# Can anyone explain why there is a force constraint in the z direction

1. Sep 16, 2012

### theBEAST

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

3. The attempt at a solution
Can anyone explain why there is a force constraint in the z direction. The pin force only affects the x and y...

Also shouldn't there be a moment in the z since the pin prevents the block from rotating cw and ccw in the xy plane?

Last edited: Sep 16, 2012
2. Sep 16, 2012

### Simon Bridge

Re: Constraints

Can the force P have a component in the z direction?

Hmmm ... they are kinda implying that P does not turn the block aren't they... and it is drawn off-axis.

I'd ask about it - possibly you were supposed to figure either that p acts through the center of mass via the pin perhaps but still.

3. Sep 16, 2012

### theBEAST

Re: Constraints

I don't see why not... The pin only prevents movement in the x and y. Since we are just looking at the pin with force P and the block I can see myself pushing the block in and out of the page.

4. Sep 16, 2012

### Simon Bridge

Re: Constraints

You don't think it is implicit in the diagram?

5. Sep 16, 2012

### theBEAST

Re: Constraints

Unfortunately I am not sure how to see it in the diagram :S

6. Sep 17, 2012

### Simon Bridge

Re: Constraints

The problem seems to be under-specified as it is written. I think your were expected to "get it" from the context. That happens a lot. You learn to use your judgement.

7. Sep 17, 2012

### theBEAST

Re: Constraints

Could you please explain how you interpreted it? What is the reasoning behind the constraint in the z direction.

8. Sep 17, 2012

### Simon Bridge

Re: Constraints

I don't think I did it by analysis ... but from the experience of having seen many such problems before.

The way to think about it is to reconstruct the question as an answer for each of the possible constraints. eg. for A.

A force in x is present between the block and the rod with the force P.

Now consider if that is true or false ...
The trouble is that I'd have got it right before you started asking questons, which suggests that I'm doing it from experience rather than evidence.

The answers seem to be saying that there is no constraining force in z, and no constraining moment in z. Which is OK for the force but clearly silly for the moment since the block cannot rotate about the z axis (there's a ramp in the way.)

That would be your objection right?

OTOH: what the diagram says to me is that there can be no net force in the z direction and no net moment about z. There can be net force/moments for the other two axis because of the slope... for instance, if the block can, in principle, rotate about the normal (to the slope) axis ... this would give it moments about x and z, but not z.

A "constraint" in this question does not stop motion, just limits it.

So, if I have this right, the question asked is pretty much the opposite of what you thought it meant.

9. Sep 18, 2012

### theBEAST

Re: Constraints

xD thanks for your help, I understand it a lot better now!

10. Sep 18, 2012

### Simon Bridge

Re: Constraints

I should have mentioned - the way it is drawn, it is implicit that P has no z component - so net translational force is zero.

Cheers.