List of Topics


Physics Feedback

by Ron Kurtus

Readers have sent in a total of 803 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




Approved fuel containers Basics of Static Electricity Australia
Torque and increased diameter Force and Torque USA
Annoying ad on page Units of Frequencies and Wavelengths USA
Using 60Hz on 50Hz circuit AC World Voltages USA
Applications of viscosity Viscosity India
Suggested resource Resources India
Update Guyana voltage AC World Voltages Suriname
Equation different in textbook Doppler Effect India
Using static electricity to charge phone Basics of Static Electricity Egypt
Uses for friction Uses of Friction Nigeria
References on electrostatic painting Uses for Static Electricity Egypt
New Triboelectric idea Static Electric Materials USA
What do the signs mean Force Between Two Objects Australia
Difference in echo times Sound Echoes Tanzania
Electricity for water heater AC Wiring USA

Next 15 letters


Basics of Static Electricity

Approved fuel containers

May 12, 2017


Hi guys

I am an ex firefighter (28 years) and are now involved in teaching fire safety. I deliver training to fuel company staff. One of the topics covers using the correct "Approved Fuel Containers" for transport of fuel (Petrol).

My question is, are the containers manufactured of a plastic that incorporates a conductive substance to dissipate electrical charge? If they are that is great but, my thoughts are they are as conductive as any other plastic container. If this is the case, how does the electrical charge dissipate when filling the fuel container on the ground. In theory it is non-conductive.

My thoughts are that all plastic will conduct electricity to a degree. Not enough to allow enough current to flow to power a light bulb if used in a circuit but will allow enough current to flow causing a static spark. I am thinking milliamps as compared to amps will flow.

I am attempting to confirm that the only difference between an approved fuel container and a non-approved fuel container is its solid construction of plastic that will not deteriorate by petrol.


Fred - Australia (28044)


Static electricity consists of electrical charges on the surface of the container. Approved Fuel Containers usually are made of a hard plastic material that does not collect charges as mush as a container made of a soft plastic. You can tell by rubbing a balloon on the container and see if it gets charges such that it sticks to some surface.

The big concern is having charges collect on the container and getting a static spark that would ignite the fuel fumes. If the container is on the ground and you are also on the ground, you are both "grounded" such that charges readily flow to the ground. But if the container is on a plastic bed of a truck when being filled, a spark may jump from the container to you are the filling tube, thus igniting the fumes.

Note that the issue isn't whether plastic conduct a little electricity, but is it whether charges on the surface will jump and cause a spark to some other item or person on the ground.

It isn't a simple situation, and I hope this clarifies things somewhat (or does not confuse the issue!)

Best wishes for success in your fire safety classes.

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Force and Torque

Torque and increased diameter

May 6, 2017


Im interested in torque on a round object.
I'd like to know if you increase the diameter of the object by say 10% how does it affect rotational torque.

Jon - USA (28038)


Since T = FR, increasing the diameter by 10% increases the radius R by 20%. Thus, the torque T would increase by 20%.

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Units of Frequencies and Wavelengths

Annoying ad on page

May 2, 2017


While on the page using Chrome on a Mac, the page is regularly refreshing and jumping around, making it very difficult to use.

The video ad in the upper right cannot be turned off, and is very distracting.

I like your material very much--you are clear, give enough information without assumptions as to how much a person knows and progress clearly. Thanks!

Stephen - USA (28033)


Thanks for the feedback.

The ads on my site are through Google AdSense and are not supposed to be annoying. I also hate going to websites where they have videos with loud sound. You typically can click the Pause button on the lower left corner to stop the sound and video.

If that does not work, let me know what company the ad is from, and I'll contact Google to block them from my site.

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AC World Voltages

Using 60Hz on 50Hz circuit

May 1, 2017


I buy a Makita hp1630k drill 220v 60hz question:can I use this drill in Europe 220v 50hz

Jani - USA (28031)


The drill should work, although it may run a little slower. It is good to contact the company customer service to see if they recommend using an adapter.

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Applications of viscosity

April 18, 2017


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

Kummari - India (28007)


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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Suggested resource

April 2, 2017



I am a writer of science topics and have a website which is about teaching the kids about science with interesting articles and easy-to-do experiments. its motive is to provide science education free of cost and free of ads to enhance the interest. I want to deliver my website to every science student. I will thankful to you, if you can check the website's physics section at and include in your physics resource page under the name "Physics for Kids" for the students.
I look forward to hear from you soon. Thank you


Abdul Wahab - India (27980)


I added a link to Fun Science for Kids at Physics Resources. Best wishes for success with your website.

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AC World Voltages

Update Guyana voltage

March 24, 2017


Re: World list of AC household voltages has an error:

Guyana 60Hz = correct but 240V? No Way.!

Some 110 or 120V . Check it out!

