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A bridge is 50m long, has a mass of 20,000kg, and rests on two pivots, A and B. The distance between A and the left side of the bridge is 10m. The distance between B and the right side of the bridge is 10m.

A lorry drives across the bridge.

i) What is the maximum possible mass of the lorry?

I worked this answer out to be 30,000kg. (10 × M = 20,000 × 15)

But I'm struggling with the second part of the question:

ii) Derive an expression for the force acting upon A and the force acting upon B as a function of x (x = the distance of the lorry from one side of the bridge).

An explanation of how you reached the answer, or some big (:P) hints on how to reach it would be much appreciated.

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I'm not asking for an answer, by the way. – Johann Moherty Mar 13 '12 at 9:50
I updated my answer, I answered the wrong part of the question. – Manishearth♦ Mar 13 '12 at 10:01
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Welcome to Physics! This is a site for conceptual questions about physics, not general homework help. If you can edit your question to ask about the specific physics concept that is giving you trouble, I'll be happy to reopen it. See our FAQ and homework policy for more information. – David Zaslavsky♦ Mar 13 '12 at 17:33

closed as too localized by David Zaslavsky♦ Mar 13 '12 at 17:32

This question is unlikely to help any future visitors; it is only relevant to a small geographic area, a specific moment in time, or an extraordinarily narrow situation that is not generally applicable to the worldwide audience of the internet. For help making this question more broadly applicable, see the FAQ.

1 Answer

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Conserve torque (principle of moments) about A when the truck is at the extreme end which is closer to A.

Oh sorry, I answered part 1 instead of part 2.

Assume the forces at A and B to be some variables. Net force on system must be 0, net torque on system must be zero. Two equations, two unknowns. Done.

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