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 the Figure 1.2 to the other Figure 1.3. This team of three horses provides 3 horsepowers of power. 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!



calculating work from power and time

You can also calculate work if you know power and time by rewriting the power equation above as: Work = Power Time For example, if you use a 1000-watt microwave oven for 20 seconds, how much work does it do? First express 1000 watts in J/s and then substitute this value for power the work equation: Work = 1000 J/s 20 s = 20,000 J

calculating power from work and time

Power can be calculated using the formula above if the amount of work and time are known. For example, assume that a microwave oven does 24,000 joules of work in 30 seconds. Then the power of the microwave is: 24000 J Power = Work Time = 30 s = 800 J/s, or 800 W Q: Another microwave oven does 5,000 joules of work in 5 seconds. What is its power? A: The power of the other microwave oven is: J Power = 5000 5 s = 1000 J/s, or 1000 W Q: Which microwave oven will heat the same amount of food in less time? A: The 1000-watt microwave oven has more power, so it will heat the same amount of food in less time.

what is power

Power is a measure of the amount of work that can be done in a given amount of time. Power can be represented by the equation: Power = Work Time In this equation, work is measured in joules (J) and time is measured in seconds (s), so power is expressed in joules per second (J/s). This is the SI unit for power, also known as the watt (W). A watt equals 1 joule of work per second. Youre probably already familiar with watts. Light bulbs and small appliances such as microwave ovens are labeled with the watts of power they provide. For example, the package of light bulbs in the Figure 1.1 is labeled 14 watts. Q: Assume you have two light bulbs of the same type, such as two compact fluorescent light bulbs like the one pictured in the Figure 1.1. If one light bulb is a 25-watt bulb and the other is a 60-watt bulb, which bulb produces brighter light? A: The 60-watt bulb is more powerful, so it produces brighter light. Compared with a less powerful device, a more powerful device can either do more work in the same time or do the same work in less time. For example, compared with a low-power microwave oven, a high-power microwave oven can cook more food in the same time or the same amount of food in less time.


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compared with a less powerful device, a more powerful device can do

a) more work in the same time.

b) the same work in less time.

c) less work in the same time.

-->  d) two of the above

if a microwave does 3000 j of work in 3 seconds, its power is

a) 9000 w.

b) 3300 w.

c) 2700 w.

-->  d) 1000 w.

if you know power and time, you can calculate work with the equation

-->  a) work = power x time.

b) work = power/time.

c) work = time/power.

d) none of the above

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