## 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 |
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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 |
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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 |