Correct Option
The correct option isIt acts vertically upwards.
Explanation
Buoyant force, also known as upthrust, is the upward force exerted by a fluid (liquid or gas) on an object that is wholly or partially immersed in it. This force arises due to the difference in pressure exerted by the fluid at different depths.
Detailed Analysis
- Pressure Gradient: In a static fluid, pressure increases with depth due to the weight of the fluid column above. Consequently, the pressure exerted on the bottom surface of an immersed object is greater than the pressure exerted on its top surface.
- Net Force Direction: Since force is the product of pressure and area ($F = P \times A$), the upward force on the bottom surface is stronger than the downward force on the top surface. The horizontal forces acting on the sides of the object cancel each other out.
- Resultant Vector: The net resultant force acts in the direction of the pressure decrease, which is opposite to the direction of gravity. Therefore, the buoyant force acts vertically upwards through the center of buoyancy.
Analysis of Incorrect Options:
- It acts vertically downwards. Vertically downwards: This is the direction of the gravitational force (weight) acting on the object, not the buoyant force.
- It acts horizontally. Horizontally: In a static fluid, horizontal forces on opposite sides of an object are equal and opposite, resulting in zero net horizontal force.
- It acts in the direction of motion. In the direction of motion: Buoyancy is a static property determined by fluid displacement and gravity; it does not depend on the direction of the object's motion.
Key Takeaway
Buoyant force is always directed vertically upwards, opposing the weight of the object, because fluid pressure increases with depth.