Work and
Power
Physics
Mrs. Coyle
What is the direction of the component
of the force that causes the sled to
move?
What component of F applied causes this
object to move?
Work
• The work done on an object by an external
constant force:
W= F d cos q
• q is the angle between the displacement d
and the force F.
Work
• Is a scalar quantity.
• Unit:
•
Joule= Newton ∙ meter
J= N∙m
• James Prescott Joule
English, 19thCentury
• When the force is in the direction of the
displacement, q=0o:
cos 0= 1 =>
W= F d and work is positive
Which force acting on the sled
does positive work?
When the force acts opposite to the direction of
the displacement, q=180o:
cos 180 o = -1 => W= F d and work is negative
Which force acting on the sled
does negative work?
When the force acts perpendicular to the
displacement,
then the work done by that force is zero.
What force(s) acting on the
sled does no work?
Question
• Does the centripetal force do work?
Meaning of Sign of Work
When the work done
by an external force
onto the system is:
+ : the system gains energy
- : the system loses energy
Example 1
• How much work was done by
the 30N applied tension force
to move the 6kg box along the
floor by 20m?
(A: 300J)
• How much work did the
weight do in the same case?
• If µ=0.2, how much work did
the friction do?
(Hint: Use Fxnet to calculate N)
(A: -136J)
Work done by Resultant
W= W1+ W2+ Wn
Since work is a scalar, the net work done by
a resultant force is equal to the sum of the
individual works done by each individual
force acting on the object.
Example 2
• How much total work is done on a sled that
moves at constant velocity?
Work is the area under
an F vs d graph.
Power,
P= W = Fdcos q
t
t
• Rate
• Scalar
• Unit : J/s = W
( Watt)
• James Watt (18th Century) developed the
steam engine.
Power
P= W = Fdcos q = Fvcos q
t
t
kiloWatt∙ hour
Does a kWatthour (kWh) measure:
Power
or
Work
Pile Driver
Example 3
A pile driver drives a pile in 10 m with a force of
2000N in 0.05s.
1. Calculate the work done by the driver. (A: 2 x104J)
2. Calculate the power of the driver. (A: 4x105W)
Example 4
• An engine moves a boat through the water at
a speed of 12m/s. The force exerted by the
engine is 5500N. What is the power of the
engine?
• Answer: 6.6 x 104 W