Equilibrium requires zero net force for constant straight-line motion

Steps:

• Recall that equilibrium means net force and net torque are zero, so the object moves at constant velocity in a straight line or remains at rest.
• Analyze each scenario: check if the underlined object (aeroplane in A/B/C, ship in D) has constant velocity without acceleration.
• Identify acceleration: changing speed, direction, or both violates equilibrium per Newton's first law.
• Conclude the option with directional change at constant speed involves centripetal acceleration.

Why C is correct:

• Changing direction requires centripetal force (F = mv²/r), so net force is not zero, violating equilibrium.

Why the others are wrong:

• A: Steady climb means constant velocity (including vertical component), so net force is zero.
• B: Constant altitude and towing imply steady horizontal motion, allowing equilibrium.
• D: Towing into harbour assumes constant velocity, with balanced forces for equilibrium.

Final answer: C