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25

W Fd

P = =

t t

Power has the units of joule per second (J/s). This unit is given the special name

of watt (w), and 1 w = 1 J/s. However, to rate the power of motors and engines

another unit the horsepower (hp) is commonly used and 1 horsepower = 746 w.

4

Work produces a change in energy. Thus power can be thought of as the

energy produced or consumed divided by the time taken, and we can write

energy produced or consumed E

Power = or P =

time taken t

From this equation we can see that

E = P ⋅ t

This formula is useful in computing the amount of energy consumed in a home.

In particular, since energy is power times time, a watt ⋅ hour (wh) is a unit of

energy. For a larger unit of energy, a kilowatt (kw) may be used and 1000 watts

⋅ hour is a kilowatt – hour (kwh).

5

Energy occurs in many forms. Two of the most fundamental forms of

energy are kinetic energy and potential energy. Kinetic energy is the energy a

body possesses because of its motion or, simply is the energy of motion. As we

learned, work requires motion, so when work causes a change in motion, there

is a corresponding change in kinetic energy. The kinetic energy of an object can

be written as 1 1

kinetic energy = ⋅ mass ⋅ (velocity)

2

or E

k

= mv

2

2 2

6

Potential energy is the energy a body has because of its position or

location or, simply, the energy of position. Work is done in changing the

position of an object, and hence there is a change in energy. For example, if a

book (mass = 1kg) at rest on the floor is lifted at a height of 1 m to the top of a

table, work is done. Work is done against gravity and this work is equal to

W = mgh

where mg is the weight of the object (w = mg) h is the height through which the

book is lifted. With work being done, the energy of the book changed

(increased) and the book on the table has energy and the ability to do work

because of its height or position. This energy is called gravitational potential

25 W Fd P= = t t Power has the units of joule per second (J/s). This unit is given the special name of watt (w), and 1 w = 1 J/s. However, to rate the power of motors and engines another unit the horsepower (hp) is commonly used and 1 horsepower = 746 w. 4 Work produces a change in energy. Thus power can be thought of as the energy produced or consumed divided by the time taken, and we can write energy produced or consumed E Power = or P= time taken t From this equation we can see that E = P ⋅t This formula is useful in computing the amount of energy consumed in a home. In particular, since energy is power times time, a watt ⋅ hour (wh) is a unit of energy. For a larger unit of energy, a kilowatt (kw) may be used and 1000 watts ⋅ hour is a kilowatt – hour (kwh). 5 Energy occurs in many forms. Two of the most fundamental forms of energy are kinetic energy and potential energy. Kinetic energy is the energy a body possesses because of its motion or, simply is the energy of motion. As we learned, work requires motion, so when work causes a change in motion, there is a corresponding change in kinetic energy. The kinetic energy of an object can be written as 1 1 kinetic energy = ⋅ mass ⋅ (velocity) or Ek = mv2 2 2 2 6 Potential energy is the energy a body has because of its position or location or, simply, the energy of position. Work is done in changing the position of an object, and hence there is a change in energy. For example, if a book (mass = 1kg) at rest on the floor is lifted at a height of 1 m to the top of a table, work is done. Work is done against gravity and this work is equal to W = mgh where mg is the weight of the object (w = mg) h is the height through which the book is lifted. With work being done, the energy of the book changed (increased) and the book on the table has energy and the ability to do work because of its height or position. This energy is called gravitational potential

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