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  Last Update: 11 March 2011

- Work and Energy -

A quick review of the fundamentals

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Suppose the engine of your car stalled while you were in line to exit from a flat, level parking lot. You try several times to restart it, but it just won't start.

Since you are a considerate person, you decide to push your car out of the way of the people behind you. You get out and go round back and begin to push on the car. Suppose also that you are a fairly strong person, so you exert a horizontal force of 100 pounds on the rear of the car. The car doesn’t move. But you are also a persistent person, so you continue to push on the car for two whole minutes, exerting the same 100 pounds of force. The car still won’t move. Although you will probably be quite tired, you will have done NO WORK.

WHY? Because WORK is defined as a FORCE operating through a DISTANCE. The car didn’t move, so although there was FORCE, there was no MOTION.

Now you get smart and release the parking brake, and, having recovered from your previous 2-minute exercise in futility, you again push the car with the same constant 100 pound force. This time the car moves, and you push it for another two minutes. It travels 165 feet during that two minutes of effort. In that case, you will have produced 16,500 foot-pounds of  WORK (100 pounds of force x 165 feet of distance = 16,500 foot-pounds).


Later that day, you are working in your shop. You need to install a 3-inch long spring into a 2-inch space. The nature of this particular spring is that it takes 600 pounds of force to compress it one inch (the "spring rate" = 600 pounds per inch).

Using a lever-operated spring compressor, you pull on the lever with a force of 100 pounds and you move the lever 6 inches, causing the compressor to squeeze the spring and shorten it by 1 inch. The spring is now pushing on the compressor with a force of 600 pounds. You have stored the WORK you did on the compressor lever (100 pounds x 6 inch = 600 inch-pounds) in the spring, in the form of ENERGY (600 pounds x 1 inch = 600 inch-pounds).

ENERGY is defined as the CAPACITY of a body to do WORK, by virtue of the position or condition of the body.

Now suppose there is a 150-pound plate of steel on your bench, resting on four blocks which are 2 inches tall (so the space between the bottom of the plate and the bench is 2 inches). You install the compressed spring into that space and locate it at exactly the CG of the plate, and release the spring compressor.

The spring will lift the steel plate 3/4 of an inch, so the spring has done WORK on the plate, thereby releasing some of the ENERGY stored in the spring.

There are many different forms of energy. There are a few which are of particular interest with respect to powerplants: kinetic energy (the energy contained in a body by virtue of its velocity), potential energy (the energy contained in a body by virtue of its position), chemical energy (energy which can be released by a chemical reaction, such as combustion), and heat energy (energy which can be used to make machines operate).

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