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Physics Feedback

by Ron Kurtus

Readers have sent in a total of 853 comments and questions on Physics issues. They are listed according to date.

You can read them to further your understanding of the subject.

List of next 15 letters




Correction on phase degrees. AC World Voltages USA
Why is copper used in a motor's stators? Electromagnetism USA
How can sound travel forever? Sound Waves USA
Static in jet airplanes Causes of Static Electricity Australia
Ohm's Law Electricity Basics India
Electrical fields and consciousness in the brain Electricity and Magnetism UK
Swinging bucket of water over your head Centrifugal Force Nigeria
Info on dry ice Dry Ice USA
Weather effects General Philippines
Graphing pendulum motion Pendulum Equations Nigeria
Does length affect period? Pendulum Equations Solomon Islands
What is the calculation of density? Density South Africa
Questions on ship and weight of air Floating Australia
Explain pressure in gases and liquids Pressure and States of Matter Nigeria
Wants to be a doctor by has problems with physics General South Africa

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First 15 letters

AC World Voltages

Correction on phase degrees.

April 6, 2016


This is a nicely done page. I suggest one correction: 3-phase AC has the phases 120 degrees out of phase. That is as much as three phases possibly can be out of phase. The statement that they are "slightly" out of phase does not describe them.

Thank you!

David Kazdan, MD, PhD
Electrical engineering faculty,
Case Western Reserve University
Cleveland, Ohio

David - USA (26910)


Thanks for pointing that out to me. I'm not sure how got the "slightly out of phase" but I updated it to 120 degrees. Also, I plan to add a page explaining the different AC power phases.

Thanks again for your input.

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Why is copper used in a motor's stators?

April 5, 2016


Hi Ron,

My question, if you don't mind, is why do I see copper used if iron in the below stator if they are weak magnets? Cost?

Thanks for the information on your site and for replying to me email if you have the time.

Richard - USA (26903)


The stator in an electric motor is the stationary part of the motor’s electromagnetic circuit. It usually consists of copper windings to create the magnetic field.

The copper is much more efficient than using iron wire.

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Sound Waves

How can sound travel forever?

March 25, 2016


Hi Mr Kurtus and School for Champions,

I am a high school student currently studying Physics. I greatly appreciate your site and would like to say thank you. However, I do have a question about physics which I hope you can enlighten me on:

1. Gas molecules are constantly moving and therefore experience frequent collisions. However, they do not lose kinetic energy upon collision - the collisions are elastic. If the collisions of particles of a material in the gaseous/liquid/solid state are elastic, why then can sound energy be lost since the particles would forever be moving with the same amount of kinetic energy and therefore propagate the sound energy over endless distances? According to your webpage, How Obstacles Affect Sound Waves, it says that "Since sound is a regular vibration of the atoms or molecules in a material, some of the kinetic energy of the waveform is lost due to molecular collisions. That sound energy is turned into random heat energy." But wouldn't this imply that the collision of molecules of the medium are inelastic and total kinetic energy is not conserved?

2. If the Law of Conservation of Energy states that energy can neither be created or destroyed, only transformed or transmitted, then how can there ever be energy in the first place? (I have tried Googling this, but to no avail. Perhaps it is a silly question, but I can't seem to wrap my head around it).

Thank you again for your website and help!

Emily - USA (26885)


Collisions between molecules in a gas are so close to being elastic that such an approximation can be made.

When sound travels through a liquid or solid, the collisions of the molecules are inelastic and some of the energy of the wave is absorbed in the material. You can witness this in the fact that sound traveling through a wall will be less in volume with a thicker wall.

Sound waves spread out from their source, reducing the amplitude of the waves with distance from the source. You can see an illustration in Sound Seems Amplified Over Water.

(Note that I need to add some pictures on the pages on sound.)

The Kinetic Theory of Matter states that matter consists of molecules and atoms in constant motion. That means that kinetic energy (KE) is everywhere. However, it can be changed into potential energy that can be released as KE, such as in a chemical compound. Thus the total energy is a sum of the KE and PE, and that total does not vary. Unfortunately, some science resources deal with energy as if it was some sort of substance.

I hope that helps to clear things up.

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Causes of Static Electricity

Static in jet airplanes

March 22, 2016



Tex - Australia (26881)


Thanks for the information.

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Electricity Basics

Ohm's Law

March 21, 2016


kw=18.5,v=440 I=? And Homs law formula

suresh - India (26877)


Ohm's Law is V = IR.

But since you have KW, it looks like you want to find Electric Power, which is P = VI. Thus, I = P/V = 18.5KW/440V

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Electricity and Magnetism

Electrical fields and consciousness in the brain

March 19, 2016


Dear School for Champions.

I am interested in the concept of both electricity (and magnetism?) being explained as the movement of electrons. If physical particles can generate non physical effects (magnetic and electrical 'fields') does this have relevance for the question of the brain generating consciousness?

Gerry - UK (26873)


Since matter has the properties of taking up space and having mass, you can see the physical effects when particles collide and such. But what is not easy to explain are the forces that act at a distance, such as electrical and magnetic forces

See Mysterious Force at a Distance for information.

