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M1 Moments hard question Watch

1. Last question of the exercise has completely baffled me. Could someone help me and tell me what is going on here. How does (Rd) being negative show that the rod will tilt? Furthermore I thought that Rd should be 0 N in this case because the rod is about to tilt about point C?
2. (Original post by scientific222)

Last question of the exercise has completely baffled me. Could someone help me and tell me what is going on here. How does (Rd) being negative show that the rod will tilt? Furthermore I thought that Rd should be 0 N in this case because the rod is about to tilt about point C?
if thats off the edexcel solution bank some of the solutions have errors
3. (Original post by scientific222)

Last question of the exercise has completely baffled me. Could someone help me and tell me what is going on here. How does (Rd) being negative show that the rod will tilt? Furthermore I thought that Rd should be 0 N in this case because the rod is about to tilt about point C?
Its the fact that RD cannot be negative that demonstrates it is about to tilt, as the system cannot be in equilibrium. It can't be negative as it points upwards (in the positive direction) in the diagram.
4. (Original post by upthegunners)
if thats off the edexcel solution bank some of the solutions have errors
There's no error here
5. (Original post by Indeterminate)
There's no error here
I was just making sure the OP knows that I wasn't in the position to say if there was or wasn't an error as I am only an Alevel student myself
6. (Original post by joostan)
Its the fact that RD cannot be negative that demonstrates it is about to tilt, as the system cannot be in equilibrium. It can't be negative as it points upwards (in the positive direction) in the diagram.
Thanks; and sorry to bother you for the third (hopefully final) time but I am also stuck on part c). Similar to the first question I asked (how would I know where the force E is meant to be acting down on the rod). You mentioned that there must be a force acting on either side of the support; in this case I took moments about C; how would I know that E sits in between C and D? I initially thought that E would sit after between D and B but was wrong.

7. (Original post by scientific222)
Thanks; and sorry to bother you for the third (hopefully final) time but I am also stuck on part c). Similar to the first question I asked (how would I know where the force E is meant to be acting down on the rod). You mentioned that there must be a force acting on either side of the support; in this case I took moments about C; how would I know that E sits in between C and D? I initially thought that E would sit after between D and B but was wrong.

This question is slightly different.
You don't need to know where the force acts, other than it is to the right of C, in order to counterbalance the force at A
8. (Original post by joostan)
This question is slightly different.
You don't need to know where the force acts, other than it is to the right of C, in order to counterbalance the force at A
Also , does "equilibrium" mean the same as "about to tilt". The question says the weight holds it in equilibrium, but the answer scheme says that it is about to tilt at C and so Rd = 0 N
9. (Original post by scientific222)
Also , does "equilibrium" mean the same as "about to tilt". The question says the weight holds it in equilibrium, but the answer scheme says that it is about to tilt at C and so Rd = 0 N
Equilibrium means that it is not tilting at all - i.e. the forces are in complete balance. Anticlockwise moments = clockwise moments etc.
10. (Original post by joostan)
Equilibrium means that it is not tilting at all - i.e. the forces are in complete balance. Anticlockwise moments = clockwise moments etc.
Is that an error on Edexcels part in the answer? The question says it is held in equilibrium yet the mark scheme says "if the system is just about to tilt about C". In equilibrium I would be considering Rd but it is ignored.
11. The solution uses the method for calculating equilibrium, so I think the idea is to show that equilibrium cannot be calculated in that situation. I did it like this (although my numbers seem a bit different?):

Posted from TSR Mobile
12. (Original post by scientific222)
Is that an error on Edexcels part in the answer? The question says it is held in equilibrium yet the mark scheme says "if the system is just about to tilt about C". In equilibrium I would be considering Rd but it is ignored.
Picture it in your mind, if it is just about to tilt, but not actually tilting about C what would be happening at D?
It would be just about to lift off of D, but not quite, yes?
This means that the force supporting at D is no longer applied as the force to support the beam is no longer required.
13. (Original post by stuart_aitken)
The solution uses the method for calculating equilibrium, so I think the idea is to show that equilibrium cannot be calculated in that situation. I did it like this (although my numbers seem a bit different?):

Posted from TSR Mobile
Except I don't think you can use the value of Rd =5N from the previous situation into this situation. Since the 12N has been added I think it affects the value of Rd
14. (Original post by joostan)
Picture it in your mind, if it is just about to tilt, but not actually tilting about C what would be happening at D?
It would be just about to lift off of D, but not quite, yes?
This means that the force supporting at D is no longer applied as the force to support the beam is no longer required.
I understand how Rd would be 0 in the situation that C is just about to tilt; but its not, the question part c states it is held in equilibrium. As you said before, equilibrium means it is not tilting at all, so how did they bring that situation into the solution
15. (Original post by scientific222)
I understand how Rd would be 0 in the situation that C is just about to tilt; but its not, the question part c states it is held in equilibrium. As you said before, equilibrium means it is not tilting at all, so how did they bring that situation into the solution
Are you familiar with
?
You see that limiting friction means it is just about to slide, but is not yet sliding, here the principle is the same with the tilt.
16. (Original post by scientific222)
Except I don't think you can use the value of Rd =5N from the previous situation into this situation. Since the 12N has been added I think it affects the value of Rd
Actually yeah you're right. I totally convoluted it, haha.

Just think about moments around C when the weight is applied.

MomentC=(12x2)-(20x0.5)=14Nm

Therefore resultant moment!

And yeah, the mention of 'negative RD' in the solution is really daft! Pointless empty nonsensical figure!

Posted from TSR Mobile
17. (Original post by joostan)
Are you familiar with
?
You see that limiting friction means it is just about to slide, but is not yet sliding, here the principle is the same with the tilt.
Ah that made it a lot clearer thanks
18. (Original post by scientific222)
Ah that made it a lot clearer thanks
NP

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