AP Physics 2 : Buoyant Force

Study concepts, example questions & explanations for AP Physics 2

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Example Questions

Example Question #21 : Buoyant Force

Which of the following statements best describes the buoyant force on an object submerged in water?

Possible Answers:

None of these accurately describe buoyant force.

The buoyant force is equal to the weight of the water displaced by the object.

The buoyant force is equal to the weight of the object submerged in water.

The buoyant force is equal to the volume of water displaced by the object.

The buoyant force is equal to the weight of all of the water.

Correct answer:

The buoyant force is equal to the weight of the water displaced by the object.

Explanation:

The correct answer is that the buoyant force is equal to the weight of the water displaced by the object.

It is an upward force that is exerted on the object because of the volume of fluid displaced by that object.

Example Question #21 : Buoyant Force

What mass of helium would be needed to lift a  balloon and a  camera off of the ground?

Possible Answers:

This will be impossible

Correct answer:

Explanation:

Combining equations:

Plugging in values:

Solving for

Example Question #21 : Fluids

A block weighing  is suspended by a string and submerged in water. The block is  times denser than water. What is the apparent weight of the block in water?

Possible Answers:

Correct answer:

Explanation:

The correct answer is 

We know that the block is five times denser than the water. This means that a weight  of the block would be the buoyant force which can be calculated as the following:

We can subtract the of weight from the original weight of the block which was 

We obtain 

Example Question #24 : Buoyant Force

A block with a volume of  sinks to the bottom of a water tank. What is the buoyant force on the block?

Possible Answers:

There is no buoyant force on the block

Correct answer:

Explanation:

The correct answer is  because although the block has sunk, there is still a buoyant force. This buoyant force is the result of the block displacing a volume of water, equal to the block's volume. The weight of the water volume displaced is  because  was displaced. The weight of  is  which is equal to the buoyant force on the block.

Example Question #25 : Buoyant Force

An  paint can of volume  is floating on top of a lake, but water is slowly leaking in. Determine the volume of water present in the can when it finally sinks.

Possible Answers:

None of these

Correct answer:

Explanation:

When the can sinks, the net force of the water in the can plus gravity will be greater than the buoyant force. So the point of sinking is when they are equal.

Solving for 

Plugging in values:

Using the density of water, 

Example Question #21 : Buoyant Force

Seawater density:

A baseball has a mass of  and a circumference of . Determine what percentage of a baseball will be submerged when it is floating in seawater after a home run?

Possible Answers:

None of these

Correct answer:

Explanation:

Solving for 

Plugging in values

If the circumference is , it is 

Example Question #27 : Buoyant Force

A ship has mass . What volume of water would it need to be able to displace in order to be able to float?

Density of seawater: 

Possible Answers:

None of these

Correct answer:

Explanation:

Solving for 

Example Question #28 : Buoyant Force

Determine the volume of water that needs to be displaced in order for a  man to be able to float.

Possible Answers:

None of these

Correct answer:

Explanation:

When floating, the net force on the man is zero. The net force is made up of buoyant force and gravity. 

Solving for volume

Plugging in values:

Example Question #29 : Buoyant Force

A ship has mass . What volume of water would it need to be able to displace in order to be able to float?

Possible Answers:

None of these

Correct answer:

Explanation:

Solving for 

Example Question #30 : Buoyant Force

A nitrogen bubble of radius  is pumped into a bathtub from rubber tubing. Determine the net force on the bubble. The density of nitrogen is 

Possible Answers:

None of these

Correct answer:

Explanation:

The mass of the bubble will also be related to density and volume:

Plugging in values:

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