Energy makes stuff happen. Energy associated with the motion of an object is called kinetic energy. Energy stored in an object due to its position is called potential energy. Energy can be converted between potential and kinetic based on an object’s motion.

**To better understand Potential vs. Kinetic Energy…**

## LET’S BREAK IT DOWN!

### Energy makes stuff happen.

Richard Feynman, one of the greatest physicists of the twentieth century, told his students that, “It is important to realize that, in physics today, we have no knowledge of what energy is. We do not have a picture that energy comes in little blobs of a definite amount. It is not that way. However, there are formulas for calculating some numerical quantity, and when we add it all together it gives … always the same number. It is an abstract thing in that it does not tell us the mechanism or the reasons for the various formulas“(Feynman, Leighton, and Sands 1965, p. 4-2). According to Feynman, it is more important to describe how energy behaves in a system than to define what energy is. One thing we know is that energy makes stuff happen, and we can calculate it very precisely when it does.

### Energy associated with the motion of an object is called kinetic energy (KE).

From your youngest age, you probably associated motion with energy. You intuitively knew that the faster something moved, the more energy it must have. A more precise term for energy of motion is kinetic energy. The amount of KE an object has depends upon its mass and its velocity (speed in a given direction). By making many observations over time, scientists determined that the KE of an object is equal to one-half its mass (m) times its velocity (v) squared, and they write the equation as:

KE = ½ × m × v²

The equation shows that KE is directly proportional to the mass of an object (KE ∝ m). As this graph shows, if we double the KE, then we double the mass, or if we halve the KE, then we halve the mass. It also shows that the relationship is linear (directly proportional, straight-line). That’s why wrecking balls are so big and heavy—more KE to destroy stuff!

Look at the relationship between KE and velocity shown in this graph. Again, we see that KE and velocity are directly proportional (KE ∝ v²), but if we increase the KE by a factor of 4, then the velocity of the object doubles (that’s the velocity times itself, not times 2). The graphed relationship is exponential (directly proportional where one of the variables is an exponent, curved line). Think of the damage a railgun can do shooting out a lightweight projectile at six thousand miles per hour without using explosives!