List of Topics


Physics Feedback

by Ron Kurtus

Readers have sent in a total of 876 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




Properties of good insulators General USA
Doppler effect Visible Light India
Time to reach the ground Derivation of Velocity-Time Gravity Equations India
Confusion on thermal energy Thermal Potential Energy USA
Static charges and slugs Static Electricity Shocks UK
Distance sound travels Sound Echoes Bangladesh
Relationship between magnetic force and electricity Electromagnetism Nigeria
Instantaneous velocity and acceleration Acceleration India
Difference between vaporization and sublimation Dry Ice Kenya
Magnetic alignment Magnetism Ghana
Reducing permanent magnetism Basics of Magnetism New Zealand
Top spin in golf with driver Golf USA
Contents of Gravity and Gravitation Gravity Nigeria
Biology vs Physics General USA
Wands free books on EMF Magnetic Materials Ethiopia

Next 15 letters



Properties of good insulators

February 1, 2019


what are some good properties of good insulators.

ruben - USA (28933)


Certainly, a good insulator should hinder outside heat transfer (hot or cold). But also, the insulator must not weigh too much or take up too much space.

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Visible Light

Doppler effect

January 31, 2019


What will happen if object travels at half the speed of light. Is doppler effect applicable for electromagnetic waves?

Pankaj - India (28930)


An object must travel at about 90% the speed of light for any noticeable effect. See Speed of Light for information.

Also see Doppler Effect Equations for Light.

I hope that helps your understanding.

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Derivation of Velocity-Time Gravity Equations

Time to reach the ground

January 27, 2019


I have seen an equation to calculate time of an object to reach the ground as
Can you please help understand how this equation has come

Srinivasan - India (28924)


The time it takes a falling object to reach the ground is t = SQRT(2h/g) where h is the height or distance to the ground.

See Gravity Time Equations for Falling Objects for information.

I hope that helps.

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Thermal Potential Energy

Confusion on thermal energy

January 15, 2019


My question is the following:

It has to do with the idea of thermal energy being a sum of both the kinetic and potential energy of a material. Because I am teaching 8th grade Science at the time, I am trying to keep this as simple as possible. However, a student asked if two samples of air (one at approx. 0 Celsius, and the other at approx 27 Celsius) have different amounts of potential energy? After reading more about it, my thought would be no, they would have the same potential energy, just different amounts.

Rebecca - USA (28916)


The definitions of thermal kinetic energy and potential energy can be confusing. I updated the whole section to try to clarify issues and make it easier to understand.

What is called Internal Energy is the sum of the Thermal Kinetic Energy and Thermal Potential Energy.

See the updated versions of Thermal Energy is Kinetic Energy and Thermal Potential Energy.

The potential energy of two sample of air at different temperatures would be approximately the same. However, since the atoms or molecules travel faster at the higher temperature, the kinetic or thermal energy would be greater for the higher temperature.

I hope that helps. Best wishes for success in your class.

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Static Electricity Shocks

Static charges and slugs

January 14, 2019


Hello. I want to find a combination of materials that will hold a static charge for as long as possible to repel slugs and snails from a planter.
I have bought a copper braid copper adhesive strip for the purpose, but I want to find out that if I was to line the inside circumference of the planter with 15mm copper pipe and induced a very low current,(not enough to kill them, but enough to see them off)how long would either the static charge last as environmental conditions would influence this as well as a battery charge. Any help would be appreciated, thanks

mark - UK (28913)


One of the methods to get rid of with static electricity is explained in: How do I Get Rid of Slugs and Snails in my Garden? listed under "Give them a shock".

The method states that since slugs leave a slime trail, by using copper tape the slugs will get small electric shocks from the static electricity caused by the copper-slime combination.

The electric charge would only hold while the slug is moving along the copper tape.

I hope that helps. Best wishes in getting rid of those pests.

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Sound Echoes

Distance sound travels

November 20, 2018


the sound of echoes is heard after 0.12 sec.what is the distance between the source and the reflecting surface when temp of the place is 30 degree Celsius

nasiba - Bangladesh (28840)


The velocity of sound in air at 30 degree C is: 349.4 m/s. (see Speed of Sound in a Gas).

