Frictional force (fluid drag)
Explanation
According to Newton's First Law of Motion, an object in motion continues to move with a uniform velocity unless acted upon by an external unbalanced force. In the context of fluids (liquids and gases), this opposing external force is known as fluid friction or drag.
Option Analysis
- Gravitational force is incorrect: Gravitational force acts vertically downwards. In the case of a boat, this force is balanced by the buoyant force exerted by the water. It does not act in the horizontal direction to oppose the boat's forward motion.
- Magnetic force is incorrect: Magnetic force requires magnetic materials or fields, which are not the primary forces acting on a standard rowing boat in water.
- Frictional force (fluid drag) is correct: When rowing ceases, the applied force driving the boat forward becomes zero. However, the water exerts a resistive force known as fluid friction (or viscous drag) against the direction of the boat's motion. This unbalanced retarding force causes deceleration, eventually bringing the boat to a halt.
- Atmospheric pressure is incorrect: Atmospheric pressure acts uniformly on the boat and the water surface. It does not create a net horizontal force to stop the boat.
Key Takeaway
Friction is a contact force that always opposes the relative motion between two surfaces or layers. In fluids, this resistance is termed viscosity or fluid drag, and it is the primary cause for the deceleration of objects moving through water when no propulsive force is applied.