Expert: Courtney Seligman - 6/8/2009

Question
often when mentioning these objects on tv,films etc the narrator will talk about the impact of a collision and describe it in terms of the equivelent to x amount of nuclear explosion,surely this sort of comparison is wrong in that nuclear,tnt or any man made eruption comes from a chemical,electrical or other scientifically produced weapon,how can a lump of rock or ice be described as having a tnt value,its inert

Answer
The only equivalence involved is the total amount of energy. If, say, an impact has a total energy E, and a nuclear explosion has an energy N, the ratio E/N is used to express how many nuclear explosions would have the same total energy.

You are correct in that the energy released by an impact, which is purely due to the energy of motion of the object (which is equal to half its mass times the square of its velocity, and is independent of what the mass consists of), is in no way like the energy released by an explosion. Even the nuclear reactions in the center of the Sun, although atomic in nature, do not produce the same kind of radiation as an atom bomb does. But if, say, the energy produced by the Sun is equivalent to a million billion hydrogen bombs going off every second (which is not far from the truth), then we could say it is 'equivalent' in the sense of how much energy is produced, even though it is totally different in every other respect.

A more mundane approach might be to consider the electrical output of a power plant. If the plant produces 100 Megawatts of power, it doesn't make any difference how it produces that power -- by nuclear fission, burning coal or natural gas, running water through a dam, or witchcraft -- it is still a 100 Megawatt power plant. The specific method of producing the energy might concern those who live near the plant, but those who use the energy don't usually care where it comes from.