Power
7.4 Power
Learning Objectives
By the end of this section, you will be able to:
- Relate the work done during a time interval to the power delivered
- Find the power expended by a force acting on a moving body
Power
Power is defined as the rate of doing work, or the limit of the average power for time intervals approaching zero,
Discuss!
Consider how you would answer these questions. Then bring this to class for a group discussion.
An elevator car (see figure) has a mass of 1008 kg and is carrying passengers having a combined mass of 874 kg. A constant friction force of 4084 N opposes its motion. The elevator descends at 3.0 m/s. What is the power delivered by the motor? (Energy must be removed to slow the elevator’s descent.)
A 2000 kg trolley is pulled up a 30° hill at 20 mi/hr by a winch at the top of the hill. How much power is the winch providing?
Practice!
| Practice 7.4.1 |
|---|
 A weightlifter exerts an upward force on a 1000-N barbell and lifts the barbell 1 meter upward in 2 seconds. Approximately how much power does the weightlifter exert on the barbell during this time? |
| (a) 1000 watts |
| (b) 500 watts |
| (c) 2000 watts |
| (d) 0 watts |
| Practice 7.4.2 |
|---|
A weightlifter exerts an upward force on a 1000-N barbell and holds it at a height of 1 meter for 2 seconds. Approximately how much power does the weightlifter exert on the barbell during this time? |
| (a) 1000 watts |
| (b) 500 watts |
| (c) 2000 watts |
| (d) 0 watts |
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