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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 most recent 10 letters




Viscosity Applications of viscosity India
Pressure How does pressure affect a liquid? Ghana
Floating Weight of two containers of water Egypt
Measure Volume of Irregularly Shaped Object Volume of a cone Zimbabwe
Relationship Between Water Pressure and Depths Is this useful for watering plants? India
Floating Questions on ship and weight of air Australia
Pressure Pressure at bottom of gasoline tank USA
Pressure Force required by a water pump South Africa
Floating Calculate the buoyancy force USA
General Measurement of altitude of an aeroplane India

Next 10

Applications of viscosity

Topic: Viscosity


April 18, 2017

Sir please explain: Applications of viscosity in our daily life in an easy way.

Kummari - India



Viscosity can be thought of as the "thickness" of a liquid or fluid. One application is in lubricating a machine. Oil or grease has a greater viscosity than water and thus it works better at reducing friction and wear between the parts.

High viscosity means the material is slow-flowing, like molasses or toothpaste. You wouldn't want them to have low viscosity, such that they were runny.

I hope that helps your understanding,

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How does pressure affect a liquid?

Topic: Pressure


February 6, 2017

Give 3 ways on how pressure acts on liquid

Adu - Ghana



Pressure acts on a liquid in all directions. Pressure can also increase the temperature. Pressure only decreases the volume a small amount, if at all. Likewise, the temperature that the liquid changes to a solid is only slightly affected by pressure.

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Weight of two containers of water

Topic: Floating


January 16, 2017

Two containers one of them has water and small piece of wood and another one has only water which one is heavier ?

Misho - Egypt



If both containers have the same amount of water in them, then the one with the wood in it would be heavier by the weight of the wood.

However, if the containers were full to the top and the piece of wood was added to one of them, some water would overflow from the container. The weight of the overflow water would be the weight of the water displaced, such that both containers would then weigh the same.

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Volume of a cone

Topic: Measure Volume of Irregularly Shaped Object


July 20, 2016

How can l calculate the volume of cone

Flourish - Zimbabwe



The equation for the volume of a cone is: V = ?(r^2)h/3 where:
-- ? is pi = 3.14
-- r^2 is the radius of the base squared
-- h is the height of the cone

Thus, if the radius of the base was 2 and the height was 6, the volume would be: 3.14 * 2 * 2 * 6/3 = 25.12

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Is this useful for watering plants?

Topic: Relationship Between Water Pressure and Depths


April 17, 2016

What is the useful of these methode,can i watering plants

muhammed - India



The experiment shows how water pressure is greater the deeper the water is. However, it really is not useful for watering plants. You want to use a watering can or a water hose with pressure to stream the water.

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Questions on ship and weight of air

Topic: Floating


February 17, 2016

Hi Ron,

I am an Australian Bridge Officer in the Merchant Marine and currently completing my final year of a Bachelor of Nautical Science and also studying a Master of Philosophy.

I'll give you some background first; This question revolves around Draught Survey. This is where a professional Draught Surveyor observes the change in a ship's draught from when she sails in and ties up alongside a bulk loading terminal empty, to when she is laden with the bulk commodity and ready to cast off her lines.
(This is done by sighting the fore, aft and midship marks with a keen eye, calculating mean draughts etc... I won't go into the process)

The specific gravity of the dock water must be taken into account with a hydrometer, and it must be one calibrated in air rather than a vacuum, else a correction be applied.

My question is this:

Air weighs around 1250g/m3 @ 20 degC @ sea level.
Presume a 300m long 50m wide ship sails in and moors alongside the wharf empty (Hatches closed) at a mean draught of 5m. She opens hatches and proceeds to load a bulk commodity until she draws 15m and then close hatches again. There is nothing on top of the hatches before or after loading. The ship is now 10m further down in the water and there is obviously an extra 300m x 50m x 10m = 150000m3 column of air over the whole ship.

A very learned chap on the subject assures me that we have to allow for the weight of this volume of air because she is floating deeper in the water, and being buoyed up by the water rather than the slightly denser air at that 10m lower height that she would experience were she theoretically on land rather than in the ocean.

I am struggling with wrapping my head around this concept. If it were on land it would seem obvious that the surrounding air would always be at the same density as the extra column of air above the deck and air floating in a medium of similar air apparently weighs nothing... That's fine, no problem there.

