Newton's First Law after link break

Steps:

• Initially, constant speed means net force zero; engine's driving force balances friction on both.
• Link breaks, removing pulling force on truck.
• Truck now has unbalanced friction, so decelerates and slows down.
• Engine's constant driving force continues to balance its own friction, maintaining constant speed.

Why B is correct:

• By Newton's first law, with no change in net force on engine (driving force equals its friction), speed remains constant; truck loses forward force, so friction causes deceleration per F = ma.

Why the others are wrong:

• A: Engine speeds up ignores that driving force already balances its own resistance alone.
• C: Truck does not stop immediately; friction causes gradual slowdown, not instant halt.
• D: Engine does not speed up, and truck does not stop immediately for same reasons.

Final answer: B