Work and Power
Decision Points
A shopping cart
full of groceries
sits in a
grocery store.
Decision Points
•You apply a force
to the same cart
of groceries.
Work
• Work
– Work is done when a force moves an object
through a distance against another force.
– Ex: Pushing a book across a table – work is
done against friction.
Work, cont’d
W  F d
When a force is applied parallel to
the direction of motion
• W and E are measured in Joules
• 1 Joule = 1 N∙m
How Work is Done
• Work can only be done if the displacement is in the
same direction as the force that causes it.
•
Is work done?
1.
2.
3.
4.
5.
6.
7.
A teacher applies a force to a
wall and becomes exhausted.
A book falls off a table and
free falls to the ground.
A rocket accelerates through
space.
A waiter carries a tray full of
meals above his head by one
arm across the room.
A power lifter lifts 80 kg
above his head.
A mover pushes a piano up a
ramp into the back of a truck.
A child pulls his dog behind
him on a sled.
1.
2.
3.
4.
5.
6.
7.
NO – no distance, so no work.
YES – work done by gravity.
YES – work done by the
rocket boosters, and rocket
moves.
NO – force is perpendicular to
the distance
YES – work is done when
weightlifter exerts an upward
force
YES - work done by mover –
horizontal component does
work
YES – work done by child –
horizontal component does
work
If a force of 100 N was exerted on an object and
no work was done, the object must have ____.
A accelerated rapidly
B remained motionless
C decreased its velocity
D gained momentum
W = F*d
W=0
and F = 100 N, so
0 J = 100 N x d
distance must be 0
It did not move!
How much work is performed when a 50 kg crate is
pushed 15 m with a force of 20 N?
F 300 J
Use the formula W = F*d
G 750 J
W = 20N*15m = 300 J
H 1,000 J
J 15,000 J
Work Practice Problems
Do letter (a) only of each “Work and Power
Practice Problems” – be prepared to share
with the class!
Power
• Power: the rate at which work is done
W
P
t
•
•
•
•
Power is measured in horsepower (Hp) or watts (W)
1 watt = 1 Joule / second
746 Watts = 1 Horsepower (Hp)
1000 Watts = 1 kilowatt (kW)
Power
W
P
t
W
F d
P

t
t
 Fv
Power
• A machine performs 500 Joules of work
over a period of 100 seconds. How much
power does the machine have?
– P= W/t
– P= 500 J/100s = 5 Watts of power
More Power….
• An electric motor lifts an elevator (full of gazelles) 9.0 m in 15.0 s
by exerting an upward force of 1.20 X 104 N. What power does the
motor produce in watts, kilowatts, and horsepower?
W F d
P

t
t

1.2 E 4 N   (9m)

 7200 W
15s
7200 W 1 kW
 7.2 kW
1
1000 W
7200 W 1 Hp
 9.65 Hp
1
746 W
Power Practice Problems
Now finish the “Work and Power Practice
Problems” – be prepared to share with the
class!
•A teacher applies
a force to a wall
and becomes
exhausted.
•A book falls off a
table and free falls
to the ground.
•A rocket
accelerates
through space.
•A waiter carries a
tray full of meals
above his head by
one arm across
the room.
•A power lifter lifts
80 kg above his
head.
•A mover pushes a
piano up a ramp
into the back
of a truck.
•A child pulls his
dog behind him
on a sled.
Exit Ticket
• Describe the kinetic energy of an object in
freefall as it is on its upward path. Address
the following:
– How does the ball’s final kinetic energy
compare to the initial kinetic energy?
– If they are different, what force is doing the
work?
– How does the direction of the force acting on
the object compare to the displacement of the
object? What effect does this have on the
kinetic energy?
Exit Ticket 2
• Describe the kinetic energy of a falling
object. Address the following:
– How does the ball’s final kinetic energy
compare to the initial kinetic energy?
– If they are different, what force is doing the
work?