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Answers to Readers' Questions on Physical Science:

Pulse Waves

The following 5 comments and questions have been sent in. They are listed according to date.

List of first 10 items

• Wind blows in one direction but wood floats in the other
• Is a pulse from a crowd dangerous?
• Difference between a ray and a wave?
• Trying to get rid of moles
• Duplicating a contrail

Next 10

Wind blows in one direction but wood floats in the other

Question

May 8, 2009

An observer is standing on the bank of river. The wind is blowing from left to right, and waves are moving from left to right. However the observer sees a small piece of wood floating in the middle of the river that is moving slowly from right to left.

Waseem - Qatar

17717

Answer

The wind causes the surface waves to move in the same direction as the wind -- that is, a left to right. But if the current of the river was moving right to left, the piece of wood could be affected by the current under the surface waves and move along with the direction of the water.

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Is a pulse from a crowd dangerous?

Question

August 24, 2006

for the pulses thats i sometimes generate and the effect of the crowd within my space. the realization if quite strong, but most fear and anger me and scares me all in one "boom" as u might say. is this dangerous?

John - Canada

11762

Answer

Such an effect can startle you and cause tension that can stay with you for a while. Although it is not dangerous, tension or stress can effect your health if it is long lasting.

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Difference between a ray and a wave?

Question

April 19, 2006

what is the difference between ray and wawe? plzzz.. ans me as soon as possible. thanks.

gurjinder - India

10878

Answer

A ray is a straight line from the source of a wave motion. Since waves spread out from the source, using the concept of a ray is a more convenient way to study reflections in a given direction. You do not have to worry about wavelength or frequency when plotting the motion or light or sound as a ray.

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Trying to get rid of moles

Question

December 31, 2005

I have recently been looking at an item in the Home and Garden supply store which is called the Vibrasonic Molechaser. It is a round tube perhaps one foot long with a removable cap on one end in which to insert four size D batteries. It will cause moles to leave the area of the sonic vibrations. There are three models, one which works for 1/3 acre, one for 1/4 acre and one for 1/5 acre. I have been having a mole problem in my yard for several years and I have tried many things to try to stop it to no avail. Is this just another method of wasting money or do you think that such a gadget actually can work?
Norm Ochsner

Norm - USA

9507

Answer

Most sonic bug and varmint chaser don't seem to work too well. But looking at the reviews on this device, it may do the trick for you. One person said it worked fine, but the other was trying to stop a gopher and didn't like the device for that job.

See the reviews at:
http://www.epinions.com/pr-Pest_Control-P3_International_Vibrasonic_Molechaser_P7906

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Duplicating a contrail

Question

April 12, 2005

Is it possible to duplicate the vapor cloudform sometimes caused by a plane traveling at the speed of sound by spinning a noisemaker around a 12 foot circumference at 6000 RPM's (the speed of sound)?

Jeff - USA

6637

Answer

The vapor trail or contrail from a high-flying aircraft is really formed through the injection of water vapor into the atmosphere by the exhaust fumes from the jet engine. If there is enough mixing between the cold upper atmosphere and the hot exhaust gases to produce a state of saturation, ice crystals will develop. That is what you see.

Note that as a object approaches the speed of sound, an air barrier builds up, known as the sound barrier.

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