Physics 10
Problem Set #4
Due Thursday, July 28
(c) Bill Watterton
As always, please answer these questions on a separate sheet of paper, and please
write "Problem Set #4" at the top.
Question #1:
Why are there two high tides each day? (A diagram will help make your answer more clear…
and in all likelihood, you will find that you have to explain why tides exist in the first place.)
Bonus bragging rights if you can explain how Galileo misunderstood the phenomenon of tides.
Question #2:
Sketch how the Earth, moon, and sun are oriented with respect to one another:
a) At full moon.
b) At new moon. (That is, when we can see no lighted part of the moon from Earth.)
c) At half moon.
Question #3:
Some people will tell you that on days near the full moon, both high and low tides are more
extreme. Is this just an urban legend, or are they correct? (Hint: your sketches from Question #2
will be very helpful here.)
If they are correct, then at what other point in the moon's phase cycle will the tides be extremely
strong?
Question #4:
Suppose that four Amperes of current are flowing from the positive terminal of your 12-volt car
battery, toward your left headlight.
a) Which direction are electrons actually flowing through that section of wire?
b) If you were going to try to figure out how many electrons are flowing through the wire per
second, which number from the above paragraph would be more important: "12 volts" or "4
amps"? Briefly explain why.
(You do not need to actually calculate the number of electrons, though if you want to, you can. If
you choose to try, you will have to look up (or remember from lecture) one additional important
fact about electrons.)
Question #5:
You are listening to an opera vocalist. If the frequency of the sound he is producing gradually
increases, what change will you notice in the way his voice sounds to your ears and/or brain?
What changes would you notice if, instead, the amplitude of his sound were to gradually increase
while the frequency stayed the same?
Question #6:
You have a know-it-all uncle who tends to get everything wrong.
(My nieces tell me they've got one of those, but oddly, I've never met the guy...)
On this occasion, he is talking to you about an old TV set with a cathode-ray tube monitor. "So
electrons are emitted near the back of the CRT," he explains, "and the magnets on the sides of the
tube magnetic fields accelerate them to high speeds."
As usual, he is wrong. But can you explain how you know he's wrong in this case? And if it's not
magnetic forces that speed up the electron beam, then what kind of force is it? And finally, if the
magnets aren't there to speed up the electrons, then what are they for?
Question #7:
"Radio waves travel a lot faster than most other sound waves," that same uncle continues to
explain. "This is because radio waves have an exceptionally short wavelength; it's also the
reason why radio waves can travel in outer space, but other sound waves cannot."
Identify at least two mistakes your uncle is making in this rant? (It should be fairly easy, since
he's making a lot of them.) Can you also identify at least one thing he said which is at least
partly true? (This is not so easy...)
Question #8:
Which travels faster: high-frequency sound waves, or low-frequency sound waves? Can you
propose a simple experiment that would check whether your answer is correct?
Question #9:
Briefly describe the experiment (conducted by Ernest Rutherford) that proved that the atomic
nucleus existed. (In other words, the evidence that overturned the “plum pudding” model of the
atom.)