Question

The net force acting is not zero on

• A. a retarding train
• B. a ball falling with terminal velocity
• C. a kite held stationary in the sky
• D. a truck moving with constant velocity
• E. a book placed on a table

Hint:

Any change in velocity (speed or direction) ⇒ net force is non-zero.

Answer 1

The Correct Option is A

Concept: Newton’s First Law
\[ \text{Net force} = 0 \Rightarrow \text{velocity constant} \] \[ \text{Net force} \neq 0 \Rightarrow \text{acceleration exists} \] ---

Step 1: Analyze each case
(A) Retarding train:
• Velocity decreasing
• Acceleration present (negative)
• Net force $\neq 0$ ✔ --- (B) Ball at terminal velocity:
• Forces balanced (weight = drag)
• Net force = 0 --- (C) Kite stationary:
• Forces balanced
• Net force = 0 --- (D) Constant velocity:
• No acceleration
• Net force = 0 --- (E) Book on table:
• Normal = weight
• Net force = 0 --- Final Answer: \[ \boxed{\text{(A)}} \]