O is the centre of an equilateral triangle ABC.F1,F2 and F3 are three forces acting along the sides AB, BC and AC as shown in the figure. What should be the magnitude of F3, so that the total torque about O is zero ?
A
F1+F2
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B
F1−F2
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C
(F1+F2)2
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D
(F1−F2)2
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Solution
The correct option is AF1+F2 Net torque about point O is,
τ=τ1+τ2+τ3
Assuming clockwise torque is positive.
∴τ=−F1d−F2d+F3d
For net torque to be zero,
F3d=F1d+F2d
∴F3=F1+F2
Hence, (A) is the correct answer.
Why this question?
When an object is in rotational equilibrium, make sure that net torque on the object is zero.