List of Topics

SfC Home > Physics > Matter > Fluids >


Feedback Comments on Fluids

by Ron Kurtus

A total of 94 comments and questions have been sent in. They are listed according to date.

List of next 10 letters




Floating How fast does the balloon rise? USA
Floating Why isn't differential pressure used in floating? USA
Floating Salt needed to float an egg USA
Floating How does a burning candle float? Canada
Floating How can salt make water denser? Hong Kong
Floating Trying to find density of piece of wood USA
Floating Is clay buoyant and cooking oil? Canada
Floating Building a boat for science project USA
Fluids How to measure specific gravity? India
Fluids Do bubbles reduce the density? USA

Next 10


First 10 letters

How fast does the balloon rise?

Topic: Floating


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



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.

Back to top

Why isn't differential pressure used in floating?

Topic: Floating


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



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.

Back to top

Salt needed to float an egg

Topic: Floating


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



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.

Back to top

How does a burning candle float?

Topic: Floating


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



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.

Back to top

How can salt make water denser?

Topic: Floating


February 28, 2007


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?


Lindsay - Hong Kong



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.

Back to top

Trying to find density of piece of wood

Topic: Floating


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



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.

Back to top

Is clay buoyant and cooking oil?

Topic: Floating


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



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.

Back to top

Building a boat for science project

Topic: Floating


October 3, 2006

I have a project for science class. I have to built a boad that will float the best when weight is added to it.
What materials will work the best? I need to have all materials and the boat built by next week.

Danielle - USA



The best type of boat would be something that is hollowed out. Try a plastic shoe box. You can add weight to it until the water comes over the sides. If the box has a sealed top on it, you can add even more weight.

Back to top

How to measure specific gravity?

Topic: Fluids


May 28, 2006

1. How is Specific Gravity measured in Liquids, Slurry and what are its units?
2. Is Specific Gravity and Density one and the same?

Hari - India



Density is the mass per unit volume. The density of water = 1 g/cc.

Specific gravity is the ratio of the density of a material to the density of water.

Measure the mass of a liquid in grams and its volume in cubic centimeters. Divide to get the density. Since the specific gravity of water is 1.0, the number of the density of a liquid is also its specific gravity (with no units).

Back to top

Do bubbles reduce the density?

Topic: Fluids


May 1, 2006

May I have a question?

suppose bubles formed on the surface of the objects that were submerging. How would the bubbles affect the measurement of densityof the objects? Would the bubbles make the density large, or small?




Bubbles typically rise to the surface because they are lighter than water. If they stick to the surface of a submerged object, they will exhibit a slight upward force, thus reducing the measured weight and apparent density. But that change would be so slight as compared to the density without the bubbles that it would barely make a difference. In fact, the upward force would be almost impossible to detect with standard measurements.

Back to top

Next 10


Hopefully, this reader feedback has helped provide information about Fluids.

Listen carefully

Resources and references

Ron Kurtus' Credentials


Sound Resources


Top-rated books on Fluids

Questions and comments

Do you have any questions, comments, or opinions on this subject? If so, send an email with your feedback. I will try to get back to you as soon as possible.

Please include it as a link on your website or as a reference in your report, document, or thesis.

Copyright © Restrictions

Where are you now?

School for Champions

Physics topics

Feedback on Fluids

Fluids topics

States of Matter


Let's make the world a better place

Be the best that you can be.

Use your knowledge and skills to help others succeed.

Don't be wasteful; protect our environment.

You CAN influence the world.

Live Your Life as a Champion:

Take care of your health

Seek knowledge and gain skills

Do excellent work

Be valuable to others

Have utmost character

Be a Champion!

The School for Champions helps you become the type of person who can be called a Champion.