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MEI Mechanics M2 Moments help

This question is from exercise 3a in the MEI Mechanics M2 textbook. It is question ten.

A simple suspension bridge across a narrow bridge consists of a uniform beam, 4m long and of mass 60kg, supported by vertical cables attached at a distance 0.75m from each end of the beam.

(i) Find the tension in each cable when a boy of mass 50kg stands 1m from the end of the bridge

Answer = 35g N and 75g N

(ii) Can a couple walking hand in hand cross the bridge safely, without it tipping, if their combined mass is 115kg?

I'm not sure what I'm doing with part two. Any suggestions? It has something to do with seeing if the bridge remains in equilibrium right, so result moment and force = 0. The answer is no, so I'm assuming the total moment is found to be a non-zero result. I just don't know how to get to it.

Reply 1

ghostwalker

Original post by jglfc

...

Diagram, if you haven't already done one.

Consider where the pivot is if the bridge were to tip.

Then where would the couple need to stand to generate the maximum moment to aid the tipping?

Then take moments about the pivot.

What is the tension in the other cable? If it needs to be <0 the bridge will tip.

Reply 2

IceKidd

If tension in other cable needs to be <0 the bridge will tip because the 'cable' would be slack and unable to provide any support, just thought id mention that so you understood why. it would actually need to exert a thrust which cables cant, but ropes can

Reply 3

ghostwalker

Original post by IceKidd

it would actually need to exert a thrust which cables cant, but ropes can

In bold: Not to the best of my knowledge, except perhaps in the Indian Rope Trick.

Reply 4

IceKidd

Original post by ghostwalker

In bold: Not to the best of my knowledge, except perhaps in the Indian Rope Trick.

Lol i got it the wrong way round, cables can but ropes cant

OP just ignore me. i shouldnt be allowed to post on here

Reply 5

ghostwalker

Original post by IceKidd

Lol i got it the wrong way round, cables can but ropes cant

OP just ignore me. i shouldnt be allowed to post on here

Sorry, but cables wouldn't normally support thrust either, you need something rigid such as a spar, or a girder, or a beam, ....

Reply 6

IceKidd

Original post by ghostwalker

Sorry, but cables wouldn't normally support thrust either, you need something rigid such as a spar, or a girder, or a beam, ....

really. In my board MEI we are taught cables can exert thrust and tension and that ropes can only exert a tension. Cables are assumed rigid for us... (not like a wire cable, but a strong stiff cable ygm?)