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Physics Feedback

by Ron Kurtus

Readers have sent in a total of 892 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 most recent 15 letters




Concerning point masses Universal Gravitation Equation USA
Attracting dust Static Electric Materials USA
Insight on Space and Time Atom as Solar System India
Friction Reference Coefficient of Rolling Friction Iran
Charge a metal bar to be a magnet Magnetism Zambia
Copyright permission Noise Cancellation USA
Resolve components Mechanics Nigeria
Reducing friction Friction USA
Calculating simple pendulum Pendulum Equations Nigeria
Sound of sea is louder at night Sound Pakistan
Role of moons in theory Atom as Solar System India
Moving the magnetic field Generating Electricity USA
Relationship between Brownian motion and Kinetic theory Kinetic Theory of Matter USA
Branches of Physics General Kenya
Velocity equations Convention for Direction in Gravity Equations India

Next 15 letters

Universal Gravitation Equation

Concerning point masses

August 8, 2020


Mr. Kurtus,

How does a champion handle criticism? :)

The "Problem with point masses" section on the "Universal Gravitation Equation" page of this website states, "A problem exists when considering point masses, and that is the situation when the separation between point masses approaches zero. In such a case, the gravitational force becomes approaches infinity. Since that is impossible, there must be some small separation at which the Universal Gravitation Equation breaks down, perhaps at quantum distances."

Allow me to address the explicit and implicit notions made, therein, in reverse order.

First, ALL distances, even in the macroscopic, are - necessarily - quantum distances. The very existence of the macroscopic is entirely dependent upon the existence of the microscopic. Therefore, all macroscopic laws, rules and functions are - fundamentally - microscopic in origin. There is only one force and one set of laws that express over scale.

And, second, the above described equation represents the expression of two point masses that must - necessarily - be accelerating toward each other. This means, at some point, those masses will merge. Thus, two point masses will become one. Therefore, it is NOT impossible for the gravitational force to approach infinity because, upon merger, it must reset to a single point mass. Thus, zero or the inversion of infinite.

Conclusion: All of the descriptive mathematics in the world will never be a greater tool and a tiny bit of prescriptive reasoning.

If you would like to review more of my work, then please make a request via email. I look forward to hearing from you.

Matthew - USA (29607)


Thanks for bringing up the issue of point masses. I corrected the Universal Gravitation Equation page to add "center of mass" to the description and eliminated "force approaches infinity."

Also, when two point masses or particles meet or collide, they do not merge but instead usually change directions according to the Conservation of Momentum law.

Best wishes for success in your studies.

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Static Electric Materials

Attracting dust

July 23, 2020


I would like to know if it is possible, and if so how, to electrostatically charge string so that it will attract chalk dust. The string can be be nylon or any other material about 1.5 mm thick.
Thank you

ALFRED - USA (29563)


Although a nylon string can be electrostatically charged by stroking it with something like a polyester cloth, the big problem with attracting chalk dust is the small surface area of the string. Just handing the string near a chalk board will only attract a small amount of the dust. Yuo might need some other configuration to do the job.

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Atom as Solar System

Insight on Space and Time

July 17, 2020



This is Yug Patel from Gujarat, India.

My topic is Space - Time Relation.

-I just got to know about it when i was wondering in my room about the topic space, time, and all such beautiful events that we get to see.

-So, as we know that we are still finding answers to the questions that what is beyond Universe or what is much small than an Atom ? It's answer could be that there is no limit of anything. 

-I want to think of a situation, here when you see an Elephant walking according to us it is walking slowly ; and if we see an Ant walking it seems quite faster. Although they both travel with same speed just the difference is reality and frame of reference.

-According to Ant world is so big but for an Elephant world is slight small if we compare it with human's frame of reference.

-Just like that an Atom is much much smaller and Universe is very very big.

So according to the situation time seems to run slow as we increase our size and time run fast as we decreases our size. Because an event like destruction of atom take just fraction of second but dying of star take billion of years.

-So according to me there could be no end of universe or nothing beyond an atom. But there could be a multiple universe dimensions as we look much smaller in an atom or beyond a Universe.

- Just imagine it your size keep increasing with more than speed of light or decreases with same speed of light, and you see that universe is just a small molecule and you more such molecule and then you see that all those molecule (universe) forms an object which can be anything and finally you see that you arrived in another dimension.

If we talk about another situation then assume that your size keep on decreasing and you see bacteria, then molecule, then atoms, and then many more this and you land on a planet like thing, which could give you an Idea that you arrived in another dimension.

So basically there could be no end of anything.

And if this is possible which can be possible because whatever we found yet are just some hypothetical theories made on our assumption.

-After all this, i wanted to ask one question that if this is true then what could be more or what i had missed ?

Thank you so much for giving your beautiful time to read my message.

I hope you would have found it interesting.

Yug - India (29558)


Thank your for your insight and ideas about the relationship between the size of objects, humans, or animals and their perception of time and motion.

Observations of things around you when you decrease or increase your size or velocity points to their relativity. Some scientists feel that time is the 4th dimension, although others believe time is an extension of each dimension of space.

It is good to delve into these matters. Best wishes for success in your studies and observations.

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Coefficient of Rolling Friction

Friction Reference

July 9, 2020


What is the reference or table of rolling coefficients of friction coefficients?

danial - Iran (29538)


"Resources and references" on the page points to Rolling friction and rolling resistance for the table.

