Consider the following statements:
- There is no net moment on a body which is in equilibrium.
- The momentum of a body is always conserved.
- The kinetic energy of an object is always conserved.
Which of the statements given above is/are correct?
1, 2 and 3
2 and 3 only
1 and 2 only
1 only
Answer is Right!
Answer is Wrong!
This question was previously asked in
UPSC CDS-1 – 2018
1. There is no net moment on a body which is in equilibrium. This is correct. A body is in rotational equilibrium if the net torque (moment) acting on it is zero. For complete equilibrium (translational and rotational), both the net force and net moment must be zero.
2. The momentum of a body is always conserved. This is incorrect. The momentum of a body is conserved only if the net external force acting on it is zero. If a net force acts on a body, its momentum changes according to Newton’s second law (F = dp/dt).
3. The kinetic energy of an object is always conserved. This is incorrect. Kinetic energy is conserved only in specific situations, such as elastic collisions where no energy is lost as heat, sound, or deformation, and when no non-conservative forces (like friction) do work on the object, and no external forces change its speed. For example, when an object falls under gravity, its kinetic energy increases.
Only statement 1 is correct.
– Momentum of a system is conserved in the absence of external forces. Momentum of a single body is conserved only if the net force on it is zero.
– Kinetic energy of an object is conserved only if no net work is done on it by non-conservative forces or if the system is isolated and interactions are perfectly elastic.