Question

Internal force can change:
A. linear momentum as well as kinetic energy
B. linear momentum but not the kinetic energy
C. kinetic energy but not the linear momentum
D. neither the linear momentum nor the kinetic energy

Answer 1

Hint:To solve this question think about some common examples which have internal forces. For example, friction between the surfaces of two bodies, tension in a rope and restoring force of a spring. Observe that the net force due internal forces is always zero. Then apply the law of conservation of linear momentum.

Complete answer:
Internal forces are the forces that are exerted within or inside a system of particles. The common examples of the internal forces are friction between the surfaces of two bodies, tension in a rope and restoring force of a spring. In the above three examples we can observe that internal forces always act in pairs. When there is a relative motion between the surfaces of two bodies, there is force of friction acting on both the bodies. The two frictional forces are equal in magnitude but opposite indirection. The tension force in a rope or the spring force pulls or pushes from the opposite direction with equal magnitudes.

Now, we can say that the internal forces act in pairs with equal magnitudes but opposite in direction. This means that the two forces will cancel out each other and the resultant will be equal to zero. Therefore, the net or resultant force of all the internal forces is equal to zero.

From the law of conservation of linear momentum, we know that when net force on a system of particles is equal to zero, the linear momentum of the system is conserved.Meaning, the net linear momentum of the system remains constant. Therefore, internal forces do not change the linear momentum of the system.

Now, let us discuss the kinetic energy of a system due to the internal forces. Let us take an example of an explosion of a particle placed at rest in two particles. The explosion occurs due to internal forces. Since the particle is at rest initially, its kinetic energy is zero. However, after the explosion the two pieces attain some velocity. Therefore, the system will gain some kinetic energy. Here, we can observe that due to the internal forces the kinetic energy of the system has changed.

Therefore, internal forces can change the kinetic energy of a system but not the linear momentum.Hence, the correct option is C.

Note: If you do not understand the example of the explosion, then you can also consider an example of a gun and its bullet. When a gun is fired, the gun exerts a force on the bullet and in return the bullet exerts a force of equal magnitude on the gun (in opposite direction). We know this from the third law of motion. The bullet is at rest but after firing the gun and bullet attain some kinetic energy.