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Charge a metal bar to be a magnet

June 15, 2020


Explain how you can charge a metal bar to be a magnet?

Mwape - Zambia (29522)


One way to magnetize a metal bar is to wrap it with wire and send current through the wore, making the bar an electromagnet. See Electromagnets.

Another way is to stroke the metal bar with another magnet in order to align its atoms.

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Noise Cancellation

Copyright permission

March 26, 2020



I am writing on behalf of "aha! Process, Inc.", to seek permission to reproduce material for which I believe you control the copyright. Below is information about the upcoming publication in which your material would appear.

Information about our publication:
• Title: Emotional Poverty Volume II
• Author: Ruby K. Payne
• Format: Paperback
• Genre: Nonfiction
• Initial print run: 5,000
• Price: $30
• Projected release: August 2020

aha! Process seeks permission to reproduce the following material:
• Item: Text and graphics
• Source title: “Noise Cancellation”
• URL:

If you approve the use, please reply promptly with an appropriate permissions credit line to be placed on the copyright page of Emotional Poverty Volume II. For example:

“Human Learning Process” graphic from Building Expertise: Cognitive Methods for Training and Performance (pp. 50–51), by R. C. Clark, 2008, San Francisco, CA: Pfeiffer. Copyright 2008 by John Wiley & Sons. Reprinted with permission.

Thank you for your consideration,
Jesse Conrad
Assistant Publications Manager
aha! Process

Jesse - USA (29395)


Yes, you have permission to reproduce the material in the School for Champions article on "Noise Cancellation".

Best wishes for success in your work.

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Resolve components

January 2, 2020


How can I get your materials, how can I know how to resolve component, the understanding

Okesanjo - Nigeria (29306)


What are you trying to do concerning this subject?

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Reducing friction

December 20, 2019


I'm doing a project to how to reduce friction, so how do you reduce friction?

sameera - USA (29300)


See Reducing Sliding Friction.

Also see Friction Experiments.

Best wishes for success in your project.

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Pendulum Equations

Calculating simple pendulum

November 20, 2019


what is the method of calculating simple pendulum

MAUAN - Nigeria (29272)


You can measure the frequency that a pendulum swings with a stopwatch.

To calculate the frequency, you must measure the length of the rod or wire and use the equation: f = [SQRT(g/L)]/2PI.

(See Equations for a Simple Pendulum.

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Sound of sea is louder at night

October 8, 2019


Why sound of sea is louder at night than day time???

Nahida - Pakistan (29214)


The loudness or amplitude of sound increases with a reduction in temperature. The air above the sea is cooler at night and thus seems louder.

See Sound Seems Amplified Over Water for an explanation.

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Atom as Solar System

Role of moons in theory

September 9, 2019


1 how would the moon plays role in the given theory?
2 what would change in the theory if e considered 9 planets?
Or change to 12 planets?>

majid - India (29165)


The Solar System model of an atom looks at electrons moving around the nucleus of the atom. However, in most solar systems, many planets also have one or more moons revolving around the planet. This presents a problem with this model, since electrons are primary particles that cannot be subdivided.

Also, according to Quantum Mechanics, the sub-atomic particles can be viewed as wave-forms.

Thus, this model is simply convenient way of looking at the Solar System.

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Generating Electricity

Moving the magnetic field

August 16, 2019


I'm a tutor. Discussing electricity with students. This came up. If you move the field around the coil, instead of the coil moving, would that generate a current? Thank you!

james - USA (29151)


The first illustration in Generating Electrical Current shows a wire moving through a magnetic field. You could easily move the magnets and keep the wire stationary for the same effect.

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Kinetic Theory of Matter

Relationship between Brownian motion and Kinetic theory

July 9, 2019


What is the relationship between Brownian motion and the Kinetic Theory of Matter?

Your content is rich on the topic.

Emmanuel - USA (29098)


The Kinetic Theory of Matter states that matter is composed of a large number of small particles—individual atoms or molecules—that are in constant motion.

On the other hand, Brownian motion is the erratic random movement of microscopic particles in a fluid, as a result of continuous bombardment from molecules of the surrounding medium. Thus Brownian motion concerns the collisions of particles in motion due to the Kinetic Theory of Matter.

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Branches of Physics

July 8, 2019


What are the branches of Physics?

kevin - Kenya (29096)


Branches of Physics include:

- Classical physics
- Modern physics
- Nuclear physics
- Atomic physics
- Geophysics
- Biophysics
- Mechanics
- Acoustics
- Optics
- Thermodynamics
- Astrophysics

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Convention for Direction in Gravity Equations

Velocity equations

June 17, 2019


Would you please Explain the sign convention of g with the following examples

A)A ball thrown vertically upwards from the ground.
B)A ball thrown vertically upwards from a tower of hight h.

Find the maximum hight, find the time of flight and also velocity of ball before it reaches the ground.

S - India (29073)


The acceleration due to gravity (g) is positive toward the ground. When an object is thrown upwards, the force of gravity slows the velocity of the object.

See Gravity Equations for Objects Projected Upward..

To find the maximum height, you need the initial velocity upwards of the object, as well as the height from which it is thrown. The same is true for the time in flight.

Also see: Equations for Objects Projected Upward

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