Question

Internal forces can change

• A. the linear momentum but not the kinetic energy.
• B. the kinetic energy but not the linear momentum.
• C. linear momentum as well as kinetic energy.
• D. neither the linear momentum nor the kinetic energy.

Answer 1

The correct option is B the kinetic energy but not the linear momentum.

According to the principle of conservation of linear momentum, if the external force acting on a system add up to zero, then linear momentum of the system remains constant. It does not depend on the internal force. It means internal force cannot change linear momentum of the system.

However, it can change kinetic energy. Consider two equal and opposite charges are kept at rest and are released free. Both charges will attract each-other (due to internal force) and they will accelerate towards each other, as a result its speed will increase and hence increase in kinetic energy.

So, internal force can change kinetic energy but cannot change linear momentum of a system.

<!--td {border: 1px solid #ccc;}br {mso-data-placement:same-cell;}--> Hence, $\left(B\right)$ is the correct answer.