Waves: Kinds of Properties
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### Lecture Description

A wave is a strange thing. Try to answer this question: What is a wave? It is not so easy to say. The best we can do for the moment is to say this: A wave is a disturbance which transmits energy. A better attack is this: let us show some waves and seeing what a wave does will tell us better what a wave is. This we call in Physics the operational point of view.

A - We have two rubber tubes - one called "empty" - the other filled with sand. I say "called empty" because it is really NOT empty. It has air in it! One end of the tube is fixed I hold the other end. Now by a slight blow with the hand I depress a bit of the tube and we see a pulse travel along the tube to the remote fixed end. This wave is a transverse wave - where the particle disturbance is at right angles to the direction of the energy propagation. If enough energy gets to the fixed and we might have a reflection. And we see the wave travel at a certain velocity. It I pull up on the tube and increase the tension the waves travels faster. With the sand-filled tube we see the wave velocity slower. It can be shown either experimentally or mathematically the velocity of a transverse wave is given by the expression V= T/m where T is the tension and m the linear density

B- We now show a different kind of wave with a spring called a SLINKY. Here we give rise to a pulse traveling along the spring in the same direction as the energy propagation. This kind of wave we call a compressional or longitudinal wave. Sound - we shall see - is a compressional wave mechanism.

C - With a piece of blackboard chalk we show a torsional wave by twisting the cylinder of chalk. Torsional waves are important - as in of a steamship shaft which delivers energy from the driving mechanism to the propeller - or in the drive-shaft of your car - where a torque is needed.

D - A dramatic showing of the propagation of a compressional wave arises in the collision of steel spheres on a track. One sphere strikes an array at rest. The compression travels with the speed of sound in steel - about 15,000 feet per second. Pretty fast !

E - Now transverse waves possess a special character: They can be polarized -just like light can be polarized. Compressional waves cannot be polarized.

F - Water waves are especially "tricky" business. They are made up of wave motions of several types but we understand these pretty well. The expression for the velocity of a water waves comes out
\
w _ ug x lambda 7 2 pi T
2 pi lambda x d
Which LOOKS awfully rough but really is not!

G - Conduction of sound energy requires a medium. And the more elastic the medium the greater the conduction. Thus it is that a metal rod conducts a compressional wave better than a rubber tube. And this is why a stethoscope works as it does. Look at one closely.

H - If sound is a compressional wave it needs something to compress if it is to be transmitted. Accordingly a bell in a bottle can be heard if there is stuff in the bottle - air - say — but if the air is pumped out no sound can be heard. So on the Moon - with no atmosphere - how will we hear each other?

### Course Description

Demonstrations in Physics was an educational science series produced in Australia by ABC Television in 1969. The series was hosted by American scientist Julius Sumner Miller, who demonstrated experiments involving various disciplines in the world of physics. The series was also released in the United States under the title Science Demonstrations.

This program was a series of 45 shows (approximately 15 minutes each) on various topics in physics, organized into 3 units: Mechanics; Heat and Temperature / Toys; and Waves and Sound / Electricity and Magnetism.