# Gre Problem # 28

1. Sep 2, 2004

### quantumworld

GRE GRE GRE... I need some help

this question is kinda of experimental question, dealing with oscilloscope, I tried to work it by calculating the beat, but it didn't work out...
may I provide the link to the question, because there is figure that follows it,
it is page 26, number 28.
http://phys.columbia.edu/~hbar/Physics-GRE.pdf [Broken]
I am really stuck at it... thought about it for a long time.

Many thanks for all efforts

Last edited by a moderator: May 1, 2017
2. Sep 2, 2004

### chroot

Staff Emeritus
You can tell the signal is one low-frequency sinusoid with another, smaller, higher-frequency sinusoid added to it. The minimum of the low-frequency component is right around 2 cm, at about 1.5 divisions (3V), while the maximum is right around 5 cm, at about 4 divisions (8V). That means the signal is 5V peak-to-peak, or has an amplitude of 5/2 = 2.5V. There is only one answer which fits this, choice D.

You can double check this answer easily. The second component certainly does have an amplitude of about 1.25V, 2.5V peak-to-peak, or a little over one division peak-to-peak. Also, there seem to be about six cycles of the high frequency component for every one cycle of the low-frequency component, so 83:500 Hz seems like the correct ratio.

You could go a step further and calculate the frequency of the low-frequency component. It appears to be about 5 cm per cycle, or about 2.5 milliseconds per cycle, or about 1/0.0025 = 400 Hz. The frequency given in choice D, 500 Hz, is probably close enough for comfort. (I wouldn't do all these checks on the test to avoid wasting time, but, if you're really not confident in your first answer, they will help.)

- Warren

3. Sep 3, 2004

### quantumworld

I could see your explanation, if I thought of the low frequency sinusoid as the average of the given graph, and the high frequency sinuoid as the wiggling of it. My problem was trying to distinguish which is which, because they are already blended.

THanks again, and please correct me if I was wrong.

4. Sep 8, 2004

### chroot

Staff Emeritus
That's a reasonable way to think of it.

- Warren