# Physics Feedback

by Ron Kurtus

Readers have sent in a total of 846 comments and questions on Physics issues. They are listed according to date.

## List of next 15 letters

### Country

How are water molecules able to escape gravity? Evaporation USA

find the slope of the pendulum graph Demonstration of a Pendulum Ghana

Center of gravity when standing on a ball Center of Gravity UK

Rural echoes Sound Echoes USA

Rubbing polycarbonate and polypropylene Static Electric Materials USA

Designed to travel faster than sound Traveling Faster than Sound USA

Rolling friction and area of contact Rolling Friction India

I need a project on physics General Nigeria

Device using small disc magnet General Canada

Electrostatic induction and rice General India

Reflecting sound from concrete wall Sound and Obstacles India

Would the Gravity book be helpful? Work Against Gravity USA

What if matter did not change its state? Changing States of Matter Pakistan

Is pressure distributed on a bench? Pressure USA

Permission to use images Pressure and States of Matter USA

Next 15 letters

First 15 letters

## How are water molecules able to escape gravity?

September 22, 2017

### Question

How are water molecules able to escape gravity? In other words, gravity withholds H2O from leaving the surface of the earth, but that same body of water evaporates and those molecules are unaffected by gravity.

Travis - USA (28256)

With water, the velocity of the molecules is not great enough to overcome their molecular attraction, and thus the material is a liquid that is kept toward the surface of the Earth. However, when H20 becomes a gas, the velocities of the molecules is much greater, such that they move about freely.

Most of those molecules velocities that are low enough to be constrained to the Earth's atmosphere. But some of the molecules of the gas or steam become accelerated due to radiation from the Sn, such that they have enough kinetic energy or velocity to escape the Earth's gravity.

Fortunately, the loss of water vapor molecules into space it very small.

## find the slope of the pendulum graph

September 21, 2017

### Question

Set up a pendulum of length: l=100.0, l=90, l=80, l=70 displace the bob vertical about its equilibrium determine the time, tandperiodic time, T for 20 oscillations and plot a graph with T2 as ordinate and L as abscissa determine the slope s of the graph.evaluate P=(4(22/7)(22/7))/s

Josiah - Ghana (28254)

You need to look at the equations in Equations for a Simple Pendulum and input your values to make a graph.

## Center of gravity when standing on a ball

September 20, 2017

### Question

If we are standing upright on the ball of one foot where would our centre of gravity be on our foot?

Athos - UK (28251)

The centre of gravity would be somewhere in the centre of the person. However, there would be a ling going from the COG to the point where the foot meets the ball.

## Rural echoes

September 18, 2017

### Question

On a nostalgia site about Freeport, Texas--where I was a child sixty years ago--someone mentioned that she remembers just going outside and shouting to hear an echo. But that no longer works. I remember being able to do that as a child, too. Can someone say a little about how echoes work. Would I be more likely to be able to hear an echo in a rural setting than in an urban one?

Cece - USA (28239)

Typically an echo is sound reflected off a flat wall. This can happen in hallways in some buildings or outside a wall. Echoes can also be heard when sound is reflected off a cliff in a mountainous area.

However, weather conditions are cause echoes in a rural area, such as a fog bank across an open field.

Another factor is that children seem to have more sensitive ears, such that they can hear sounds and echoes that an adult cannot.

## Rubbing polycarbonate and polypropylene

September 14, 2017

### Question

Will Poly-carbonate and polypropylene create static when rubbed together?

Jason - USA (28236)

It is possible that some static electrical charges could be produced, however the effect would be minimal.

## Designed to travel faster than sound

September 12, 2017

### Question

How are objects designed to travel faster than the speed of sound?

Ali - USA (28230)

Aircraft that are designed to travel faster than sound usually have a relatively sharp nosecone to better cut through the air resistance and reduce the shock wave. Also, they have their wings slopped back to reduce that resistance and allow the air to flow around them.

## Rolling friction and area of contact

September 11, 2017

### Question

Is rolling friction depend upon area of contact
How to prove that if yes or no with an
Explanation

Rohan - India (28226)

The coefficient of rolling friction, ?r, isn't a simple number as in the case of sliding friction for hard materials. Instead, it is proportional to the width of the wheel and inversely proportional to the radius. In the case of soft wheels and tires, ?r the surface area of the tire on the ground is also a factor.

See: Coefficient of Rolling Friction.

## I need a project on physics

September 8, 2017

### Question

I need project on physics

Abubakar - Nigeria (28222)

See our short list of Physics Experiments for some examples and ideas.

## Device using small disc magnet

September 6, 2017

### Question

Good morning Ron. My son and I are developing a low tech medical device that relies on the attraction between a small disc magnet 20 mm x 2 mm and a similarly sized flat washer. We're having a problem with the amount of attraction between the two and need to increase it and are thinking about increasing the size of the magnet or the washer but are constrained in doing this due to physical dimensions. Is there anything readily available such as a flat washer or disc with a higher than normal iron content that would cause the magnet to be more strongly attracted to it? Many thanks!

You might try a Neodymium disc magnet. They are small, strong and inexpensive and can be bouht through Amazon.com or other outlets.

I hope that helps. Best wishes for success with your device.

## Electrostatic induction and rice

September 5, 2017

### Question

Hello sir...

Electrostatic induction or electrostatic generator that can stick with plain dry rice or not ???

daksha - India (28213)

An example of rice being affected by static electricity can be explained in Rice grains sticking to a polythene bag.

## Reflecting sound from concrete wall

September 3, 2017

### Question

what frequency can a concrete or a stone reflect back.

Kapil - India (28210)

Within the range of human hearing (20 Hz - 20,000 Hz), the higher frequencies are reflected much better than the lower frequencies. Of course, much depends on how smooth the concrete surface is.

## Would the Gravity book be helpful?

September 1, 2017

### Question

Hi, I will be working on a physics project this fall semester and was wondering if your book would be helpful in assisting with the project and/or project ideas.

Thank you,

Ashley - USA (28207)

The advantage of the book is that everything is in one place, such that it is easier to navigate. However, all the material on Gravity and Gravitation is on the website. You can pick and choose what you want to see.

Best wishes for success in your Physics project.

## What if matter did not change its state?

August 25, 2017

### Question

Explain that what happened if there would be no change in states of matter?

Thanks

Aamish - Pakistan (28197)

If all matter stayed in its existing state or phase, ice would not melt to become water, and water could not boil into steam. Also, there would be no evaporation of water. Just these factors alone would affect living beings in a short time.

## Is pressure distributed on a bench?

August 15, 2017

### Question

This article of you on pressure helped me greatly. But I still have some confusions. Such as-

If we apply force with the surface of our hand on a big bench then will the force be distributed throughout the whole area of the bench or it will just act on the portion of the bench equal to the area of our hand?

Dise - USA (28183)

The pressure on the bench will be just on the area where you push with your hand. Think of how a pillow will compress where you push your hand on it.

However, the force you apply can move the whole bench.

## Permission to use images

August 14, 2017

### Question

I would like to use the phase diagram figures that you have on this page: http://www.school-for-champions.com/science/matter_states_pressure.htm

in an article that I am writing for the Health Physics Society. I understand that I need permission to use the figures. Is that possible that I could have permission to use your figures?

Thanks so much,

Linda - USA (28181)

Yes, you have permission to use the diagrams provided you cite the source.

Best wished for success in your article. Let me know when it is published.

## Misc

Do you have any questions, comments, or opinions on this subject? If so, send an email with your feedback. I will try to get back to you as soon as possible.

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