It's the fact that the vessel is submerged further in the water that throws a spanner in the works for me... The authority aforementioned claims that the ship now apparently weighs an extra 150000m3 x 1.25kg = 187500kg or 187.5t because of the air and we should take this off the measured amount of cargo loaded lest the customer at the other end be charged for 187.5t of air!

I don't necessarily disagree with him but am unconvinced...

I've tried to make this question clear, here's hoping you grasp what I'm getting at.
At the least it is an interesting query!
An answer would be greatly appreciated.
Let me know if you need clarification on any of the information provided.

Here's a similar concept; A ship founders and slowly sinks down into the depths of the ocean; As she sinks further and further down, she doesn't weigh any more as the buoyant force of the water is pushing in on all sides and the bottom as well as pushing down from the top, proportional all the way down until she hits the bottom. This particular bottom is quite soft and silty mud and she hits it with reasonable force and so embeds herself half her own height into the mud (exaggerated for effect!). Does she now weigh more because the buoyancy of the water isn't pushing up from the bottom and half the height of the sides anymore?



Chris Felan.
Tasmania, Australia.

Chris - Australia



If you would use a barometer and measure the air pressure when the ship is not yet loaded and then when it is 15 m lower in the water, I don't think you would see much difference in the barometric pressure. That pressure represents the weight of the air column above. Any change in air density would be negligible.

I have never heard of anyone charging extra for any additional weight of air above the ship. That would be like changing according to the atmospheric pressure of the day.

On your sinking ship question: The ship sinks because its density is greater than that of the water. Its weight in water is its weight in air less the volume of water displaced. When it sinks into the mud, the mud is pushing on the bottom of the boat according to the depth of water. Now the weight is its weight in air less the amount of water and mud displaced. Thus, it would weight less. However, it would be more difficult to pull it up, due to the suction action of the mud.

I hope these ideas help.

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Pressure at bottom of gasoline tank

Topic: Pressure


December 24, 2014

Hello Mr. Kurtus,
I am trying to solve a math problem that is beyond my cope of knowledge. I want to measure the force in pounds per square inch in an open-top tank, the kind you see at an oil refinery. Assuming tank dimensions of 60 feet wide by 40 feet high, and that the tank is half full of gasoline@6.2 lbs per gallon (7.5 gallons per cu.ft.), what is the formula I can use to arrive at answer? I teach special education at a local high school and do not have a physics background. I am trying to help my students understand the physical forces at work in the refinery business.

William - USA



Often problems are stated in a confusing manner, supposedly as a way to make students think.

I believe you are looking for the pressure at the bottom of the tank. The reason it has an open top is to prevent the build-up of pressure from the gasoline fumes.

For the liquid, Pressure = Force/Area. In this case, the force would be the weight of a column of the liquid, and the area would be the cross section area of the column.

The diameter of the tank is of no concern, only the height of the liquid comes into play.

Since gasoline weighs 6.2 lbs/gal and there are 7.5 gal/cu ft, a cubic foot of gasoline weights 6.2 x 7.5 = 46.5 lbs. Now a 40/2 foot high 1 cu foot column of gasoline would weigh 20 x 46.5 = 930 lbs. The pressure per square foot would then be 930 lbs/sq foot.

But you want to find the pressure per square inch. So you divide 930 by 144 (12 x 12) to get 6.46 lbs/sq inch.

That is quite a problem and can be confusing. I hope this explains it.

Best wishes with your class.

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Force required by a water pump

Topic: Pressure


March 16, 2014

when you are using water pump like rotary pump, what force the water from the dam into the pump?

elled - South Africa



The water pressure from a dam at the pump is related to the depth of the water at that point divided by the cross section area of the entry to the pump.

The force that the pump must create is the water pressure times the cross section area.

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Calculate the buoyancy force

Topic: Floating


May 22, 2012

How can I calculate the buoyancy force. Give me the rule .
please be quick

Mark - USA



Buoyancy is the upward force on an object in a fluid that keeps it afloat. For an object floating in water, that force is equal to the weight of the water displaced by the object.

That is the volume of the part of the object under water times the density of water.

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Measurement of altitude of an aeroplane

Topic: General


December 26, 2011

Measurement of altitude of an aeroplane

Himangshu - India



An altimeter device is used in an aeroplane to determine its altitude.

From the ground, you can use a rangefinder to approximate the altitude.

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