NEET 2022 Physics Fluid Dynamics MCQ Question

✅ Correct answer: D — B

NCERT states that for a sphere falling through a viscous medium:

• Initially, it accelerates due to gravity.

• As velocity increases, the retarding viscous force increases because

  $$F_v=6\pi\eta av$$

• Eventually, viscous force + buoyant force = gravitational force.

• Net force becomes zero, so acceleration becomes zero.

• The sphere then moves with constant terminal velocity. (Sathee)

Therefore the $v-t$ curve must:

$$\boxed{\text{start from }0\rightarrow\text{increase}\rightarrow\text{become horizontal}}$$

That corresponds to curve B in your diagram.

❌ Why other options are wrong

• A: Straight line → constant acceleration. ❌ But acceleration decreases as viscous force increases.
• C: Velocity eventually decreases → not possible here. ❌ The ball approaches a constant terminal velocity.
• D: Velocity falls toward zero → incorrect. ❌ It continues falling at terminal velocity.

📌 NCERT line/concept

As velocity increases, the retarding force also increases. Finally, viscous force plus buoyant force becomes equal to gravity; net force and acceleration become zero, and the sphere descends with constant velocity.

🧠 NEET Trick

Viscous falling sphere:

$$\boxed{v\uparrow,\quad a\downarrow,\quad \text{finally }v=\text{constant}}$$

So:

$$\boxed{\text{Ans = D (B)}}$$