# Recent content by CPW

1. ### Isotope decay via alpha and beta particle emissions

What does your textbook say happens during alpha decay? (How does the atomic number Z and the mass number A change?) What does your textbook say happens during beta minus decay? (How does the atomic number Z and the mass number A change?)
2. ### Electric field due to a flat hollow disk

I agree. Nice work. But with y axis up in your figure, shouldn't you include a minus sign in your vector answer?
3. ### Electric field due to a flat hollow disk

It appears to me you are on the right track, but is the problem asking for the E field at the center of the semicircles? Can you explain why sinθ and not cosθ?
4. ### Three charges attached to the vertices of a triangle

You will need to use math. Solving a similar example problem, with the algebra and trig being performed on symbols (saving the numerical substitution for the last step), may provide you with the insight you need for this homework problem. See this solution, and don't just read it. Instead...
5. ### B Pre 2019 definitions for an Ampere, how is this useful in practice?

Good question. Consider the size of one Newton of force: about equivalent of holding up an apple in your hand in Earth's gravitational field. A human-sized force. Is your concern about the SI definition of the unit for electrical current that the forces involved are so much smaller? Sensitive...
6. ### Curvilinear... isn't that a strange word?

Hi PF. I'm over ten years out from graduate school in physics, and I still enjoy re-reading my textbooks. I came across the word curvilinear, and thought how strange a word it is; a juxtaposition of curved and linear, which are at times understood to be opposites. The textbook was describing...
7. ### Stargazing M - M as the distance modulus, and a question about the distance ladder

Hi phyzguy. Yes. The gap in my knowlege about this topic is no longer there. I think I just needed to settle on the fact that the period - luminosity relationship of cephiad variable stars was a discovery. A revealing of a fact of nature, discovered by Henrietta Leavitt. The comments and...
8. ### Stargazing M - M as the distance modulus, and a question about the distance ladder

Thank you for the explanation.
9. ### Stargazing M - M as the distance modulus, and a question about the distance ladder

I'm attempting to understand fully the distance ladder we use in astronomy to determine the distance to stars that are too far away for parallax to work. I understand we calibrate to a standard candle data of period vs luminosity for the cepheid variable stars in a group. Then from knowing the...
10. ### Question of Rotational motion

In determining the reaction force on the cylinder by the wall, consider that there is no translational acceleration of the center of mass of the cylinder.
11. ### I A Puzzle Involving the Moon's Orbital Motion

This delightful puzzle you posted reminds me of a question I like to ask my work colleagues (non physicists): "So, the sidereal period of Earth's rotation is not 24 hours. It is 23 hours and 56 mins. Shouldn't that fact mess up our time keeping within a few days?" I'm always impressed with...
12. ### Galileo's tower of Pisa experiment

I have two textbooks from when I took this course (over a decade ago): Fowles & Cassiday and also Thornton & Marion. In the first book, the drift to the East for free fall (example calculation for experiment on Earth's surface at latitude 45 deg), is explained in terms of the Coriolis force and...
13. ### B Can a pool ball come off a cushion faster than it hits it?

There are other KE of rotation + KE of translation examples to elucidate the above explanations to your question. Consider the rolling down an incline of a sphere vs hoop vs solid disc. Thinking of that example helps me understand how a billiard ball's spin prior to collision with the cushioned...
14. ### B Radon testing my first basement (already has a mitigation system)

Just received the long-term test results: 3.8 pCi/L, just below the EPA limit of 4 pCi/L. The test kit was in the basement for 105 days, and the report states the uncertainty in the result is +/- 15%. My wife and I intend to let our kids play in this unfinished basement some. I see that there...
15. ### What is the total amount of paving in the US?

While commuting I'm listening to The Great Courses "How the Earth Works" lecture series, and in one lecture the professor stated that the amount of paving (asphalt on the ground, buildings, concrete, etc.) in the United States is now greater than the area of the state of Ohio. The lectures were...