Physics · · 4 marks
A uniform beam of weight 240 N and length 3.0 m rests on two supports, A at the left-hand end and B 2.0 m from A. A load of 160 N is placed 2.5 m from A. Calculate the magnitude of the force exerted by support B on the beam. [4]
Mark scheme
- Weight of the beam acts at its centre, 1.5 m from A
- Taking moments about A: (240 × 1.5) + (160 × 2.5) = R(B) × 2.0
- 360 + 400 = 2.0 R(B)
- R(B) = 380 N
Take moments about A so the unknown force there drops out — choosing the pivot well turns two unknowns into one. "Uniform" tells you the weight acts at the centre.