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Physics Feedback

by Ron Kurtus

Readers have sent in a total of 785 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




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
Superhero trying to outrun villan Force Affects Motion UK
Weight of two containers of water Floating Egypt
Calculating speed of light for spectral line Doppler Effect Equations for Light Ireland
More electrons than protons Electrostatic Induction Phillippines
Static electricity and combing hair Electricity Canada
Reducing static on a trampoline Reducing Static Electricity Shocks USA

Next 15 letters


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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AC Wiring

Need to update home voltages

January 30, 2017


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.

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LED lights

Change frequency of LED light

January 29, 2017


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.

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Dry Ice

Using CO2 for supercritical extraction

January 29, 2017


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.

Thank you in advance,

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.

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Force Between Two Objects

Gravitational force

January 28, 2017


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.

See Law of Universal Gravitation for more information.

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.

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Fluid Friction

Reducing aerodynamic drag

January 28, 2017


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.

See Factors in Changing Fluid Friction for more information.

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Force Affects Motion

Superhero trying to outrun villan

January 18, 2017


Hi there, I am researching for a comic book and was hoping you could answer a question I have. If an object accelerates in one direction (a) then suddenly reverses applying the same force in the opposite direction (b), is it possible that if travelling fast enough the sudden change in direction (a to b) would have a spring board effect increasing the acceleration and overall speed when continuing in direction (b)? How fast would the object need to be moving? This scene will be set in space and has a superhero character that is trying to find a way of outrunning his counterpart. Many thanks.

Karl - UK (27814)


Acceleration means that the superhero is continually increasing in speed. This could happen until he reached the speed of light.

If a the same force was applied in the opposite direction, the superhero would slow down until he stopped and then started to accelerate in the opposite direction.

It would be better to suddenly accelerate in a sideways direction, which would fool the villain, who would continue going along until he adjusts his direction of motion.

I hope that helps. Best wishes for your comic book.

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Weight of two containers of water

January 16, 2017


Two containers one of them has water and small piece of wood and another one has only water which one is heavier ?

Misho - Egypt (27810)


If both containers have the same amount of water in them, then the one with the wood in it would be heavier by the weight of the wood.

However, if the containers were full to the top and the piece of wood was added to one of them, some water would overflow from the container. The weight of the overflow water would be the weight of the water displaced, such that both containers would then weigh the same.

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Doppler Effect Equations for Light

Calculating speed of light for spectral line

January 15, 2017


Hope you can help us. We are trying to calculate the velocity of a wavelength using a line profile.
We don't know what we are doing :-(
We know the rest wavelength 36.4680 nm, the line centre is 36.462 nm and c.
The anwser we are getting is 299785064.75!!!
Can the velocity of the wavelength be near the speed of light?
What is the correct formula?

Thank you on behalf of the LYIT astronomy society.

Seamus - Ireland (27806)


You need to be exact in your terminology. The "velocity of a wavelength" is incorrect. You should be looking at the velocity of light for a given wavelength.
In space, the light travels at the same speed for all wavelengths. When the radiation is passing through a material, the speed varies according to the wavelength or frequency.

If the source of the light is moving, the speed of light remains constant, but the wavelength can shift according to the direction and velocity of the source.

The profile of a spectral line is affected by the various velocities of the particles emitting that specific wavelength. The Doppler Effect causes the slight shifts in wavelength from the ideal situation.

I hope that helps.

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Electrostatic Induction

More electrons than protons

January 12, 2017


what is the overall charge of an object that has 18 more electrons than protons?

jeffrey - Phillippines (27799)


Negative ions may have a few more electrons than protons. It would be very unusual to have 18 more electrons than protons. However, since electrons have a -1 charge, the overall charge of the object would be negative 18 or -18.

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Static electricity and combing hair

January 1, 2017


Why would it be better to comb hair with aluminum comb rather than plastic comb to avoid build up of static charge?

Thank you

Sam - Canada (27770)


Human hair and aluminum combs both tend to give up electrons, while a plastic or hard rubber comb attracts electrons and thus attracts the hair. See Materials that Cause Static Electricity.

Also see Controlling Flyaway Hair from Static Electricity.

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Reducing Static Electricity Shocks

Reducing static on a trampoline

December 23, 2016


My trampoline has a safety net surrounding it.I put my water hose on jet spray and cleaned the net.That got rid of the static instantly. Now the kids can play on it in peace. The accumulation of dust on the screen was the problem.

Ernestine - USA (27578)


Thanks you for your information on solving the problem of build-up of static electricity on a trampoline.

I am sure our readers will appreciate knowing about your method.

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