# Solve Forces & Couples at Point G - Moment & Resultant Calculations

• ual8658
In summary, the problem is saying that the resultant force cannot withstand a couple, which means it cannot withstand rotation.
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.

PhanthomJay
Simon Bridge said:
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?

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?

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

Simon Bridge said:
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?

ual8658 said:
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?

Simon Bridge said:
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?

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.

CWatters said:
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!

## 1. What is the difference between a force and a couple?

A force is a push or pull that acts on an object, while a couple is a pair of forces that act in opposite directions on an object, causing it to rotate.

## 2. How do you calculate the resultant force at Point G?

The resultant force at Point G can be calculated by finding the vector sum of all the forces acting on the object at that point. This can be done using trigonometry and vector addition.

## 3. What is the moment of a force?

The moment of a force is the turning effect of that force around a pivot point. It is calculated by multiplying the magnitude of the force by the perpendicular distance from the pivot point to the line of action of the force.

## 4. How do you find the moment of a couple?

The moment of a couple can be found by multiplying one of the forces by the perpendicular distance between them. This distance is known as the arm of the couple.

## 5. Can you solve forces and couples at Point G if the object is in motion?

Yes, the principles of force and moment calculations at Point G still apply even if the object is in motion. Additional factors such as velocity and acceleration may need to be considered, but the basic principles remain the same.

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