ASVAB (Armed Services Vocational Aptitude Battery)Mechanical Comprehension (MC)Medium
A small boat is moving at a constant velocity across a calm lake. If the boat's engine suddenly stops, but the boat continues to glide for a distance before stopping, which of Newton's Laws best explains why the boat eventually stops?
- ANewton's Second Law (F=ma)
- BNewton's Third Law (Action-Reaction)
- CThe Law of Conservation of Momentum
- DNewton's First Law (Law of Inertia)
Show answer & explanationAnswer & explanation
Correct answer: A. Newton's Second Law (F=ma)
While Newton's First Law explains why the boat would *continue* to move without external forces, it stops because of an *external force* acting against its motion. This external force (water resistance/friction) causes a deceleration (acceleration in the opposite direction), which is best described by Newton's Second Law (F=ma), where F is the net force causing the change in motion.
Why the other options are wrong
- B. This law describes interacting forces between two objects, not the cause of an object coming to rest due to a single external resistive force.
- C. While related to motion, the stopping of the boat is primarily about the forces causing deceleration, which is covered by Newton's Second Law.
- D. This law explains why it *would keep moving* in the absence of external forces, not why it stops due to them.
Newton's Second Law (F=ma)
States that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass (F=ma). It explains how forces cause changes in motion.
- A net force causes acceleration (change in velocity).
- If net force is zero, acceleration is zero (constant velocity or rest).
- Explains why objects start, stop, speed up, slow down, or change direction.
Memory trick: Inertia wants to stay; Force makes it stray; Action-Reaction is the way.