# Reaction forces

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#1
Hi
I dont understand why in the following diagram there is no reaction at C and is There a reaction at A if they did not take moments about that point?
THanks
0
3 years ago
#2
(Original post by 111davey1)
Hi
I dont understand why in the following diagram there is no reaction at C and is There a reaction at A if they did not take moments about that point?
THanks
There is a reaction at C on the rod from the wall but usually you only show forces acting on one body in M1/M2 force diagrams. In this question you only care about forces acting on the beam because it is these forces that you will use to form a moments equation. The reaction at C is acting on the rod and not the horizontal beam so it is irrelevant for the working in this question.

This isn't to say that you couldn't show the forces acting on the rod instead and take moments for the rod, but it wouldn't be useful if you're trying to find the centre of mass of the beam. Plus you don't know the weight of the rod.

If you draw this diagram I would recommend including the reaction forces acting on the beam at A (this would be a horizontal and vertical reaction force at A since it's hinged). Solutions won't always show these forces unless they're involved in the working but you should always have them in your diagram.
0
#3
(Original post by notnek)
There is a reaction at C on the rod from the wall but usually you only show forces acting on one body in M1/M2 force diagrams. In this question you only care about forces acting on the beam because it is these forces that you will use to form a moments equation. The reaction at C is acting on the rod and not the horizontal beam so it is irrelevant for the working in this question.

This isn't to say that you couldn't show the forces acting on the rod instead and take moments for the rod, but it wouldn't be useful if you're trying to find the centre of mass of the beam.

If you draw this diagram I would recommend including the reaction forces acting on the beam at A (this would be a horizontal and vertical reaction force at A since it's hinged). Solutions won't always show these forces unless they're involved in the working but you should always have them in your diagram.
Thanks I understand that at hinges there will be horizontal and vertical resistance to motion so the reaction can be considered as vertical and horizontal components and that if something is resting against a surface there will be a normal reaction force. But For the reaction at C i don't really understand how would you draw the reaction if you wanted to take moments? Would you not need the mass of the rod?
0
3 years ago
#4
(Original post by 111davey1)
Thanks I understand that at hinges there will be horizontal and vertical resistance to motion so the reaction can be considered as vertical and horizontal components and that if something is resting against a surface there will be a normal reaction force. But For the reaction at C i don't really understand how would you draw the reaction if you wanted to take moments?
You're not told in the question how the rod CD is connected to the wall at C so you can't really say what the reaction looks like. If it is freely hinged then there would be a vertical and horizontal reaction force.

But as I said in my last post, the force on the rod at A is irrelevant for this question so you don't need to think about what it looks like or draw it on your diagram. Do you understand this?
0
3 years ago
#5
(Original post by 111davey1)
But For the reaction at C i don't really understand how would you draw the reaction if you wanted to take moments? Would you not need the mass of the rod?
Yes you would need the mass of the rod if you were to take moments for the rod. So in this question you ignore the forces on the rod and only consider forces on the beam.
0
#6
(Original post by notnek)
Yes you would need the mass of the rod if you were to take moments for the rod. So in this question you ignore the forces on the rod and only consider forces on the beam.
Thanks so for M2 you just need to consider forces acting on the body in question
0
3 years ago
#7
(Original post by 111davey1)
Thanks so for M2 you just need to consider forces acting on the body in question
Yes normally you only consider forces acting on one body and it's often obvious in the question which body that should be. Sometimes you might need to consider two different bodies but for that I'd recommend drawing separate force diagrams.
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