m along a level road. How
much work is done?
W=Fxd
Step 1: List the given and unknown values
F = 120 N
d= 50, 000 m
W=?
Step 2: Enter the known values into the
equation and solve.
W = F x d W = 120 N x 50,000 m
W = 6,000,000 N•m = 6,000,000 J
Now you try….
How much work is done on a 16 N
sack of potatoes when you lift the
sack 1.5 m?
Solution
How much work is done on a 16 N sack of
potatoes when you lift the sack 1.5 m?
W=Fxd
W = 16 N x 1.5 m
W = 24 J
Let’s try another problem…
A rope is thrown over a beam, and one
end is tied to a 300 N bundle of
lumber. You pull the free end of the
rope 2 m with a force of 400 N to lift
the lumber off the ground. How much
work have you done?
Problem Solved…..
A rope is thrown over a beam, and one end is
tied to a 300 N bundle of lumber. You pull the
free end of the rope 2 m with a force of 400 N to
lift the lumber off the ground. How much work
have you done?
W=Fxd
W = 400 N x 2 m
W = 800 J
Calculate how fast work is done by
calculating power.
Power the rate at which energy is transferred.
Calculating Power
To calculate power (P), you divide the amount
of work done (W) by the time (t) it takes to do
that work, as shown in the following equation:
P=W
t
The unit used to express power is joules per
second (J/s) … also called the watt. One watt
(W) is equal to 1 J/s.
Calculate the Power
A stage manager at a play raises the curtain by doing
5,976 J of work on the curtain in 12 s. What is the
power output of the stage manager?
Step 1: Write the equation for power.
P=W
t
Step 2: Replace the variables with the known values
and then solve the equation for power.
P = 5,976 J
12 s
P = 498 W
Now you try….
If you do 50 J of work in 5 s, what is
your power?
Now you try….
If you do 50 J of work in 5 s, what is your power?
Step 1: Write the Equation
P=W
t
Step 2: Replace the variables with the known values
and then solve the equation for power.
P = 50 J
5s
P = 10 J/s or 10 W
Try another problem ….
If you did 100 J of work in 5 s, what was your
power output?
Try another problem ….
If you did 100 J of work in 5 s, what was your
power output?
P = 100 J
5s
P = 20 W