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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




Fluid Applications Cooling a hydraulic press India
Fluids What is plasma supposed to be? Canada
Fluids Thank you for your web-site! USA
Floating Change in density with change in state USA
Floating What does buoyancy mean? Turkey
Fluids Experiment showing water pressure with depth USA
General Can a peanut or walnut explode in lemonade? USA
Fluid Applications Difference between pressure and fluid New Zealand
Floating You should have more activities to do USA
Floating Acceleration of a balloon after weight released USA

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First 10 letters

Cooling a hydraulic press

Topic: Fluid Applications


August 8, 2010

In this hydraulic press, smaller piston is to be move up and down in order to move bigger piston continuously . so heat may be develop how to controll the heat?

i want to increse the force on bigger piston by applying same force and same area of both piston. so i am going to put 5 or more smaller piston of same area. Can it be possible?

sabin - India



You can cool the smaller cylinder similar to cooling an automobile or motorcycle engine, using cooled water or moving air.

See: for some information on the subject.

The force from the larger piston is due to the ratio of the area of the larger cylinder divided by the smaller. It you use five small cylinders, you are actually increasing the area of the small portion. To increase the force, either increase the top area of the larger piston or decrease the area of the smaller piston.

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What is plasma supposed to be?

Topic: Fluids


December 26, 2009

I thought that the state of matter is only solids liquids and gases. What is plasma supposed to be???

Abdo - Canada



A plasma is a very hot gas in which many atoms have electrons stripped away and are ionized. Since it has different characteristics than a solid, liquid or gas, many scientists consider a plasma a state of matter.

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Thank you for your web-site!

Topic: Fluids


December 24, 2009

Thank you for your web-site! I love the name "School for Champions" because it sends a very encouraging message to your "readers"! I graduated from college about 25 years ago and am now pursuing a new career as a Diagnostic Medical Sonographer. I absolutely love learning and am confident I will be great in the field. However, I have had very little exposure to physics and find it a bit intimidating. My first physics course had to do with doppler physics. I spent quite a bit of time reading information about sound waves, echos, etc. I found the material understandable and could conceptualize each subject you covered! Thank you so much! It is late and I am sleepy, but I just had to let you know that I appreciate your web-site and will be back!

Carol - USA



Thanks for the feedback. I'm glad the material was useful to you. Best wishes for success in your new career. I am sure you will be a champion.

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Change in density with change in state

Topic: Floating


November 27, 2009

How does the density of something change when when it changes states of matter? for example, will something become more dense if it changes from a liquid to a solid, or from a liquid to a gas?

are there any exceptions to this for water?

please explain this in terms an 8th grader can understand. thank you.

John - USA



Density is mass divided by volume. When steam turns to water, it takes up much less room or volume. Thus, water have a greater density than steam.

When water freezes, at first it expands or increases its volume. Thus at that point water has a greater density than ice. But as ice gets colder, it contracts and becomes more dense than water.

It has to do with how close the molecules are to each other. The change in state usually changes the distance of the molecules.

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What does buoyancy mean?

Topic: Floating


June 7, 2009


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

I love your site by the way

- Turkey



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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Experiment showing water pressure with depth

Topic: Fluids


January 19, 2009

In the experiment with water shooting out of a milk carton through holes at different depths, do you measure the horizontal distance that the milk squirts out of the hole before it starts falling down to the tabletop, or the total distance the water travels before landing on the tabletop (including after it stops moving out horizontally and falls to the tabletop)? The reason I am asking is because when we did this experiment at home,the water squirting out of the lowest hole squirted out with more force and went further horizontally before starting to fall down to the tabletop, but once the total distance was measured after it fell to the tabletop, it didn't go as far as the water that squirted out of the higher holes. The water squirting out off the topmost hole went a total further distance from the milk carton by the time it fell down to the tabletop, even though it squirted out of the hole with less pressure and did not go as far initially horizontlly before starting to fall down to the tabletop. We thought we saw things wrong, so we even took pictures and lo and behold, this happened time and time again. Can you please explain this to me? Thanks so much. As we are planning to use this for a science fair experiment, we would like to understand what happened. Your help and explanation is greatly appreciated.

Rivkah - USA



What you are looking for is the velocity of the water as it leaves the respective holes. The velocity would be proportional to the water pressure, provided the holes were the same size.

The distance the water goes horizontally is a result of the force of the water. Its velocity is v = d/t. You can easily measure the horizontal distance from the base of the carton to the point the water hits the table.

The time it takes is a function of the height of the hole and the force of gravity. From the lesson on Gravity Equations for Falling Objects at:

t = SQRT(2h/g), where h is the height of the hole from the table and g is the acceleration due to gravity. (Be sure to keep the units consistent.)

Measure d and calculate t to determine v and show that the greater the height of the hole from the table, the less the force of the water and thus its velocity.

Your final solution shows that the greater the depth of the water to each hole the greater the velocity of the water squirting out the hole.

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Can a peanut or walnut explode in lemonade?

Topic: General


May 7, 2008

Can a peanut or walnut explode in a glass of lemonade? Is it possible?

Diana - USA



The peanut or walnut must be airtight for it to possibly burst or explode. Usually they have cracks that allow air pressure to escape. I have heard that walnuts will fizz in lemonade.

It would be easy to try, but I feel the idea is an urban myth that has been going around the Internet.

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Difference between pressure and fluid

Topic: Fluid Applications


September 26, 2007

how can we differentiate between pressure and fluid?

renuka - New Zealand



A fluid is a liquid or gas. Pressure is a force per area in the fluid.

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You should have more activities to do

Topic: Floating


September 21, 2007

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

Sarah - USA



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

Topic: Floating


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



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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