http://www.ck12.org Chapter 4. Motion and Forces
Work = 1000 J/s×20 s = 20,000 J
You can watch a video about calculating work and power at this URL:
http://www.youtube.com/watch?v=dXurKNVhatA
Horsepower
Sometimes power is measured in a unit called the horsepower. For example, the power of car engines is usually
expressed in horsepowers. One horsepower is the amount of work a horse can do in 1 minute. It equals 745 watts of
power. Compare the horsepowers in theFigure4.74 to the otherFigure4.75.
FIGURE 4.74
This team of three horses provides 3 horsepowers of power.FIGURE 4.75
This big tractor provides 180 horsepowers of power.Q: If the team of horses and the tractor do the same amount of work plowing a field, which will get the job done
faster?
A: The tractor will get the job done faster because it has more power. In fact, because the tractor has 30 times the
power of the six-horse team, ideally it can do the same work 30 times faster!
Summary
- Power is a measure of the amount of work that can be done in a given amount of time. Power equals work (J)
divided by time (s). - The SI unit for power is the watt (W), which equals 1 joule of work per second (J/s).
- Power can be calculated from work and time using the equation:.
- Power may be measured in a unit called the horsepower. One horsepower is the amount of work a horse can
do in 1 minute, which equals 745 watts of power.
Vocabulary
- power: Measure of the amount of work that can be done in a given amount of time.
- watt (W): SI unit for work, equal to 1 joule of work per second.