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IGCSE Moments

Could you explain why we use the principle of moment formula for part b of this question to calculate the weight of the workman?

I thought we only had to use that formula when an object is in equilibrium. But in this case, the floor board has tipped so it is not at equilibrium?

https://imgur.com/vy9jhDQ

(edited 11 months ago)

Reply 1

Joinedup

I think the question really wants to know what the minimum weight for the workman that causes the contact force at Q R and S to be zero.

I think the diagram is a bit exaggerated because it wants you to realise that the fulcrum is going to be the corner of the joist P rather than the centre of the joist.

Reply 2

S0303

Original post by Joinedup

I think the question really wants to know what the minimum weight for the workman that causes the contact force at Q R and S to be zero.
I think the diagram is a bit exaggerated because it wants you to realise that the fulcrum is going to be the corner of the joist P rather than the centre of the joist.

I'm not sure if I've got it just yet..
Are you saying that in actuality because the board will just be slightly pivoted we can use the principle of moments?
But how can we call this a system in equilibrium if there is even a slight movement of the board? One moment has to be greater than the other in order for the board to move?

Reply 3

Joinedup

Maybe he waved his arms about like a tight rope walker or slightly shifted his centre of mass when he felt the board begin to move and the board is static at the instant the picture was taken?

I dont think there's any other way of tackling the question with gcse level concepts and the question doesn't have enough information for anything other than statics.