# 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.

## List of next 15 letters

### Country

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

Need to update home voltages AC Wiring USA

Change frequency of LED light LED lights USA

Using CO2 for supercritical extraction Dry Ice USA

Gravitational force Force Between Two Objects USA

Reducing aerodynamic drag Fluid Friction Uganda

Next 15 letters

First 15 letters

## New Triboelectric idea

February 20, 2017

### Question

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.

## What do the signs mean

February 19, 2017

### Question

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
Thanks

Aric - Australia (27888)

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

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.

## Difference in echo times

February 19, 2017

### Question

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.

## Electricity for water heater

February 19, 2017

### Question

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.

## Questions on electromagetism

February 17, 2017

### Question

DC Electromagnet.
1. Ratio of Core area & Poles face area.
2. What is ATTRACTION POWER?
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
ELECTRO-PERMENANT MAGNETS

tks. rgds, Vish

Viswanatan - India (27886)

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

## Noise comes on every day

February 16, 2017

### Question

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.

## How do electromagnets work in railway signals?

February 14, 2017

### Question

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.

## How does pressure affect a liquid?

February 6, 2017

### Question

Give 3 ways on how pressure acts on liquid

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.

## Placing electromagnet inside another electromagnet

February 5, 2017

### Question

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.

## Advantage or application of friction

February 1, 2017

### Question

ameh - Nigeria (27852)

See Uses of Friction for some examples and uses.

## Need to update home voltages

January 30, 2017

### Question

On your page AC Wiring I encourage you to change all instances of "110" to "120" when referring to household voltage, since it is set (in the US) to 170 Vp, which ends up being approximately 120 VACrms.

Noji - USA (27841)

Thanks for pointing that out. Although 110 VAC is often used, the actual average between 110 and 120 is 117 VAC. But 120 is more commonly used. I changed the pages on AC from 110 VAC to 120 VAC.

I also updated the higher voltage from 220 to 240 VAC.

## Change frequency of LED light

January 29, 2017

### Question

How can I change the frequency of an LED lifght?
Thank you

Chris - USA (27836)

Right now, LEDs come in only three colors: red, green, and blue. A combination of them is a white LED.

Each color has a specific electronic configuration. Unfortunately, engineers have not found a way to vary the output frequency of an LED.

## Using CO2 for supercritical extraction

January 29, 2017

### Question

I would like to discuss some commercial uses for Dry Ice and it's conversion to Co2 for supercritical extraction.
Please let me know if you are available to discuss.

Gary - USA (27838)

Dry ice is a convenient source of CO2. Allowing the gas from the dry ice to heat to about 31 degree C should help in the supercritical extraction process.

I hope that helps. Unfortunately, I am not an expert in this field.

Best wishes for success in your work.

## Gravitational force

January 28, 2017

### Question

1. Two objects attract each other with a force of 1×10^-8 N when separated by 20 cm. If the total mass of the objects is 5 kg what is the mass of each?

2. A spacecraft is on a journey to the moon. At what point between these two bodies does the gravitational force exerted on the spacecraft by the earth exactly balance gravitational force divided by the moon? At this point what is the net gravitational force on the spacecraft? assuming no other gravitational forces act.
I'd really love some help figuring these out!! Thank you!

Emily - USA (27831)

In the gravitational force equation, let "m" equal the mass of one object and "5 - m" be the mass of the other object. Thus, the equation is:

F = G(m)(m-5)/R^2
Thus: F(R^2)/G = m^2 - 5m
Use the quadratic formula to solve for m.
Note that R must be in meters.

The second problem is solved by using the forces from the Earth and from the Moon on the spacecraft: GmM/r^2 = GmE/R^2
where m is the mass of the spacecraft, M is the mass of the Moon, r is the distance from the Moon to the spacecraft, E is the mass of the Earth, R is the distance from the Earth to the spacecraft. Also D = r + R the distance between the Earth and the Moon. That involves a lot of Algebra.

The net force is zero, since the gravitational forces cancel out.

I hope that helps.

## Reducing aerodynamic drag

January 28, 2017

### Question

how to reduce aerodynamic drag

fatmil - Uganda (27833)

Usually, the shape of the object determines much of the aerodynamic drag. Streamlining can reduce that drag. Also, surface conditions can reduce some of the drag.

## Misc

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