# F=ma help

1. May 10, 2007

### brianthewhitie7

Force=mass X acceleration is used for objects larger than 10^-10 but what formula is used for particles smaller than 10^-10?

Also when squaring light in E=MC^2 what would light be? - 9 X 10^35?

2. May 10, 2007

### StatMechGuy

What are the units that you are talking about so that we can get a sense of what's exactly 10^-10 is.

3. May 10, 2007

### Staff: Mentor

Yes, you need to specify which system of units you are using, and stay consistent in your calculations. The two main unit systems that Physicists use are cgs and mks:

http://en.wikipedia.org/wiki/Units_of_measurement

4. May 10, 2007

### G01

If your question is what is the equivalent of Newton's Second Law on the quantum scale, then the answer that most would give is that it is the Schrodinger Equation.

Give some units and you'll probably get a more detailed answer.

5. May 10, 2007

### Tomsk

In QM, the closest you get to F=ma (or F = dp/dt = -dV/dx) is F = d<p>/dt = -<dV/dx> where <> denotes the average value recorded over many repeated runs. This comes from the Schrodinger eqn. This is a weird result, which basically means classical physics exists as a limit of quantum physics, that is, you get a classical world when you consider large enough quantum systems, with large quantum numbers. There is no exact line you can draw between the quantum and classical world either, it's a sliding scale, sort of. (I think that is supposed to be quite a sticky subject though!) So we don't really need to know what your 10^-10 is measured in. ;) Although you should just use SI, it is the basically the standard nowadays. XD