Question

Assertion: Internal forces cannot change linear momentum.
Reason: Internal forces can change the kinetic energy of a system
A. Both assertion and reason are correct and reason is the correct explanation for assertion
B. Both assertion and reason are correct but reason is not the correct explanation for assertion
C. Assertion is correct but reason is incorrect
D. Assertion is incorrect but reason is correct

Answer 1

Hint: Firstly, we know that the rate of change of linear momentum is equal to the external force. We also know that internal force doesn’t depend on the linear momentum. Total energy remains conserved so internal energy is used to convert potential energy to kinetic energy. So, we can say that reason is not the correct explanation for the assertion.

Complete step by step solution:
Internal forces will be pairwise. They will not change the external force acting on an object. Momentum will be conserved.
We know that ${F_{external}} = \dfrac{{dP}}{{dt}}$ here, ${F_{external}} = $ external force applied to a system and we also know that rate of change of linear momentum equals to the external force.
So internal force doesn’t even change the velocity of the centre of mass which is given by the net external force acting on the system.
So only external forces can change the linear momentum not the internal force.
Moreover, internal forces can never ever change the total mechanical energy (kinetic energy potential energy) of an object. But they can only just convert a part of kinetic energy to potential energy. So they can change the kinetic energy but total energy remains conserved.
But when internal energy changes kinetic energy can change. If an object at rest can explode into multiple fragments and KE changes.
Hence the required solution is both assertion and reason are correct but reason is not the correct explanation for assertion (Option- “B”)
So, we can say that internal forces cannot change linear momentum that doesn’t mean that internal forces can change the kinetic energy of a system.

Hence, option “B” is correct.

Additional Information: Internal forces are forces exchanged by the objects in the system. And external force means applied force.

Note:
Internal forces occur pairwise, sum of all internal forces =0. These cannot change the momentum of the system.
When the only forces doing work are internal forces, energy changes forms from kinetic to potential (or vice versa). So, one may think why the option “D” is not correct because the reason is not the correct explanation for the assertion.