Equilibrium of forces on rising bubble

Steps:

• Directions: Upthrust U acts upward (buoyancy); weight W acts downward (gravity); viscous drag V acts downward (opposes upward motion).
• Constant speed means net force is zero (Newton's first law).
• Vertical equilibrium: U upward equals total downward forces, so U = V + W.
• Relative sizes: U is largest; V and W are smaller, both downward.

Why A is correct:

• Shows U upward and equal to sum of downward V and W, satisfying force balance for constant velocity.

Why the others are wrong:

• B: Drag V shown upward, ignoring opposition to motion.
• C: U smaller than W + V, implying deceleration.
• D: All forces equal size, violating U = V + W.

Final answer: A