The rate at which work is done, or the rate at which energy is converted from one form to another is the

The units of power are watts (W) where one watt is one joule per second.

Since work done = energy converted = force x distance, we may express power as:

Power = force x distance/ time

and you should see that this is equal to force x velocity. So power may be expressed as:

A lorry of mass 2000 kg moving at 10 m s

What power must the engine produce if the lorry is to travel up a hill of 1 in 10 at a constant speed of 10 ms

Kinetic energy of lorry = ½ x 2000 x 100 = 105 J = Fx 12.5

Therefore: F = 8000 N

On the hill, height risen per second = 1 m and distance travelled along the slope = 10 m.

Potential energy gained by lorry per second (taking g = 9.8 Nkg

Work done against friction per second = 200x 10 =2000J.

Total energy required per second = 21600W= 21.6 kW.

Power of your heart | = 1W |

Power of a rowing eight at the start | = 8 HP = 8x746 W = 5968 W = 6 kW |

Power of a family car | = 20 - 100 kW |

Power of a World War II fighter | = 680 HP = 507 kW |

Power of a power boat | = 112 kW at v= 20 ms^{-1} |

Power of a coach | = 200 - 300 kW |

Power of a Lynx helicopter | = 670 - 835 kW |

Power of a cross channel ferry | = 28 000 HP = 20 MW |

Power delivered by a power station | = 1000 MW |

The following data refers to some production model cars.

Use it to calculate:

(a) the mean acceleration from 0-60 mph

(b) the maximum kinetic energy

(c) the force produced by the engine at top speed

0 - 60 mph | Max. power | Mass | |

Car 1 | 11.85 s | 51 kW | 1300 kg |

Car 2 | 7.25 s | 110 kW | 1700 kg |

Car 3 | 13.55 s | 77 kW | 1750 kg |

See also:

Energy sources

Kinetic energy

Energy

Work