Moments and couples

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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.

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Simon Bridge
Homework Helper
A couple is a pair of anti-parallel forces producing a rotation, but not translation.
https://en.wikipedia.org/wiki/Couple_(mechanics)#Forces_and_couples

It means it prevents translation but not rotation.
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?

Simon Bridge
Homework Helper
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?

CWatters
Homework Helper
Gold Member
Perhaps think about how rivets work (I mean one on its own).

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?
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?

Simon Bridge
Homework Helper
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?
Have you ever seen a rivet before?
Do you know how they work?

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

CWatters
Homework Helper
Gold Member
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.

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.
Thank you!