Thought processes in the brain are apparently caused by electrical impulses, as well as electrical fields. Computers have been designed to simulate many of the things the human brain can do, but consciousness is still something to be achieved, if possible.

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Centrifugal Force

Swinging bucket of water over your head

March 12, 2016


What is responsible for the inertial force in the earth?

2, i want more explanation why water did not fall down when swing.

Fidelis - Nigeria (26858)


Inertia is a property of matter or objects that they tend to go in a straight line. When the direction changes--like swinging something in a circle--the inertial force tries to keep it in a straight line.

If you swing a bucket of water in a circle such that it goes over your head, the water will pull outward from the inertial force. If you swing fast enough, the water will not fall downward because the inertial force is greater than gravity.

the swinging bucket is an interesting experience to try.

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Dry Ice

Info on dry ice

March 11, 2016


To whom this may concern,

I am a sixth grader at Kennedy Middle School. Our school is participating in something called Genius Hour which allows us to research a topic that interests us. We are required to interview a professional who is knowledgeable about our topic. My topic is dry ice.
If you could please answer the questions below as soon as you get the chance, it would be greatly appreciated. Thank you for taking time out of your day to respond to this email.

Thank you for your time,

Gianna G.

My questions are:

Where was dry ice first found?

How long does dry ice last in water?

What gas is given off of dry ice?

How is dry ice created?

What is inside dry ice?

How long can you touch dry ice with bare hands?

Can you make dry ice?

Gianna - USA (26856)


Dry ice was first observed in 1835 in France. It is frozen carbon dioxide (CO2). When it melts, it turns to CO2 gas. You should only hold dry ice a fraction of a second. Special equipment is used to make dry ice.

See our lesson on Dry Ice is the Solid State of Carbon Dioxide for more information. Also see Dry Ice Wiki for detailed info.

Best wishes for success in your report.

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Weather effects

March 2, 2016


1. What conditions affect temperature and pressure?
2. How cold water currents and warm water currents affect the climate of a location?
3. Explain the conditions:
a. A mountain near the equator has snow on its peak throughout the year.
b. A desert exists on the eastern side of a mountain range.

Jesus Ramon Neopolo - Philippines (26832)


Weather patterns, sunlight and altitude affect temperature and pressure. Weather and temperature is affected by ocean current temperatures.

At high altitudes, the temperature gets colder. That is why a mountain near the equator can have snow at its peak.

Since most rainstorms come from the west toward the east and since mountains can block the clouds, the eastern side of mountain ranges often have desert conditions.

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Pendulum Equations

Graphing pendulum motion

February 29, 2016


hello. why would a graph of length against T square in a simple pendulum not pass through the origin.

- Nigeria (26818)


The equation for the period of a simple pendulum is T = 2pi*SQRT(L/g), as seen in Simple Pendulum Equations.

When the length is zero, the period is also zero.

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Pendulum Equations

Does length affect period?

February 28, 2016


Does the length of the "string" affect the period?
Does the period change with the angular amplitude of the string?

Svelt - Solomon Islands (26815)


The equation for the period in seconds of a pendulum is: T = 2pi*SQRT(L/g). You can see that if the length L increases, the period T also increases.

This equation works for small angles, and the period does not change with angle. However, at larger angles the equation becomes complex.

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What is the calculation of density?

February 23, 2016


calculation of density

sello - South Africa (26803)


The density of an object is its mass divided by its volume.

Since the density of water is 1.0, you can find an object's density relative to water by comparing its weight to the weight of an equal volume of water.

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Questions on ship and weight of air

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 (26788)


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 and States of Matter

Explain pressure in gases and liquids

February 16, 2016


explain vapour
pressure and explain the principles of pressure
measurement in liquid and in gases

okpu - Nigeria (26784)


Vapour is the gaseous state of matter. It is like steam.

Meters can compare water or air pressure with some standard. A barometer is an example of a meter that measures air pressure.

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Wants to be a doctor by has problems with physics

February 11, 2016


Hey,I am a grade 12 student doing math IT physics and life science...I Get very good marks in all my subject except physics. though I practise it everyday of school,I get 50% at the end of the semester.I would like to be a doctor but I am panicking now because I do not know how to tackle this subject.please if there's anything that I can do of which you know of. help

caroline - South Africa (26772)


Some courses in Physics emphasize problem solving using the various equations. It is almost more of a math exercise than having knowledge of how things work.

That said, it is good to do the homework exercises to be able to solve problems in your exams. You might try doing your homework as if it was an exam, solving problems quickly and without referring to the book.

Take notes in class, because often the instructor gives hints on what will be in the tests. You can review your notes before doing homework and taking tests.

Also, ask your teacher for some ideas on how you can do better in the class. Often they will give ideas that will help.

If you want to become a doctor, you certainly want to get good grades in all your subjects. Note that chemistry and life sciences are more important than physics for becoming a doctor.

Best wishes in getting good grades in all your classes.

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