The buoyant force.
Explanation
The phenomenon of a body floating on a liquid is governed by the principles of hydrostatics and static equilibrium. For a body to remain at rest (float) on a liquid surface, the net force acting on it must be zero, in accordance with Newton's First Law of Motion.
Detailed Analysis:
- Weight of the Body: This is the gravitational force acting vertically downwards on the body due to the earth's attraction.
- Buoyant Force (Upthrust): According to Archimedes' Principle, when a body is immersed wholly or partially in a fluid, it experiences an upward force exerted by the fluid. This force is known as the buoyant force.
- Condition for Floating: For a floating body to be in equilibrium, the downward weight of the body must be exactly balanced by the upward buoyant force. Mathematically, \( \text{Weight} = \text{Buoyant Force} \).
Analysis of Incorrect Options:
- The atmospheric pressure. Atmospheric Pressure: While atmospheric pressure acts on the exposed surfaces, it is not the primary force responsible for counteracting the body's weight to maintain flotation.
- The viscous drag. Viscous Drag: Viscous force (fluid friction) comes into play only when there is relative motion between the body and the fluid. Since a floating body is in static equilibrium, viscous drag is zero.
- The surface tension. Surface Tension: Surface tension acts along the interface of the liquid. While it can support very small, light objects (like a needle or an insect), it is negligible for macroscopic bodies compared to the buoyant force.
Key Takeaway:
A body floats when the buoyant force (upthrust) exerted by the liquid is equal in magnitude and opposite in direction to the weight of the body.