High School Physics

# Buoyancy & Buoyant force – Questions & answers

Last updated on May 14th, 2022 at 01:35 pm

In this post, we publish a bundle of Questions and answers related to Buoyancy and Buoyant force.

## Why does a stone sink but a wood float?

A stone is denser than water. This means, if you compare the mass of a stone to the mass of the same volume of water, the stone’s mass will be greater. Therefore its weight will also be greater, so, (Wstone – Wwater) will be positive, and there will be a downward net force. Therefore the stone will move downward through the water until it rests on the bottom.

Wood, on the other hand, is less dense than water. If the wood is pushed underwater then the value of (Wwood – Wwater) will be negative. There will be an upward force on the wood and it will rise.

How far will wood rise? As some of the wood rises above the level of the water the volume and the mass, and therefore the weight, of the water displaced will be reduced. When the weight of the wood and the weight of the water displaced are equal, the net force on the wood will be zero and it will no longer move.

## Why is a stone easier to lift when it is in water?

Even though the stone sinks in water because it is denser than water, there is still an upward buoyant force on it given by Wstone – Wwater. This makes the net downward force on the stone less than the actual weight of the stone.

Hence, a stone is easier to lift when it is in water.

## Floating usually refers to a liquid such as water, but can anything float in a gas?

Remember that there is a difference in pressure of any fluid between the bottom and top of an object in it given by Pbottom = Ptop + dgh. Even though the density of a gas, d, is much smaller than that of a liquid, there is still a pressure difference, and therefore buoyancy is present in gas also.

## When a hot air balloon is launched, at some height the balloon stops rising – why?

A gas can be compressed, and so the density of the atmosphere decreases as you rise. When a hot air balloon is launched, the air within it is heated and it will rise. As it rises the density of the air decreases, and so the buoyant force decreases.

At some height the weight of the balloon and basket equals the weight of the air displaced and the balloon stops rising.
The operator of the balloon can go higher by making the gas in the balloon hotter, and thus less dense.

## What happens to the helium balloon that a child releases?

If the balloon is tied tightly, the balloon will expand as its altitude increases. This expansion is caused by the lower atmospheric pressure at higher altitudes. Eventually, the helium’s volume increases so much that the rubber balloon breaks, releasing the helium.

## How much water does a ship need in order to float?

To keep a ship afloat it has to displace just enough water to equal its weight. Therefore, if a ship were to enter a canal that was just a bit larger than the size of the ship’s hull, it would float as long as there was a small film of water around the entire hull of the ship.

## What happens to the buoyancy of the ship when cargo and passengers are added?

When cargo and passengers are added to a ship its weight increases. As the ship’s weight increases it sinks further into the water, displacing a greater weight of water.

If it sinks so far that water can spill into the ship, increasing its weight even more, the ship sinks to the bottom of the water.

The amount the ship sinks in the water as a result of cargo and passengers can be critical for navigation and maneuverability. Large cargo and cruise ships have numbers on the bow of the ship that indicate how far the distance is between the waterline and the bottom of the ship. This distance is called the ship’s draft.

If the ship has a 6-meter (20-foot) draft and the water is only 5.5 meter (18 feet) deep, cargo and passengers must be unloaded to allow the ship to rise.

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