Question
Question: Two men support a uniform horizontal beam at its two ends. If one of them suddenly lets go, the forc...
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
Decrease.
Explanation
Solution
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When the support at B is withdrawn, taking torque about A,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
aCM = (α)(AC) = (3g/2l) l/2 = 3g/4.
Let N = force exerted on the beam at A.
∴ mg – N = maCM = m(3g/4) or N = 1/4 mg.