# B Basic question - balance of forces...?

1. Apr 26, 2017

### tbaker

I'm trying to find some basic physics information. Suppose you have a (massless) beam, supported at two points, 10 feet apart. You have a force (m*g) of 100 newtons resting on that beam, say 4 meters from the left point (6 meters from the right)... How much force does that exert on each point, and how do you calculate it...? It seems this would be a simple physics example but I can't seem to find any info on an example like this. I'm working on a trailer hitch problem and the info would be useful...

Thanks.

2. Apr 26, 2017

### tbaker

Sorry, I meant the beam is 10 meters long...

3. Apr 26, 2017

### pixel

Try looking at the torque about each support, keeping in mind that the net torque about any point must be zero.

4. Apr 29, 2017

### tbaker

Thanks, but I was looking for a more specific 'example'. I'm not a student trying to get an answer for a homework problem... thanks anyway.

5. Apr 29, 2017

### jbriggs444

That's not how we roll here. For spoon-fed answers you will need to look elsewhere. The expectation is that you will put in some effort, show your work and we will try to help when you get stuck.

So, can you pick a point on the beam and write down expressions for the torque about that point from each of the three forces on the beam?

6. Apr 29, 2017

### tbaker

So I think I've got it. It's been a while since I've done a physics problem.

So there's a beam (massless), 10 feet long. At each end is a support. A 100 pound weight is 2 feet from the left support (8 feet from the right). The system is in static equilibrium. Call the 100 pound weight F3. The left support F1, the right support F2. So F3 = F1 + F2. Using torque (I'll simplify it as FxD). So, since F1 and F2 are also in equilibrium with each other, F1*D1 = F2*D2, or F1*2 = F2*8.

So, F1 = 4*F2
So, F2 + 4F2 = 100
5F2 = 100
F2 = 20 lbs.
F1 = 80 lbs

Anyhow, I think I have what I need to get an idea what I'm trying to figure out.

Thanks.

7. Apr 30, 2017

### pixel

That looks correct.

8. May 1, 2017

Thanks pixel