# Homework Help: Question Concerning F=ma and FBDs.

1. Oct 17, 2005

### lawtonfogle

Ok, I know that, using a FBD, one is susposed to use $$\sum F = m a$$
My question is what order do you put the forces that $$\sum F$$ equal
An example might help me explain my question.
Lets take a block of wood that is supported by a string. The only forces acting on it are $$mg$$ and $$T$$
So...
$$\sum F = ma$$
$$T - mg = ma$$
or
$$mg - T = ma$$.
I know if $$a = 0$$ it does not matter, but how does one decide what order to put th forces in when there is acleration?

Last edited: Oct 17, 2005
2. Oct 17, 2005

### lawtonfogle

The $$\sigma$$ should be a capital sigma.

I fixed it.

Last edited: Oct 17, 2005
3. Oct 17, 2005

### Päällikkö

It depends on how you choose the positive and negative directions.
T - mg = ma1, you've chosen the positive direction upwards.
mg - T = ma2, you've chosen the positive direction downwards.

T - mg = ma1 = -(mg - T) = -ma2

In the case that mg is greater than T, the acceleration's negative when you've chosed the positive direction upwards, and positive when you've chosen the positive direction downwards. The magnitude does not change.

4. Oct 17, 2005

### lawtonfogle

So on an Atwood machine, I should chose it so that both masses are the same way written (both T - mg, or vice versa), or so that the greater force is first.