# [PD] fft beginner question

Mathieu Bouchard matju at artengine.ca
Fri Nov 23 05:45:24 CET 2007

```On Wed, 21 Nov 2007, Martin Peach wrote:

> Only the kinetic energy can be measured directly.
> The potential energy can only be calculated.

So how do you measure the kinetic energy without calculating?

> There is no real instrument that can measure the potential energy

So, where is the potential energy that allows atoms to fall apart?
Apparently it's stored as extra mass in the nucleus. If you snap a
uranium235 with a neutron, it becomes a krypton92, a baryum141 and three
neutrons. If you account the masses more precisely, which are not the
above numbers, you get:

235.043930 + 1.008665 - 91.926156 - 140.914411 - 3*1.008665

Which is 0.186033... some fraction of a particle. Why is it? It's because
that energy weighs something! 0.000000000000000000000000308915 gram of
matter (above numbers were in nuclear units), times the square of the base
lightspeed (299792458 metres/second), is about 0.00000002776 joule of
energy, according to Einstein's most famous equation. This can't be an
(ordinary) particle, because nucleons are all around 1 unit each, and
electrons are all below 0.001 unit each.

Is throwing a ball from a cliff different? (apart from differences of
scale!). I don't quite know. I know that Special Relativity says kinetic
energy increases mass, but I don't know (or don't remember) what it is