THE TURNING EFFECT OF FORCES
3 marks · NormalA bridge is supported at both ends. A heavy truck stops near the left end of the bridge. Explain, using the principle of moments, how the upward forces at the left and right supports change as the truck moves from the left end to the centre of the bridge.
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The upward force at the left support increases when the truck is near the left end. As the truck moves toward the centre, the upward forces at both supports become more equal. This is because the moment produced by the weight of the truck about each support changes depending on its position: a load closer to one support increases the force required from that support to balance the moments.
Must show: upward force greatest under load, forces equal at centre
Common mistakes: Saying forces are always equal
Show mark scheme and worked solution
Marking points
M1 · 1 mark · Describes that the left support provides more upward force when the truck is near it.
M2 · 1 mark · States that as the truck moves to the centre, forces at both ends become equal.
A1 · 1 mark · Explains this is due to the principle of moments: the nearer support provides a greater moment to balance the truck’s weight.
Worked solution
When the truck is at the left end, the left support provides a larger upward force to balance the downward moment produced by the truck’s weight. As the truck moves to the centre, the distance to each support is equal, so both supports share the load evenly (forces equal). This follows from the principle of moments: the support nearer to the load produces a larger moment to balance the truck.