# Beam support force question

1. Jul 18, 2011

### enveng2013

1. The problem statement, all variables and given/known data
Ok so I have a beam with the forces shown below. There is a 3 m span between the supports (A&B) and both of the supports are 1.5 m from the end of the beam. The 60 lb force is .75 meters left of support A and .75 m from the left end of the beam. P is an unknown force which I must solve for at the right end of the beam. I am allowed 33.75 lb on the left support and 93.75 lb on the right (this came from a stress equation I'm pretty confident those numbers are right).

2. Relevant equations

No real relevant equations except moment and force.

3. The attempt at a solution
So the force at A should be described by the moment around B divided by the distance from B to A. Thus giving a beam where the moment forces are zero.

FA=(P*1.5-60*3.75)/3

Then force B can be related to A because the forces in the y direction must be zero or the beam would move

Thus FB=(60+P)-FA or FB=(60+P)-(P*1.5-60*3.75)/3

I've tried plugging in the maximum forces for both but I can't seem to get an answer that works for both supports.

If you guys could help me or even steer me in the right direction that would be great.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

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2. Jul 18, 2011

### SteamKing

Staff Emeritus
You've got to write all of the equations of equilibrium in order to obtain correct solutions for RA and RB. That means the sum of the forces = 0 and the sum of the moments = 0.
As the problem is stated, you have three unknowns (P, RA, and RB) and have shown only one moment equation (sort of).

3. Jul 18, 2011

### enveng2013

I actually just looked at a solutions manual. I got the answer right from what I had. It just didn't seem right to me for some reason.

4. Jul 18, 2011

### SteamKing

Staff Emeritus
If only Life came with a solutions manual.

5. Jul 18, 2011

### enveng2013

They're not always useful. Sometimes though, it's nice to know what you got was right.