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# Mechanics moments question watch

1. Can anyone help with question 8?

I am not sure where to start.

Thanks a lot.
Scan0008.pdf
2. (Original post by anonmechanics)
Can anyone help with question 8?

I am not sure where to start.

Thanks a lot.
Scan0008.pdf
draw a big diagram for before and after the movement of the weight, label all relevant distances. Label the mass*gravity of the AB beam anywhere (since it will act at the same point in the before and after diagram so it will "cancel out" when it comes to manipulating the equations.)

Hint: Take moments about D so you don't have to worry about the reaction at D
3. I drew a diagram and think the two equations would be: Before: (C*a)=(W*x) and after: (C*a)=(W*(b+x)) where x is the distance from D to W.
Is that right? if so I'm not sure where to go from there?
4. (Original post by anonmechanics)
I drew a diagram and think the two equations would be: Before: (C*a)=(W*x) and after: (C*a)=(W*(b+x)) where x is the distance from D to W.
Is that right? if so I'm not sure where to go from there?
very close, ive highlighted the part you should rethink, if you are letting the reaction at C equal 'C' before the movement of the mass then why do you think you cant let the reaction at C after the movement equal 'C' also?
5. (Original post by DylanJ42)
very close, ive highlighted the part you should rethink, if you are letting the reaction at C equal 'C' before the movement of the mass then why do you think you cant let the reaction at C after the movement equal 'C' also?
Oh because the reaction increases at C when the weight shifts.

So if the initial reaction at C is R1 and after the shift is R2 then:

(R1*a) = Wx so x = R1a/W

(R2*a) = W(b+x)

--------

R2*a = Wb + W(R1a/W) = Wb + R1a

Cancel a and you get R2 = Wb/a + R1.

And thats right I think?

Thanks very much - you helped a lot and explained well.
6. (Original post by anonmechanics)
Oh because the reaction increases at C when the weight shifts.

So if the initial reaction at C is R1 and after the shift is R2 then:

(R1*a) = Wx so x = R1a/W

(R2*a) = W(b+x)

--------

R2*a = Wb + W(R1a/W) = Wb + R1a

Cancel A and you get R2 = Wb/a + R1.

And thats right I think?

Thanks very much - you helped a lot and explained well.
perfect, well done thanks, glad I could help

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