IGCSE Physics / The turning effect of forces
Forces on a beam supported at both ends
Edexcel International GCSE Physics (4PH1), chapter 5: The turning effect of forces, specification point 1.33P. Free lesson.
Read the revision notes · IGCSE Physics past papers
A light beam supported at both ends has two upward support forces that balance the load. You can describe how the forces change as the load moves and calculate them using moments.
How the supports share the load
A light beam is one whose own weight can be ignored. The upward forces from supports A and B always add up to the weight of the load. The support nearer the load pushes up harder, because it takes a greater share of the load. With the load in the middle, each support provides half its weight. As the load moves towards A, the force at A increases and the force at B decreases. With the load directly over A, A provides all the upward force and B provides none.
How the forces change as the load moves
For an 800 N person on a 4 m plank, the force at A falls steadily as the person walks from A to B. The force at B rises steadily. Their total stays at 800 N, so the support forces change in straight lines on a graph. At A, the force at A is 800 N and the force at B is 0 N. In the middle, each force is 400 N. At B, the force at A is 0 N and the force at B is 800 N. The support nearer the person provides the larger force.
How to calculate the support forces using moments
Take moments about one support. The force at that support acts at the pivot, so its distance is zero and it has no moment. The load's moment balances the moment from the other support. Measure every distance from the point you take moments about.
How to describe the forces as a load moves
When a load moves towards one support, that support's upward force rises and the other support's force falls. Their sum remains equal to the load's weight. If a painter walks from A towards B, the force at A decreases and the force at B increases. State that the two forces always add up to the painter's weight.