| Contents for this page | Related topics |  |
|---|---|---|
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Definition of power Power required to overcome forces at constant velocity Additional questions |
Projectiles Conservation of momentum in two dimensions Energy Work |
 Data Glossary |
| Learning Outcomes | ||
| After studying this section, you will (a) understand and be able to apply the concept of power and (b) be able to solve numerical problems involving power. | ||
The rate at which work is done or at which energy is transferred is called POWER (usually given the symbol, P) . It is the work done (or energy transferred) in unit time, thus
where Δt is the time interval during which the work was performed. Power is a scalar quantity and its unit is the WATT, (W) (named after James Watt, the inventor of the steam engine). One watt is the power consumed when one joule of work is done per second (
).
The power that is required by a vehicle to overcome forces encountered while in motion are of great practical interest, as this relates to fuel consumption and hence to the cost of operating the vehicle.
Let -F be the sum of the forces opposing the motion of the vehicle. Then, the force required to keep the vehicle at a constant velocity, v, will be F. The work done in covering a distance, s, will be w = Fs.
If the vehicle is moving at a constant velocity, v, it will cover that distance in a time equal to s/v seconds.
Therefore, the average power put out by the engine of the vehicle will be the product of the total force needed to overcome drag and friction, and the constant velocity of motion.
This is discussed elsewhere. Electric appliances, light bulbs, and so on, are rated in watts (or kilowatts), this being the energy that they consume in unit time. Thus, a 100 watt bulb will consume 100 J.s-1. In one hour, the bulb will use up 100 x 3600 = 360 kJ. One pays for electric energy in terms of units called KILOWATT HOURS, kW.h. This is the amount of energy that will be consumed by a 1000 watt appliance being used for one hour. The kW.h is a unit of energy, not power, as an examination of the dimensions will clearly show. It is equivalent to 3.6 MJ (megajoules).