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Mechanical Advantage and Efficiency

Science, Grade 6


Table Of Contents: Mechanical Advantage and Efficiency

1. Mechanical Advantage
The amount of help a machine provides is measured by its mechanical advantage. This can be determined by comparing the output force to the input force. In this example, the lever with an input force of one Newton and an output force of three Newtons, provides a mechanical advantage by tripling the output force.
2. Calculating Mechanical Advantage
The formula for mechanical advantage is output force divided by input force. This lever has an input force of two Newtons and an output force of ten Newtons. It has a mechanical advantage equal to five. A person using chopsticks exerts three Newtons of input force over a short distance, while the output force decreases to one Newton over a greater distance. The chopsticks have a mechanical advantage of .33.
3. Efficiency of Machines
In an ideal situation, the total amount of work done by a machine should equal the amount of work put into it. However, due to friction, the output force is usually less than the input force. Machines with greater efficiency do a better job of overcoming the force of friction. For example, a pulley with 90% mechanical efficiency uses 90% of the work input to lift a load, while the other 10% is used to overcome friction.
4. Calculating Mechanical Efficiency
To calculate mechanical efficiency, divide work output by work input and multiple by 100. For example, you do 3,000 joules of work pushing a box up a ramp. The work done by the ramp is 2,400 joules. The mechanical efficiency of the ramp is 2,400 divided by 3,000, times 100. This equals 80%. That means the other 20% of the work input was used to overcome friction.
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