Download Slide 1

Aim: How can we explain work
and power?
Do Now:
A man pushes on a wall with a
force of 50 N for 60 s. Has he
done any work?
Work
The force acting upon an object
to cause a displacement
There must be a displacement
Force and displacement must
be in the same direction
Scalar quantity
Work or No Work?
A student applies a force to a wall and
becomes exhausted. NO WORK!!!
A calculator falls off a table and free
falls to the ground. WORK!!!
An out of shape Army soldier hangs on
a pull-up bar for 10 seconds and can’t
do a single pull-up. NO WORK!!!
A rocket accelerates through space.
WORK!!!
W = Fd
Units
Joule = N ·m
J=
James Prescott Joule
1818-1889
A 5 kg box is pushed with a force
of 20 N over a distance of 4 m.
How much work was done?
W = Fd
W = (20 N)(4 m)
W = 80 J
The same 5 kg box is now lifted a
vertical distance of 4 m. How
much work was done?
W = Fg d
W = mgd
The force required to lift
an object is equal to the
object’s weight
W = (5 kg)(9.8 m/s2)(4 m)
W = 196 J
Area under a force - displacement
graph is equal to the WORK done
by the force
Calculate the work done:
W = bh
W = (5m)(10 N)
W = 50 J
10
F (N)
5
d (m)
Power
•The rate of doing work
Rate means divide by time
•Scalar quantity
James Watt
1736-1819
Units:
A force of 50 N is applied to an
object which gives it a constant
velocity of 10 m/s. At what rate is
work being performed?
A 680 N student runs up a
flight of stairs 3.5 m high in
11.4 s. On a second run, the
same student completes the
same stair run in 8.5 s.
a) What is the work done by the
student?
W = Fd
W = (680 N)(3.5 m)
W = 2380 J
b) What is the power developed for
the 11.4 s run?

c) Compare this power to the power
developed during the 8.5 s run
Power and time are indirectly related
Time increases, power decreases
The 11.4 s run developed less power