# M1 Moments question

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#1
A uniform rod AB has length 5m and weight 20 N
The rod is resting on supports at point C and D where AC =2m and BD=1m

a) find the magnitudes of the reactions at C and D

I've done this part.

A particle of weight 12 N is placed on the rod at point A

b) show that this causes the rod to tilt about C

The answer given takes the moment about C and finds that Rd is negative which suggests an anti clockwise moment at C. I don't get how a reaction can be negative, plus how does the negative reaction show an anti clockwise moment about C??

A second particle of weight 12N is placed on the rod at E to hold it in equilibrium
c) how far must E be from A?

I get how to approach this but how do I know approximately where to place E on my diagram without finding the exact answer?
1
2 years ago
#2
(Original post by dont know it)
A uniform rod AB has length 5m and weight 20 N
The rod is resting on supports at point C and D where AC =2m and BD=1m

a) find the magnitudes of the reactions at C and D

I've done this part.

A particle of weight 12 N is placed on the rod at point A

b) show that this causes the rod to tilt about C

The answer given takes the moment about C and finds that Rd is negative which suggests an anti clockwise moment at C. I don't get how a reaction can be negative, plus how does the negative reaction show an anti clockwise moment about C??

A second particle of weight 12N is placed on the rod at E to hold it in equilibrium
c) how far must E be from A?

I get how to approach this but how do I know approximately where to place E on my diagram without finding the exact answer?
Things are negative if they point in the opposite direction. Generally a reaction force holds things in equilibrium, so it would have to be pointing downwards to maintain equilibrium. If not, it will rotate?
Is there a figure which shows the ABCD labelling. I'm presuming A is on the left? If so, the anticlockwise rotation should be fairly clear?
1
#3
(Original post by mqb2766)
Things are negative if they point in the opposite direction. Generally a reaction force holds things in equilibrium, so it would have to be pointing downwards to maintain equilibrium. If not, it will rotate?
Is there a figure which shows the ABCD labelling. I'm presuming A is on the left? If so, the anticlockwise rotation should be fairly clear?
Yeah you're right. Thanks. Could you help me with part c) please.

https://activeteach-prod.resource.pe...b_sm2_ex4e.pdf

The diagram's on the link above(q4)
0
2 years ago
#4
(Original post by dont know it)
Yeah you're right. Thanks. Could you help me with part c) please.

https://activeteach-prod.resource.pe...b_sm2_ex4e.pdf

The diagram's on the link above(q4)
A key part is to set the stuff to 0, i.e. work out where things are which would create a 0 reaction force. That will be the boundary. So they evaluate where it would be if the reaction force at D and C is zero. Then reason about which side of these values would produce a positive reasction force and hence equilibrium.
Is that ok? All the maths is in the figure.
0
#5
(Original post by mqb2766)
A key part is to set the stuff to 0, i.e. work out where things are which would create a 0 reaction force. That will be the boundary. So they evaluate where it would be if the reaction force at D and C is zero. Then reason about which side of these values would produce a positive reasction force and hence equilibrium.
Is that ok? All the maths is in the figure.
I think I get what you're saying but I don't get how you'd know where to place E on the diagram without knowing the answer. So for example in the diagram they gave, they put point E before the pivot D. Is there a way to know before hand you'd place it there or do I just place it anywhere e.g. after the pivot D since the answer will turn out the same.
0
2 years ago
#6
(Original post by dont know it)
I think I get what you're saying but I don't get how you'd know where to place E on the diagram without knowing the answer. So for example in the diagram they gave, they put point E before the pivot D. Is there a way to know before hand you'd place it there or do I just place it anywhere e.g. after the pivot D since the answer will turn out the same.
Will have a look more carefully later if noone else has ... got a few jobs to do :-)
0
2 years ago
#7
(Original post by dont know it)
I think I get what you're saying but I don't get how you'd know where to place E on the diagram without knowing the answer. So for example in the diagram they gave, they put point E before the pivot D. Is there a way to know before hand you'd place it there or do I just place it anywhere e.g. after the pivot D since the answer will turn out the same.
In the first part, they want to determine where R_D is zero.

You've already determined that without this weight, the rod would rotate anticlockwise about C, so it must occur to the right of C to provide a clockwise (about C) moment. Forget about where D is (rub it out of the diagram), it is taken out of the equation because you want R_D to be zero. Hence all you need to do is draw it to the right of C and work out the unknown distance.

Then you consider the case that you've put it too far to the right (past D) and the reaction forces switch, i.e. clockwise rotate about D and R_C = 0. It must be to the right of D otherwise there is no way the R_C could be zero. Hence just set up the distance to the right as the unknown and solve the equation.

As before, the numbers are in the model solution, but do you understand the scenario?
1
2 years ago
#8
but why are you making reaction force of D zero then ?
0
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