Happy to see a comprehensive AC voltages listing.


Jozef - Suriname (27956)


Thanks for the input. The standard AC voltage in Guyana is 220 or 240V. However, there are many areas in the country that use 110VAC.

I updated the list to reflect state both voltages.

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Doppler Effect

Equation different in textbook

March 2, 2017


In textbook eq. is given in terms of v, but in your lesson its given in terms of lambda.

Rajendra prasad - India (27919)


The velocity of a waveform--especially for light or sound--is often designated as "c" instead of "v". Lambda represents the wavelength.

You can also rearrange the equation to be wavelength = velocity/frequency.

Also see Doppler Effect Equations for Sound.

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Basics of Static Electricity

Using static electricity to charge phone

February 24, 2017


if i somehow generated this static electricity, how would i use it for turning on something ?
and how much electricity needed to make a phone charger work for example ?

nada - Egypt (27904)


Static electricity is static and does not move like DC or AC electricity.

Most phone chargers use alternating current (AC) electricity to charge the batteries. However, there are some new chargers that use the wireless electricity signals to charge the batteries.

Static electricity can be used to generate sparks. That might be able to turn something on. But also, the sparks can cause damage to electrical devices.

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Uses of Friction

Uses for friction

February 23, 2017


List 12 uses of friction and disadvantages of friction

ade - Nigeria (27902)


There are some items listed on the Uses of Friction and Problems Caused by Friction pages.

You can look around you to see many other examples of uses and disadvantages that are similar.

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Uses for Static Electricity

References on electrostatic painting

February 20, 2017


I am Working on A project On High Voltage Engineering and i need any Reference talking about Electrostatic Painting
Best Regards

Mahmoud - Egypt (27892)


Some websites explaining the basics of electrostatic painting are:

Electrostatic coating

Electrostatic Spray Painting Basics Explained

How Does Electrostatic Spraying Work?

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

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Static Electric Materials

New Triboelectric idea

February 20, 2017


Good Day to you sir,I have taken an interest on your webpage because I also work on Triboelectric generation techniques of my own that are considered totally outside of the box thinking. If you would be interested, I would very much like a moment of your time. THANK YOU

Benjamin - USA (27893)


It is good that you are coming up with some new applications.

I'm not sure if I could help you, but I'd be glad to take a look at your idea.

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Force Between Two Objects

What do the signs mean

February 19, 2017


Hey I'm Aric and I'm trying to work out the gravitational attraction between the Sun and the Earth for homework
I just wanna know what the times, divide, plus and minus mean
Like as in;
* = Times?
/ = Divide/Fraction
- = Minus
and so on

Aric - Australia (27888)


The equation F = GMm/R^2 means G times M times m divided by R squared or R times R.

To answer your question:
a*b means a times b or multiply a times b
a/b is a divided by b
a - b is a minus b or subtract b from a

The big thing is to find the mass of the Sun and of the Earth, as well as the distance between them.

I hope that helps. Best wishes in getting the problem solved.

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Sound Echoes

Difference in echo times

February 19, 2017


a helcopter is hovering at an altitude of 200 m above the surface of a lake. a speaker on the helcopter is sending out soundwaves,which are reflected from both the surface of the water and the bottom of the lake. if the difference in arrive times of the two echoes is measured tobe 0.24s, what is the depth of the lake?
(the atmopheric temperature is 20centgreds)

Toroma - Tanzania (27890)


Most of the sound would be reflected at the surface of the water. Only a very small amount of the sound would go into the water. Also much of the sound would be reflected at the surface when trying to go back into air. In other words, the amount of sound reflected off the bottom of the lake probably could not even be detected.

The speed of sound in air is 343 m/s. So, it would take 200/343 = 0.58 seconds for the sound to go from the water to the helicopter.

The speed of sound in water is 1484 m/s. The sound would have to go to the bottom of the lake and then back up, and then it would take 0.58 s to reach the helicopter. For the difference to be 0.24 s, the depth would be 1/2 of 1484 * 0.24 = 178.08 m.

I hope that helps.

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AC Wiring

Electricity for water heater

February 19, 2017


I just found that I have current in the system from the hot wire of the water heater to ground . On one 110 with it removed from the breaker I get a reading of 1.7 volts to ground. The other reads .8 volts to ground. We went to checking because the power bill has been high and though that the water heater might have gone back. This is a older home and has had several changes over years. What would be the best way to find out what is going on.

Thanks for you help

Ben - USA (27891)


Often older homes have their electrical wiring installed or fixed incorrectly. I'm not sure how you could get readings of only 1.7 volts unless you have your voltmeter set wrong or the voltages are not for heating the water.

Although you probably hate to spend the money, it is best to get an electrician take a look at things. There may be some other reasons your power bill has been high, especially if the increase is recent.

Best wishes on solving your problem.

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