The distance traveled in 0.12 sec is then 41.9 m. Thus, the distance between the source and reflecting surface is 1/2 of that or about 21 meters.

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Relationship between magnetic force and electricity

October 23, 2018


my question is, what is the relationship between magnetic force and electricity?

Abdulrahman - Nigeria (28756)


See Moving Electrical Charges Create Magnetic Field. It shows that when electrical charges move, they create a magnetic force field. Likewise, a magnetic force can create electricity. See Basics of Electromagnetism.

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Instantaneous velocity and acceleration

October 15, 2018


what is the difference between instantaneous acceleration and instantaneous velocity?

sona mariam - India (28749)


Instantaneous velocity is the velocity of an object in motion at a specific point in time, as opposed to the average velocity.

Average acceleration is the change in velocity over a period of time. Instantaneous acceleration is the acceleration of an object in motion at a specific point in time.

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Dry Ice

Difference between vaporization and sublimation

October 11, 2018


Differentiate between vaporization and sublimation

Kevin - Kenya (28745)


Vaporization is the process that occurs when a chemical or element is converted from a liquid or a solid to a gas.

When a liquid is converted to a gas, the process is called evaporation or boiling.

When a solid is converted to a gas, the process is called sublimation. This is what happens with dry ice (frozen carbon monoxide gas).

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Magnetic alignment

October 8, 2018


what is in the iron that makes magnetic substance attracted by it

obed - Ghana (28741)


It is the alignment of the electrons. See: Factors Determining Magnetic Properties.

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Basics of Magnetism

Reducing permanent magnetism

October 6, 2018


If you have a weak permanent magnet and wind a multi turn coil around it and then pass an electrical current through the coil, can you negate the magnet strength of the magnet?
Will a specific current be required to reduce the magnet strength to exactly zero?

Charlie - New Zealand (28738)


You can demagnetize a permanent magnet by hitting it with a hammer or heating it to a sufficient temperature. Both cases disrupt the alignment of the atoms.

The problem with using a coil is that the atoms can be realigned into a different direction, thus still maintaining the magnetism. The only way to reduce the strength of the magnet is to get some or all of the atoms in random orientation.

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Top spin in golf with driver

September 6, 2018


How can you impart top spin with a driver so that it can roll farther?

Ted - USA (28692)


You probably want a top spin so that the ball will roll further. You can do that by either topping the ball when you hit it or by changing your stance such that you hit the ball on the upswing.

However, most golf experts say that you will get greater distance with a driver by hitting the ball flush.

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Contents of Gravity and Gravitation

July 23, 2018



Can you provide the table of contents in your physics book Gravity and Gravitational? I would like to know what I'm to learn in the book before buying to prepare for my O level examination.

Thank you.

Damilare - Nigeria (28636)


You can look at the web pages Gravity and Gravitation for listings of the topics in the book.

Although it is nice to have an actual book that you can refer to, the website lessons provide another choice, especially to be able to search and jump to different topics.

I hope that helps. Best wishes for success in your O-level examination.

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Biology vs Physics

June 19, 2018


Hi. I am 14 and I’m in 8th grade. I finished middle school this year. And this fall I will be in high school. Next semester I will be taking biology is it harder than physics ?

Zoe - USA (28590)


Biology often consists of many fact about the various plants and animals. Physics often includes math and equations.

Typically, Biology is offered in the sophomore year in high school, while Physics is taken in the senior year. Both are interesting subjects and are worth taking.

Best wishes for success (and fun) in high school.

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Magnetic Materials

Wands free books on EMF

May 30, 2018


Hello I am bereket, age 20 and I'm from Ethiopia. I want to ask you how I can get free online books on EMF.

Bereket - Ethiopia (28571)


There are some free downloads on EMF from Free Books - EMF.

I'm not sure how extensive they are, but they might give you what you want.

Best wishes for success in your studies.

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Next 15 letters


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