# Internal forces

1. Nov 26, 2013

### princejan7

1. The problem statement, all variables and given/known data

http://postimg.org/image/4lvunjeoz/

Solution: http://postimg.org/image/v1d7gilef/

When you have to calculate the internal forces at a point, how do you decide which forces are included in the free body diagram?

For D, why isn't the rectangular bit of the distributed load included? Doesn't it affect D?

2. Nov 26, 2013

### Staff: Mentor

Before you start addressing the internal forces, you should first have determined the unknown external forces and moments (if possible). Then, you draw a free body diagram that includes your cross section of interest. You try to choose a free body that includes as few of the external forces and moments as possible; this reduces the amount of work you need to do, but doesn't affect the answer.
Yes. But its effect is captured by the other external forces that are actually acting on the free body you have chosen.

3. Nov 26, 2013

### princejan7

But the question says that D is located to the left of point B. Wouldn't that mean the rectangular distributed load is also actually acting on the body at D?

4. Nov 26, 2013

### Staff: Mentor

Only the insignificant part between B and D.

5. Nov 26, 2013

### princejan7

oh ok, but then what about the diagram for point E?
The triangular distributed load is included in the diagram even though the important part is quite a bit to the left?

6. Nov 26, 2013

### Staff: Mentor

That's OK. Your free body diagram does not include the part to the left. And, as I said earlier, the effect of the rest of the triangular distributed load is accounted for by the reactions at B and A. To prove this to yourself, use the free body to the left of E instead of the one to the right of E, and see if you get a different result for the internal forces and moments at E.