Page:Electricity (1912) Kapp.djvu/72

68 and the distance $$R$$), which, during the flight of this projectile, has been stored in it in the shape of kinetic energy. By a well-known law of mechanics the kinetic energy stored in a projectile is given by the product of half its mass and the square of the velocity. Since the mass and energy are known, the velocity can be calculated.

Let us apply, by way of illustration, this principle of equivalence between potential and kinetic energy to the calculation of the velocity with which a meteorite strikes our earth. The potential of gravity of the earth on a point on its surface is

$$V = \frac{fM}{R}$$

where $$M$$ is the mass of the earth and $$R$$ its radius. The energy stored in a meteorite of mass $$m$$ is therefore

$$E = \frac{fMm}{R}$$

which may also be written in the form

$$E = \frac{fMm}{R^2}R$$

But $$\frac{fMm}{R^2}$$ is nothing else than the weight of the mass $$m$$, and we thus find that the