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Physics Feedback

by Ron Kurtus

Each day readers send in questions and feedback concerning the various lessons in the School for Champions website. A total of 797 letters have been answered.

List of most recent 15 letters




Stating number with exponents Force Between Two Objects Philippines
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
Questions on electromagetism Basics of Magnetism India
Noise comes on every day Noise Reduciton UK
How do electromagnets work in railway signals? Electromagnetic Devices India
How does pressure affect a liquid? Pressure Ghana
Placing electromagnet inside another electromagnet Electromagnets USA
Advantage or application of friction Friction Nigeria

Next 15 letters

Force Between Two Objects

Stating number with exponents

March 20, 2017


I would like to know why is it in the given example above about gravitational force:
Ex. 6.67*10^-11 Nm^2/kg^2 when multiplied to 4.39*10^47 kg^2 will become 2.93*10^37Nm^2?

Where did the 10^37 came? What is the explaination of that?
I would like to know the exact explaination of that problem.

Thank you.

Ivy - Philippines (27945)


(6.674*10^?11)*(5.974*10^24)*(7.349*10^22) = 292.68*10^35

However, the standard practice to show numbers with exponents is to put the answer in the form of 2.93*10^37. You increase the exponent by 2 when you move the decimal point (divide 293 by 100).

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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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Basics of Magnetism

Questions on electromagetism

February 17, 2017


DC Electromagnet.
1. Ratio of Core area & Poles face area.
3. If three sides of magnet is covered with non magnetic material will there be more force on pole faces.
4. Construction & operation details of

tks. rgds, Vish

Viswanatan - India (27886)


See our lessons on Electromagnetism. They should help answer some of your questions.

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Noise Reduciton

Noise comes on every day

February 16, 2017


Dear Sir, please can you help me with this.
I have a building site across the road from my house.
I arrived home one day to a sound which was like my washing machine emptying out.
Since then the sound has changed. The Council monitored low voltage sound waves in the house and a sound consultant monitored 40 to 60 hertz.

Could it be that instead of hearing something emptying I actually heard something being connected to my pipes.

The sound now changes at the same time every day and I get brief interference on my TV at this time.

My main problem is that it gives me headaches and hurts my ears.

Could this be from a drainage/sump pump? Please can you give me some advice. Thank you.

Ann - UK (27884)


I am sure such a noise would be annoying to you. If they are building something across the street from your house, it could be that they are turning on some equipment at the same time each day. They fact that your TV has brief interference points to some electrical switching on or off.

It would be worthwhile to go to the building site and ask if they turn on some equipment at that certain time. This could give you a clue as to the cause of the noise.

It is possible that they have something connected to water or sewage pipes that reach your house. It could be some vibration from equipment pressing against the pipes or even pumping or draining from the pipes.

You might have to ask a few people there, and hopefully someone will give you a clue as the the cause of the noise.

I hope that helps in solving your problem. Best wishes for getting rid of that noise.

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Electromagnetic Devices

How do electromagnets work in railway signals?

February 14, 2017


How electromagnet works in railway signals?

Harsh - India (27879)


Railway signals can be flashing lights or a moving barrier.

An electromagnet could be used as a solenoid to turn on or turn off the railway signals.

The barrier is usually moved by an electric motor.

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How does pressure affect a liquid?

February 6, 2017


Give 3 ways on how pressure acts on liquid

Adu - Ghana (27860)


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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Placing electromagnet inside another electromagnet

February 5, 2017


I have a new found curiosity for electromagnets. I'm not a scientist or even a student... just curious. What would happen if your electromagnet core contained another electromagnet?

Ron - USA (27857)


If the core is movable, you would have a solenoid of sorts. (See Solenoids are Electromagnetic Devices).

Instead of having a movable iron core or magnet core, you would place an electromagnet there. This would allow movement of the core to be finely tuned. However, I don't think I've even seen such a configuration.

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Advantage or application of friction

February 1, 2017


advantage or application of friction

ameh - Nigeria (27852)


See Uses of Friction for some examples and uses.

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