# Moments and couples

1. Oct 21, 2016

### ual8658

• Member advised to use the homework template for posts in the homework sections of PF.

I know how to calculate the resultant moments and forces but in the solution, it says to turn into a force couple system at point G, and then solve for where the resultant should be on the line FG, and then GH.

My question is, what is the question even saying in the first place when it states that the rivet cannot withstand a couple. Isn't a couple a pair of parallel forces that produces motion? And as a result I don't understand what the solution means.

2. Oct 21, 2016

### Simon Bridge

3. Oct 22, 2016

### ual8658

I get that but when the problem says the rivet can't stand a couple, what does that mean? Does it mean that it can't stand rotation?

4. Oct 22, 2016

### Simon Bridge

I just told you - and you said "I get that"... how about applying scientific method to decipher the problem?
What are the possibilies? How could you test them?

5. Oct 22, 2016

### CWatters

Perhaps think about how rivets work (I mean one on its own).

6. Oct 22, 2016

### ual8658

Ok so the rivet will prevent translational motion but not rotational. But does this mean any rotation at all whether the rotational axis is on the rivet or not? And thus after obtaining the force couple system at g, you move the resultant in such a way that it cancels out the moment at g?

7. Oct 23, 2016

### Simon Bridge

Have you ever seen a rivet before?
Do you know how they work?

8. Oct 23, 2016

### ual8658

Honestly no. This problem has confused me a lot. What I've interpreted so far is the resultant force must act on the rivet because it will be the only thing preventing motion? And since it can't withstand a couple, it basically cannot withstand rotation?

9. Oct 25, 2016

### CWatters

Correct.

_One_ rivet will act like a single bolt. It can clamp two plates together so there could be some friction resisting rotation but a line of rivets would be much more effective in resisting rotation. If there was a line of rivets all but one would have to shear for there to be rotation.

10. Oct 25, 2016

Thank you!