# Impossible Question

1. Sep 22, 2009

### you_of_eh

From the following defining equations, determine the dimensions of the names physical quantities. Use L to represent the dimension length (distance), T to represent time and M to represent mass.

a) force, F: F=ma, (where m is mass and a is acceleration)
b) pressure, p: p=F/A, (where F is a force (see previous question) and A is an area)

-I don't need the answer it's just that I can't even attempt to solve the question as I have no idea what the question is asking for.

a) F=M*d(L/T)/dT ?
b) p=M*d(L/T)/dT/L^2 ?

..that is all that I could come up with.

2. Sep 22, 2009

### Staff: Mentor

Welcome to the PF. You are close... For a), just clean up what you have, and leave out the d symbols. The units of a change in lenght are still length. Does that help?

3. Sep 22, 2009

### you_of_eh

OK yea I get it..

a) F = M(L^3)(T^2)
b) p = ML/T^2

4. Sep 22, 2009

### Staff: Mentor

Much closer. But you messed up a division in a) (I didn't check b).

Hint -- The unit of force is a Newton. Look up what the sub-units are that make up a N.

5. Sep 23, 2009

### you_of_eh

Well I checked it a couple times..seems correct to me.

6. Sep 23, 2009

### Zhalfirin88

Try plugging in actual units. M = kg, L = m, T = s

For a) you're saying F = M(L^3)(T^2)

which means, N(Newton) = kg * m3 * s2, and that is close, but not right.

Edit: Also, b) is wrong. Again, p = ML/T^2 is close, but not right.

You're saying, p = kg * m / s2

Last edited: Sep 23, 2009