Statics question 1

[frankyg]

Hi,

This is probably a very basic question, but it's been a long time since I took statics.

I have a body with 1 known force and three unknown, all acting in the Y direction. Can I figure this out by summing the Y forces and the moments at 2 different points, or is this ambigious? Based on what I am getting, and from what I think I remember from statics, you can't solve this problem. Am I correct on this?

Thanks

 

[handleman]

i don't care what assumptions are made in the direction of the reactions

What if the "reactions" are not reactions at all?

I accept your solution, but with some pretty big reservations.

Here are just a few of the assumptions you've made:

1. All unknowns are reactions at supports. This actually conflicts with the wording of every statement made by the OP. All unknowns are referred to as "forces" or "loads".

2. All "loads" that you've assumed to be supports are rigid.

3. The "loads" that you've assumed are supports are capable of acting in the opposite direction that the OP stated that they act.

4. Without the applied 120lb load on the left end there is no preload at the supports. I.E. the beam is not already clamped.

5. Without the applied 120lb load, the beam is actually in contact with each support. I.E. there is no deformation required to make the beam contact one of the rigid supports.


Unless the OP is trying to solve a problem from a textbook, I don't think any one of these four assumptions has any support from the information provided. If any one of them is false your solution is invalidated, but you didn't state a single one until your "solution" post, and even then you didn't really recognize that you had made them.

 

[rb1957]

pretty much as you've stated handleman, all of your points were assumed, intuitively, possibly incorrectly, but with the limited information provided i think reasonably.

1) i think that's semantics
2) yeap, supports are assumed rigid, as opposed to elastic foundations.
3) yeap, supports are assumed to be able to react load in either direction.
4) yeap, assumed no initial preload
5) teap, assumed no initial deformation.

whilst we're getting picky, i assumed no weight either.

 

[handleman]

I guess I really sort of got bogged down in details rather than making my intended point. What I was trying to get across is that we can't just make a bunch of simplifying assumptions without listing them and then tell someone that their problem is an easy one to solve. If the OP were talking about a problem from a textbook, I'd say every one of the assumptions is pretty valid, because textbook problems are simplified on purpose to stress the application of a single principle. However, I certainly hope this wasn't a textbook problem.

 

[zekeman]

Quote"rb1957 (Aerospace) 11 Sep 07 12:47
ok zekeman, ASSUMING the problem is elastic ... it is solvable ... the body with the forces acting on it may not be a classical beam but the method outlined applies ...

and JStephen ... as a planar problem, it can be solved. For example IF it is a classic beam on three supports."
I still submit it is NOT solvable by the socalled classical methods, since they imply some type of restraint on the system. In your analysis you are asuming that all of the loads points do not move which is NOT in the statement of the problem.
Just consider the FBD THE OP posted and assume the right end force is zero;. You still get a valid solution. Now give it any other value and you still get a valid solution. The result of two "valid" solutions suggests that there are NO valid solutions. Yor problem is that you solved the problem assuming zero for one of the forces, get a deflection at that point and then by superposition you push at thaty point to reverse the deflection which implies that you are restraining the system to have no deflection under the loads, which is NOT a condition given by the OP.

 

[rb1957]

zekeman,
i have assumed that the three reaction points are restrained, since the OP doesn't say they are elastic. If they are elastic the problem is just as solvable (tho' quite a bit more complicated).

my solution to the problem, given my assumptions as both you and handleman are want to point out, is a correct solution to a beam on three rigid supports. this may not be hte OP's problem, but let him say so, eh ?