# Problem with learning how to use equations of motion

1. Sep 15, 2008

### StingerManB

Here is my problem: I am new to physics all together, NEVER a physics course at all. I am in a Calculus Based physics class, placed there due to the A's I made last semester in calculus (and for other scheduling conflicts)
The problems involving the equations of motion are giving me trouble. Setting up the problems and which equation to use is my biggest problem.
I am just looking for suggestions to help me improve. I have tried to just do more problems, but I get stuck right at the beginning.

Any help at all or suggestions for me?

2. Sep 15, 2008

### atyy

Use every equation that comes to your mind, as long as you know it's right (ie. guess). Count the number of unknowns you have, and count the number of equations. When you have enough equations, solve for the unknowns.

Sometimes it helps to write the same equation in different forms, eg. F=ma=m(dv/dt)=m(d2x/dt2)

A second way is to use dimensional analysis. Look at the dimensions (length, mass, time, etc.) of the unknown, look at the knowns. Put the knowns together in some combination so that their combined dimensions will match the dimensions of the unknown. This will at least give you the form of the equation. This is dangerous, because the answer is not unique - eg. m1/m2 has no dimensions so it can enter in any combination without changing the dimensions of your equation - so you must justify the exact equation properly after using this to guess its form.

As for physical intuition about which equation to use first - practice makes perfect.

Last edited: Sep 15, 2008
3. Sep 30, 2008

### Wecht

Remember to use the equation Vfx^2 = Vix^2 + 2ax(xf-xi) Helpful when you're not given the time.

4. Sep 30, 2008

### Wecht

Also, if you give me some specific examples of what you don't understand, I can help you more.