Calculating Moment of a Force

[joe465]

Homework Statement

Crates are placed on a framework for temporary storage. Calculate the moment of the force exerted by the crate around support A.

Homework Equations

Moment = force * distance

The Attempt at a Solution

The framework is 14 m in length and the crate is 2m away from Support A. What is the force acting down on the crate? Is it just the weight of the crate because of gravity or is there something else i am overlooking? The weight is 200KG

 

[joe465]

I have attached a photo to help understanding.

Is the force acting down on the crate:

F=mg

F=200*9.81
F=1962

Now times this by 2 to get the moment of the force since the crate is 2 metres away from A.

3924Nm?

Is this correct as the crate seems to cover 2 metres?

Thanks in advance.

 

[ehild]

Homework Statement

Crates are placed on a framework for temporary storage. Calculate the moment of the force exerted by the crate around support A.

The question is the moment of force (torque).

ehild

 

[joe465]

Thanks for your reply, isnt that what i have answered? Since moment=force * distance

 

[joe465]

Anyone help?

Cheers, Joe

 

[ehild]

Sorry, I did not notice that you multiplied the force by 2 m. Your answer is correct if the centre of mass of the crate is at 2 m horizontal distance from A. So the figure is not quite correct.

ehild

 

[joe465]

What distance would u say this was classed as?

I could only guess, that the centre of gravity acts through the centre of the crate which would work out as 1 metre, would you say that was correct?

The course material is a poor form of instruction

Thanks, Joe

 

[ehild]

The problem says nothing about the size of the crate. I think you can take that the middle 2 m from A, so 3924 Nm has to be correct.

ehild

 

[joe465]

Thankyou very much for your help

Joe

 

[joe465]

Thankyou very much for your help

Joe

 

[thegreenlaser]

Is there any more information about how the table is supported by the ground? It seems to me that the question is nearly impossible to answer without more information about the supports A and B. I would assume that A and B are both exerting a verticle force. If A and B are both capable of exerting a moment, then you have 4 unknowns and the system is statically indeterminate. If either is exerting a horizontal force, then I don't think you would even need a moment at A at all. If A (and ONLY A) is able to exert a moment on the system and neither support is giving a horizontal force, then I'm pretty sure your answer is correct, but that seems like a pretty big assumption for them to ask you to make.

Edit: Actually, now that I think about it, I'm pretty sure you can solve it so that A and B are only exerting upward forces, and there's no moment at all. I think either the answer is zero, or the system is statically indeterminate, or you have some very weird table legs that don't offer any upward force.

 

[joe465]

All the information im given is:

On reaching the next level the crates are placed on a framework for temporary
storage.
Calculate the moment of the force exerted by the crate around support A.

Joe

 

[thegreenlaser]

Are you sure there's no implied information, or something they said before about table legs and what support they offer? For example, in my statics class, the prof always drew the supports in a way that we would know exactly what support reactions existed at the support so that she wouldn't have to write it out in the question every time.

I've attached a free body diagram of my interpretation. Even my free body diagram makes some assumptions, and it's still indeterminate. You only have 2 useful equations, but you have three unknowns. The thing is, there's absolutely no reason, based on the information you've given, to assume that the moment at B is zero, so adding that diagram kills it even more. Adding horizontal forces to both legs doesn't really change anything, since they just cancel each other out. That system is indeterminate based on my FBD unless one of the forces/moments shown is zero. Since it seems more reasonable that a table leg would be providing an upward force as opposed to a moment and no upward force, it would make sense to me that the moment at A is zero. That's a pretty poor argument, though, even if it ends up being right.

 

[joe465]

I think the idea of the question is just to test the knowledge to calculate the moment of a force and pretending Support B isnt present in order to do this, i agree in a sense that you have to make presumtions which is disgusting

 

[thegreenlaser]

I think the idea of the question is just to test the knowledge to calculate the moment of a force and pretending Support B isnt present in order to do this, i agree in a sense that you have to make presumtions which is disgusting

If that's the case, I would argue against it. The assumption that the leg is providing no support is a pretty unreasonable one. If they want you to assume that, then they should have made some indication of that. Can you ask your prof if that assumption is supposed to be made?

 

[joe465]

Its a distance learning course which doesn't help, so getting a reply from a tutor takes ages, i asked a few quesitons last start of last week, still don't have a response as yet, maybes on holiday.

 

[thegreenlaser]

Oh wait... hold on. I just re-read the question. They want the moment that the crate exerts on point A, not the total moment reaction at A. Wow... I think I just epically failed there. Terribly sorry about that.

In that case, it wouldn't matter whether the moment created by the crate is canceled out by a force at B or a moment at A, it just matters what moment is created by the crate, in which case I don't know what's wrong with your answer. Sorry again for not reading the question properly...

 

[ehild]

Only the moment of the force exerted by the crate around the support A was the question, not the forces exerted by the supports. The crate exerts a normal force, which is equal to its weight and the line of this force passes through the CM of the crate. The mass of the crate is 200 kg, and it is also given that the crate is at 2 m distance from support A. You can ask, which part of the crate is there - the left side, the right side or the middle. If the middle, then it is all right.

ehild