Question

Internal force can change:

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

Answer 1

Answer: (C)

the kinetic energy but not linear momentum

$\text{Step 1: Kinetic Energy}$

Work energy theorem states that:

$\text{Work done by all forces(External + Internal) = Change in Kinetic Energy}$

Therefore, Internal forces can change the kinetic energy.

$\text{Examples:}$ Opposite charges accelerate towards each other, Bullet fired form a Gun, Bomb blast. In all these cases, Kinetic Energy increases due to internal forces only.

$\text{Step 2: Momentum}$
Internal forces always acts in pair (equal and opposite). So net force will be zero.
From newton's second law $F_{net}=\frac{dP}{dt}$.

Since, Net internal force is always zero, So change in momentum is also zero. i.e. Momentum will remain conserved.

Therefore, Internal force can not change linear momentum.

Hence, Option C is correct

$\text{Note:}$ Total energy of system will always be conserved (Fundamental law)