Understanding the Upward Force in Water: Buoyancy

When a body is in water, it experiences several forces. One significant force acts upwards, opposing the force of gravity which acts downwards. This upward force is what makes objects feel lighter in water and allows some to float. The question asks to identify this specific upward thrust.

Analyzing the Options for Water Borne Forces

Let's look at the provided options and understand what each force represents:

• Force of gravity: This is the force exerted by the Earth (or any large mass) on the body, pulling it downwards towards the center of the Earth. This is not an upward thrust in water.
• Resistive force: This is a force that opposes motion through a fluid, such as water. It is also known as drag force. While present in water, it generally opposes the direction of movement, not necessarily providing a constant upward thrust.
• Propulsive force: This is a force generated by the body itself or an external mechanism to create movement through the water (like the force generated by a swimmer's strokes or a boat's propeller). This is an active force for propulsion, not the inherent upward thrust provided by the water itself.
• Buoyant force: This is the upward force exerted by a fluid on a body immersed in it, whether partially or fully. It is caused by the pressure difference between the bottom and top of the submerged part of the body, with higher pressure at greater depth. This force directly opposes the weight of the body and provides the upward thrust.

Identifying the Correct Upward Thrust in Water

Based on the definitions, the force that specifically provides an upward thrust to a body immersed or partially immersed in water is the buoyant force.

This force is a fundamental property of fluids and is described by Archimedes' Principle, which states that the buoyant force on an object is equal to the weight of the fluid displaced by the object. Mathematically, the buoyant force ($F_B$) can be calculated using the formula:

\( F_B = \rho_{fluid} \times V_{submerged} \times g \)

Where:

• \( \rho_{fluid} \) is the density of the fluid (water in this case)
• \( V_{submerged} \) is the volume of the submerged part of the body
• \( g \) is the acceleration due to gravity

This upward buoyant force is responsible for phenomena like floating and explains why objects appear lighter in water.

Conclusion: The Buoyant Force

The force that provides the body an upward thrust in water borne condition is the buoyant force. It is a natural result of being immersed in a fluid and acts vertically upwards, opposing gravity.

Revision Table: Forces in Water

Additional Information: Understanding Buoyancy

Buoyancy is the ability or tendency of something to float in water or other fluid. The buoyant force is the force that enables this. The magnitude of the buoyant force depends on the volume of fluid displaced by the object and the density of the fluid. A denser fluid exerts a greater buoyant force for the same submerged volume. If the buoyant force is greater than or equal to the weight of the object, the object will float. If the weight of the object is greater than the buoyant force, the object will sink.