Power is defined by how quickly energy is delivered or received. The unit od measurement is the watt. It is abbreviated by a capital "W" because it is named after James Watt. It is modeled mathematically by
Since the change in energy equals the work, the formula can also be written as
This concept of power is not limited to mechanical power. Any time power is discussed, it can always be described energy over time. In the United States, an often used consumer unit of power is the horsepower. It is abbreviated by the letters hp.
Below is information found on the Ford's web site for the version of the Ford Focus that is sold in the United States. Take note of the power's units.
Below is the German version of the Ford Focus. This clip is from Ford's German web site. Note Germany's power units.
 This German site posts the power in kiloWatts, (1 kW = 1000 W.)
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| Practice 18 |
| A 1200 kg car is traveling at 20.0 m/s when is applies its brakes for 4.0 seconds before coming to a stop. How much power is exerted by the brakes? |
| A 1200 kg car is traveling at 20.0 m/s when is applies its brakes for 4.0 seconds before coming to a stop. How much power is exerted by the brakes? |
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| Practice 19 |
| A 0.250 kg ball is dropped off a balcony 18.0 m high. How much power does gravity exert to pull the ball to the ground? |
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A 0.250 kg ball is dropped off a balcony 18.0 m high. How much power does gravity exert to pull the ball to the ground?
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| Practice 20 |
| A 500 kg mass is dropped from rest. The mass is connected to a pulley that is attached to a generator powering a 100 W light bulb, (exactly 100 W). If the generator is to power the bulb for exactly 1 hour then how high must the weight be dropped from. Hint: Because of the connection to the generator, the pulley will not accelerate downwards. ...So you cannot use freefall motion, (kinematics,) to solve. |
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A 500 kg mass is dropped from rest. The mass is connected to a pulley that is attached to a generator powering a 100 W light bulb, (exactly 100 W). If the generator is to power the bulb for exactly 1 hour then how high must the weight be dropped from. Hint: Because of the connection to the generator, the pulley will not accelerate downwards. ...So you cannot use freefall motion, (kinematics,) to solve.
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To find the power exerted by or on a body when many forces are acting on the body at the same time, focus on the one force. To find the power of a specific force, calculate the work done by that force. To do this, look at the work done by that force while ignoring all the other forces. For example: A jet was flying with all of the forces acting on it that are shown below.
To calculate the work done by the lift force, all the other forces are ignored, as indicated below.
The work is the (lifting force)(y), where "y" is the distance parallel to the lifting force's direction. The power of this force is found from
The time can be measured directly or calculated by other means, such as kinematics.
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