## Work and Energy

• Energy is a measure of the cost to perform some action.
• The SI unit of energy is the Joule:
```                               m^2
1 Joule = 1 kg * -----
s^2
```
One Joule is a very small amount of energy.
• Work is the energy spent by a force acting through a distance:
```             Work    =  (force) * (distance parallel to force)
```
or, in general
```             Work    =  (force) * (displacement) * cos(theta)
```
where theta is the angle between the direction of the force and the displacement.
• The unit of work is Newtons times meters, which turns out to be the same as Joules.
• When a force acts in the same direction as displacement, it performs positive work (speeds things up).
• When a force acts in the direction opposite to displacement, it performs negative work (slows things down).
• Kinetic energy is another sort of energy, associated with moving objects:
```               KE    = 0.5 * (mass) * (speed)^2
```
• One can change the kinetic energy of an object by doing work on it:
```          work done on object  =   change in kinetic energy

=   KE(final)      -    KE(initial)

2                   2
=  0.5 * M * v     -   0.5 * M * v
f                   i
```

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