Question

Two men support a uniform horizontal beam at its two ends. If one of them suddenly lets go, the force exerted by the beam on the other man will 

(a) remain unaffected 

(b) increase

(c) decrease 

(d) become unequal to the force exerted by him on the beam

Answer 1

Correct Answer is: (c) decrease

When the beam is supported at A and B, the force exerted by each man = mg/ 2.

When the support at B is withdrawn, taking torque about A,

τ = (mg) l/2 = Iα = (ml²/3) α

or α = 3g/2l

The instantaneous linear acceleration of the centre of mass is

a_CM = (α) = 3g/ 2l) l/ 2 = 3g / 4.

Let N = force exerted on the beam at A.

∴ mg - N = ma_CM = m(3g/4) or N = 1/4 mg.