Question

The motion of the centre of mass depends on:

• A. total external forces
• B. total internal forces
• C. sum of (a) and (b)
• D. either (a) or (b)

Hint:

Internal forces cannot change the motion of the centre of mass; only external forces can. This is why explosions do not affect the centre-of-mass motion in space.

Answer 1

The Correct Option is A

Step 1: Recall the principle of motion of the centre of mass. The motion of the centre of mass of a system is governed by Newton’s second law applied to the system as a whole. Step 2: Identify the forces affecting the centre of mass. Only external forces affect the motion of the centre of mass.

Internal forces occur in action–reaction pairs and cancel out.
They do not affect the overall motion of the system.
Step 3: Evaluate the options.

(A) Total external forces — correct ✔
(B) Total internal forces — incorrect
(C) Sum of (a) and (b) — incorrect
(D) Either (a) or (b) — incorrect
Step 4: Final conclusion. The motion of the centre of mass depends only on the total external forces acting on the system. \[ \boxed{\text{(A)}} \]