Doubt in Rigid body Dynamics

#1

If I find the net torque about B and equate it with zero i get length to be equal to 1/2m but the answer is 1m.
However if find net torque about com then I get the correct answer 1m. Where did I go wrong?
If the rod is not rotating shouldn't the torque be zero at all points???

#2

No. If a body is in complete equilibrium, then and only then can you apply torque abput any point to 0.
But, if a body is only in rotational equilibrium, you may equate torque only about the com to 0.

Why?

We write angular momentum of a body about a given point O as

L vector = M(Ro/cm X Vo/cm) + I(com)omega,
All vector quantities.

When you differentiate this vector, if the first term is not equal to 0, dL/dt = torque, is not equal to 0.

1 Like
#3

Thanks