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Answers to Readers' Questions on Physical Science:

Floating

The following 41 comments and questions have been sent in. They are listed according to date.

List of first 10 items

• What does buoyancy mean?
• You should have more activities to do
• Acceleration of a balloon after weight released
• How fast does the balloon rise?
• Why isn't differential pressure used in floating?
• Salt needed to float an egg
• How does a burning candle float?
• How can salt make water denser?
• Trying to find density of piece of wood
• Is clay buoyant and cooking oil?

Next 10

What does buoyancy mean?

Question

June 7, 2009

hi

I still couldn't understand what buoyancy means. Can you help me out soon because I have finals coming out. Can you explain it the way a fifth grader would understand please

thanks
I love your site by the way

- Turkey

17870

Answer

Buoyancy is an upward force on an object that is in a liquid or gas. If the upward force is greater than the weight of the object (or its downward force), then the object will float.

If you push a block of wood under water, you can feel the upward force or buoyancy on the block. It is caused because the block of wood weighs less than the water it displaces, as well as the difference in water pressure on the top of the block and bottom.

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You should have more activities to do

Question

September 21, 2007

you should have more activities to do on this website it would be more fun interesting to learn!!!!!!

Sarah - USA

14423

Answer

Thanks for the suggestion. We are working on adding more activities and tests. They will be added soon.

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Acceleration of a balloon after weight released

Question

August 16, 2007

Dear Ron Kurtus, my apology to leave out the direction of velocity in my previous question, that is a 360kg balloon moving HORIZONTALLY at 12m/s and is 30m above ground. What is the acceleration of the balloon when a 16kg sandbag was thrown out from the moving balloon? How should i analyse the problem using potential / kinetic energy & forces? thank you

Mark - USA

14206

Answer

The altitude and sideways motion of the balloon are irrelevant and are added to the problem to try to confuse the students.

The weight of the balloon provides a 360kg force downward, and its buoyancy provides a 360kg force upward, resulting in the balloon remaining at a constant altitude. If you remove 16kg from the weight of the balloon, the upward force would be 16kg more than the downward force. Using F = ma, you can calculate the upward acceleration.

But note that kilograms are sometimes stated as mass and sometimes as weight. If the 16kg is mass, the calculation is straightforward. If the 16kg is weight, you need to divide by 9.8 m/s^2.

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How fast does the balloon rise?

Question

August 14, 2007

a 360kg balloon which is 30m above ground travels at 12m/s. a sandbag weighs 16kg is thrown off from the balloon so that the balloon can gain height. my question is how do we find the acceleration of the balloon. thank you

Mark - USA

14194

Answer

The 30m above ground has nothing to do with the problem. I assume the balloon is moving upward at 12 m per second and that it is filled with a gas like helium. The problem here is that the balloon would not go up at a constant speed, but instead would be accelerating. Something is wrong with this problem, since the information is incomplete.

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Why isn't differential pressure used in floating?

Question

July 5, 2007

I don't understand why that when it comes to what floats a ship, no one mentions differenial pressure. Like a battleship that draws 40 ft of water, there is about 20 psi pushing up on the bottom. That upward pressure is there when the ship is at sea or in a drydock with any amount of clearance between the ship and the drydock. Differenia pressure works the same pushing up on a ship or pushing on a hydrolic ram.

van - USA

14025

Answer

If the ship had a flat bottom, the upward pressure would be about 32 psi for a ship that draws 40 feet of water. Measuring the area of the bottom and multiplying by the pressure, you could find the weight of the ship.

Since a battleship has a V-shaped hull, the upward pressure varies with depth and can be determined by integrating over the shape. That can be fairly complex and difficult to do. You might use the difference between pressure measurements or differential pressure to help determine the value.

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Salt needed to float an egg

Question

April 2, 2007

How much salt do you need in tap water to float an egg that just came out of the refridgerator? Please give the answer to me quick, I have a science fair coming up soon.

Sarah - USA

13479

Answer

You need to measure the temperature of the water and the egg, because the amount of salt varies with temperature. If this is a science fair project, you really need to do the experiment to find out the amount for the given conditions.

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How does a burning candle float?

Question

March 4, 2007

we are doing a science project on how and why things float do you think you could answer our question? why do floating candles float higher in the water as they burn? thanks

- Canada

13307

Answer

As far as I can see, the ratio of the candle above water and below water would remain the same as the candle burned. It might look like it is floating higher, because there is less candle under the water after it has burned some.

For example, if the candle burned to one half the size, the amount under the water would be one half that it was previously.

Since you would probably put a weight on one end of the candle to keep it up right in the water, it would actually be a little lower than before because of the weight.

I've never tried this experiment, so there may be some trick for something I am not aware of, if it really does fold higher in the water.

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How can salt make water denser?

Question

February 28, 2007

Hi,

I want to ask 2 questions. =)

1) Why and how can salt makes the water denser?
2) If possible, can you give me an example of 2 different kinds of fruits (should weigh about the same)that one is denser than the other?

Thanks

Lindsay - Hong Kong

13264

Answer

Salt dissolves in water and its atoms become ions and locate between the water molecules. That means there is extra mass or weight in a given amount of water.

Pick up an orange and an apple. They are about the same size, but the orange is heavier than the apple, and thus it is denser. Density = mass divided by volume.

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Trying to find density of piece of wood

Question

January 31, 2007

Okay I understand that you cannot get the density of wood by water dispurtion because it is not heavy enough to dispurse the water in volume but if it an irregular shape piece of wood how would measure the density????

ann - USA

13022

Answer

Fill a container to its top with water. Carefully hold the irregular piece of wood under the water, perhaps using something like a thin piece of metal that has little volume. Measure the volume of the water that runs out of the container. That is the volume of your piece of wood.

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Is clay buoyant and cooking oil?

Question

January 30, 2007

If you were to put clay in cooking oil would it be buoyant and why? i know why but i wanna a clearer answer

a - Canada

13008

Answer

If the clay was less dense than the cooking oil, it would be buoyant. The density is the mass divided by the volume. I don't know the type of clay used, so I don't know its density. But if the clay was hollow, such as a small boat, it might easily float.

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