Transcript Chapter 5 - cloudfront.net

Chapter 5
5-4 Power
 Define power
Objectives
 Calculate power
 Explain the effect on machines the effect of machines on work
and power
What is power
 Power is the rate of work done in a unit of time. It can be
misunderstood by most of the students. They think that more
power full machine does more work. However, power just shows
us the time that the work requires. For example, same work is
done by two different people with different time. Say one of them
does the work in 5 seconds and the other does in 8 seconds. Thus,
the man doing same work in 5 seconds is more power full. The
shorter the time the more power full the man.
Mathematically, it is computed using the following equation.
Power
The standard metric unit of power is the Watt. As is implied by
the equation for power, a unit of power is equivalent to a unit
of work divided by a unit of time. Thus, a Watt is equivalent to
a Joule/second. For historical reasons, the horsepower is
occasionally used to describe the power delivered by a
machine. One horsepower is equivalent to approximately 750
Watts.
Example Find the power of the man who pushes the box 8m with a force of 15N in a 6seconds.
Example#1
 A tired squirrel (mass of approximately 1 kg) does push-ups by
applying a force to elevate its center-of-mass by 5 cm in order to
do a mere 0.50 Joule of work. If the tired squirrel does all this work
in 2 seconds, then determine its power.
Example#2
 Solution:
The tired squirrel does 0.50 Joule of work in 2.0 seconds. The
power rating of this squirrel is found by
P = W / t = (0.50 J) / (2.0 s) = 0.25 Watts
 When doing a chin-up, a physics student lifts her 42.0-kg body a
distance of 0.25 meters in 2 seconds. What is the power delivered
by the student's biceps?
 solutioh
Example#3
To raise her body upward at a constant speed, the student
must apply a force which is equal to her weight (m•g). The work
done to lift her body is
W = F * d = (411.6 N) * (0.250 m)
W = 102.9 J
The power is the work/time ratio which is (102.9 J) / (2
seconds) = 51.5 Watts (rounded)
Student
guided
practice
 Do worksheet
 The expression for power is work/time. And since the expression
for work is force*displacement, the expression for power can be
rewritten as (force*displacement)/time. Since the expression for
velocity is displacement/time, the expression for power can be
rewritten once more as force*velocity. This is shown below.
Power
Homework
 Do problems 1-4 page 175
Closure
 Today we learned about power
 Next we are going to learn about momentum and collisions