PHY132S Waves - Class 2 - Wednesday January 7 2009

Today we nearly finished Chapter 20, atlhough our discussion of §20.7 - The Doppler Effect will continue next class. Also, as announced, I have dropped the discussion of decibels from §20.6: this topic is not part of the syllabus for the course.

Mistake!

After the class in Convocation Hall, some bright students pointed out a mistake in the first clicker question. The wording of the question was:

A weight is hung over a pulley and attached to a string composed of two parts, each made of the same material but one having four times the diameter of the other. The string is plucked so that a pulse moves along it, moving at speed v1 in the thick part and at speed v2 in the thin part. What is v2 / v1?

1. 1
2. 2
3. 1/2
4. 1/4

Although many people said the right answer is 2, and I agreed with them, it is not the right answer! The reason is that the mass per unit length for a string of length L is equal to L * 4 pi_ r^2. So the thicker string as 16 times the mass per unit length compared to the thin string.

In the posted version of the PowerPoint, I have changed the wording to:

A weight is hung over a pulley and attached to a string composed of two parts, each made of the same material but one having two times the diameter of the other. The string is plucked so that a pulse moves along it, moving at speed v1 in the thick part and at speed v2 in the thin part. What is v2 / v1?

This new version is what was used in the evening section of PHY132S.

You may wish to know that the source of this question is a big-gun Physics professor and educator from Harvard named Eric Mazur. I should have actually thought about this question instead of just accepting his question and answer. Similarly, you should think about things that I say, since it is certain that I will be making more mistakes too.

The MP102 Section

We had some technical problems in MP102, which derailed my attempts to keep this section "in sync" with the Convocation Hall section. I will try very hard to get them back in sync next week.

We did not discuss the index of refraction (pg. 619) or the Doppler Effect (Section 20.7).