12-1: Work, Power, and Machines
Monday, December 14, 2015
7:36 AM
Key Terms:
1. work – the transfer of energy to a body by the application of a force that causes the body to move in
the direction of the force.
2. power – a quantity that measures the rate at which work is done or energy is transformed.
3. mechanical advantage – a quantity that measures how much a machine multiplies force or distance
I. What Is Work?
Work is done only when force causes a change in the position or the motion of an object in the
direction of the applied force.
a. Work is measured in joules
i. Because work is calculated as force times distance, it is measured in units of newtons times
meters. These units are also called joules.
Ch12 Work and Energy Page 1
II. Power
Running up steps requires the same amount of work as walking, but running is more exhausting.
The amount of time it takes to do work is an important factor when considering work and
machines.
a. Power is measured in watts
i. Power is measured in SI units called watts (W). A watt is the amount of power required to do 1J of
work in 1s.
ii. Be careful not to confuse the abbreviation for watts, W, with the symbol for work, W.
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III. Machines and Mechanical Advantage
a. Machines multiply and redirect forces
i. Machines can change the direction of an input force.
ii. Machines can also increase or decrease force by changing the distance over which the force is
applied.
b. Different forces can do the same amount of work
i. Machines can make doing work easier by increasing the distance over which force is applied.
ii. As a result, the force required at any point is reduced.
iii. Therefore, a machine allows the same amount of work to be done by either decreasing the
distance while increasing the force or by decreasing the force while increasing the distance.
c. Mechanical advantage tells how much a machine multiplies force or increases distance
i. Mechanical advantage is defined as a ratio between the output force and the input force. It is also
equal to the ratio between the input distance and the output distance, if friction is ignored.
ii. A machine with a mechanical advantage greater than 1 multiplies the input force.
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ii. A machine with a mechanical advantage greater than 1 multiplies the input force.
i. Useful for lifting heavy objects.
iii. A machine with a mechanical advantage less than 1 does not multiply force, but increases
distance and speed.
i. When you swing a bat, your arms and the bat together form a machine that increases
speed without multiplying force.
Quiz: Match each term in the left column with the correct description in the right column. Then write the
equation and the SI unit for each item.
Left column
Right column
1. Work
a. the amount that a machine multiplies force or distance
2. Power
b. the rate at which work is done
3. Mechanical advantage
c. what is done when a force makes an object move
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