Physics Essentials For Dummies by Unknown

Physics Essentials For Dummies by Unknown

Language: eng
Format: epub

Chapter 7: Putting Physics to Work 83

Where does the kinetic energy go when friction is involved? If a block is sliding along a horizontal surface and there’s friction, the block goes more and more slowly until it comes to a stop.

The kinetic energy goes away, and you see no increase in potential energy. What happened? The block’s kinetic energy dissipated as heat. Friction heated both the block and the surface.

You now know the ins and outs of kinetic energy. So how do you calculate it?

Breaking down the kinetic

energy equation

The work that you put into accelerating an object — that is, into its motion — becomes the object’s kinetic energy, KE.

The equation to find KE is

KE = 1⁄2 mv 2

Given a mass m going at a speed v, you can calculate an object’s kinetic energy. Say, for example, that you apply a force to a model airplane in order to get it flying and that the plane is accelerating. Here’s the equation for force:

F = ma

You know that force equals mass times acceleration, and you know from the previous sections in this chapter that the work done on the plane, which becomes its kinetic energy, equals the following:

W = Fs cos θ

Assume that you’re pushing in the same direction that the

plane is going; in this case, cos θ = 1, and you find that W = Fs = mas

You can tie this equation to the final and original velocity of the object (see Chapter 3 for that equation) to find a: v 2 – v 2 = 2





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