Resultant force requires acceleration per Newton's second law

Steps:

• Newton's second law states resultant force equals mass times acceleration (F_net = ma).
• Acceleration occurs if velocity (speed or direction) changes.
• Analyze each case for acceleration.
• Identify nonzero acceleration only in circular path.

Why B is correct:

• Circular motion at constant speed changes direction, producing centripetal acceleration (a = v²/r), so resultant force provides this via F_net = ma.

Why the others are wrong:

• A: Constant speed in straight line means constant velocity, so zero acceleration and zero resultant force.
• C: Constant velocity uphill means no change in speed or direction, so forces balance and resultant force is zero.
• D: Stationary car has zero velocity and acceleration, so resultant force is zero.

Final answer: B