Rotational Motion MCQs for NEET — Physics Questions with Answers

As per the NCERT text: '(2) The total torque, i.e. the vector sum of the torques on the rigid body is zero, $\sum \vec{\tau_i} = 0$.' This is the condition for rotational equilibrium. If the total torque on the rigid body is zero, the total angular momentum of the body does not change with time.

The NCERT text states: 'In a number of problems all the forces acting on the body are coplanar. Then we need only three conditions to be satisfied for mechanical equilibrium. Two of these conditions correspond to translational equilibrium... The third condition corresponds to rotational equilibrium.'

The text clearly states: 'Since consideration of rotational motion does not apply to a particle, only the conditions for translational equilibrium (Eq. 6.30 a) apply to a particle. Thus, for equilibrium of a particle the vector sum of all the forces on it must be zero.'

The NCERT text mentions: 'Thus, for equilibrium of a particle the vector sum of all the forces on it must be zero. Since all these forces act on the single particle, they must be concurrent. Equilibrium under concurrent forces was discussed in the earlier chapters.'

The text defines partial equilibrium: 'A body may be in partial equilibrium, i.e., it may be in translational equilibrium and not in rotational equilibrium, or it may be in rotational equilibrium and not in translational equilibrium.'

Referring to Fig. 6.20(a) and its description: 'The moment of the forces at A and B will both be equal in magnitude (aF), but opposite in sense...The net moment on the rod will be zero. The system will be in rotational equilibrium, but it will not be in translational equilibrium; $\sum F \neq 0$.'

The NCERT text states: 'If the total force on the body is zero, then the total linear momentum of the body does not change with time. Eq. (6.30a) gives the condition for the translational equilibrium of the body.'

The text explains: 'One can show that if the translational equilibrium condition [Eq. 6.30(a)] holds for a rigid body, then such a shift of origin does not matter, i.e. the rotational equilibrium condition is independent of the location of the origin about which the torques are taken.'

The NCERT text clarifies: 'Eq. (6.30a) and Eq. (6.30b), both, are vector equations. They are equivalent to three scalar equations each. Eq. (6.30a) corresponds to $\sum F_{ix} = 0$, $\sum F_{iy} = 0$ and $\sum F_{iz} = 0$. Similarly, Eq. (6.30b) is equivalent to three scalar equations $\sum \tau_{ix} = 0$, $\sum \tau_{iy} = 0$ and $\sum \tau_{iz} = 0$.'

The text states: 'The total torque on the body may not vanish. Such a torque changes the rotational state of motion of the rigid body, i.e. it changes the total angular momentum of the body in accordance with Eq. (6.28 b).'