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# Moments M1 <-- Very Annoying Question. watch

1. A plank of wood AB has ass 12kg and length 5m. It rests in a horizontal position on two smooth supports. One support is at the end A. The other is at the point C, 0.5m from B. A girl of mass 30kg stands at B with the plank in equilibrium.

a) By modelling the plank as a uniform rod and the girl as a particle, find the reaction on the plank at A.

R = 19.6N

The girl gets off the plank. A boulder of mass mkg is placed on the plank at A and a man of mass 93kg stands on the plank at B. The plank remains in equilibrium and is on the point of tilting about C.

b) By modelling the plank again as a uniform rod, and the man and the boulder as particles, find the value of m.

Reaction at C = 392N
Taking anticlockwise moments about B: -1/2R_C + (2.5 x 12g) + 49m
1/2R_C = 294 + 49m
R_C = 392,
so 196 = 294 + 49m
m = -2?

the books answer: m = 5

can somebody PLEASE tell me where i've gone wrong? its part b) where im stuck. the whole tilting at support C has me confused. it means that the reaction at A is 0, but i wasnt sure what to equate the moment about B to, so i just equated it to 0 because i saw the word equilibrium.

im desperate. +ve rep up for grab

thank you all!
2. b)

4.5m+2x12=0.5x93
4.5m=22.5
m=5kg

I assume you got the right answer for part a?

Let me know if you want me to explain more.
3. wow u made it look so simple lol..

do u know where i went wrong? im still going through my working trying to find out how i ended up with the wrong answer. it doesnt matter where u take moments right?
4. (Original post by dumb maths student)
A plank of wood AB has ass 12kg
5. hahaha i didnt even notice that :P
6. (Original post by CHEM1STRY)
dearey me lol :P
7. (Original post by CHEM1STRY)

lol same, and I noticed it's you again!!!
8. (Original post by dumb maths student)
A plank of wood AB has ass 12kg and length 5m. It rests in a horizontal position on two smooth supports. One support is at the end A. The other is at the point C, 0.5m from B. A girl of mass 30kg stands at B with the plank in equilibrium.

a) By modelling the plank as a uniform rod and the girl as a particle, find the reaction on the plank at A.

R = 19.6N

The girl gets off the plank. A boulder of mass mkg is placed on the plank at A and a man of mass 93kg stands on the plank at B. The plank remains in equilibrium and is on the point of tilting about C.

b) By modelling the plank again as a uniform rod, and the man and the boulder as particles, find the value of m.

Reaction at C = 392N
Taking anticlockwise moments about B: -1/2R_C + (2.5 x 12g) + 49m
1/2R_C = 294 + 49m
R_C = 392,
so 196 = 294 + 49m
m = -2?

the books answer: m = 5

can somebody PLEASE tell me where i've gone wrong? its part b) where im stuck. the whole tilting at support C has me confused. it means that the reaction at A is 0, but i wasnt sure what to equate the moment about B to, so i just equated it to 0 because i saw the word equilibrium.

im desperate. +ve rep up for grab

thank you all!
First of all, I'm not sure what you have done. How did you get C to be 392? How'd you get -1? What does R_C mean? What does -1/2R_C mean?

I did it, and I got M to be 5. I took moments about C though. I dont know why you are taking moments about B, you're just making it harder for urself. If you take moments about a point where there is an unknown, then it will rule it out.

Take it at point C and see what you get.

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