Buoyancy Force (Upthrust Force)

Buoyancy:

When an object is immersed in fluid, it experiences an upward force. This upward force exerted on a body is known as buoyant force, and this property is called buoyancy.

Buoyant force depends on two factors:

1. Volume of the object: The buoyant force is greater if the volume of the submerged object is larger.
2. Density of the liquid: The greater the density of the liquid, the greater the buoyant force.

If an object’s weight is less than the buoyant force acting on it, then the object floats. If an object’s weight is greater than the buoyant force acting on it, then the object sinks.

• A given volume of a denser substance is heavier than the same volume of a less dense substance. Therefore, the density of an object also affects whether it sinks or floats.
• The force due to the gravitational attraction of the earth acts on the bottle in the downward direction. So the bottle is pulled downwards.
• But the water exerts an upward force on the bottle. Thus, the bottle is pushed upwards.
• We have learnt that the weight of an object is the force due to the gravitational attraction of the earth.
• When the bottle is immersed, the upward force exerted by the water on the bottle is greater than its weight. Therefore, it rises up when released.

1. Aim: To observe how buoyant force affects whether an object floats or sinks in water.

2. Requirements: A plastic bottle with a tight lid, water, and a plastic hollow ball (optional).

3. Procedure

• Place an empty plastic bottle with a closed lid in water and observe it floating.
• Push the empty bottle underwater and note how it resists sinking and floats back up.
• Fill the bottle with water, close the lid, and release it. Observe that it floats just below the surface but does not sink to the bottom.
• Optionally, repeat with a hollow plastic ball for comparison.

Balanced and unbalanced Buoyant force

4. Conclusion: The empty bottle floats because its weight is small compared to the buoyant force from the water. When filled with water, the bottle's weight increases but does not sink because the buoyant force (Fb) still balances the force of gravity (Fg). This experiment demonstrates that buoyant force acts upward, balancing the object's weight and allowing it to float inside the water.

1. Aim: To observe how the shape of an object and the density of water affect whether an object sinks or floats.

2. Requirements: A piece of thin aluminium sheet, a bucket of water, salt, and lemon.

3. Procedure

• Dip a thin aluminium sheet in water and observe it sinking.
• Shape the aluminium into a small boat and place it on the water; it now floats.
• Note that an iron nail sinks, but a large steel ship floats due to its shape, which increases surface area and buoyant force.
• Float a lemon in water, and it sinks. Stir two spoons of salt into the water, and observe the lemon floating.

4. Conclusion: A flat aluminium sheet sinks, but when shaped like a boat, it floats because the shape increases the upward buoyant force. Objects float more easily in denser liquids, like salty water, because the greater density increases the buoyant force. Therefore, swimming is easier in seawater, and a lemon floats when salt is added to the water.