Questions tagged [homework-and-exercises]

Applies to questions of primarily educational value - not only questions that arise from actual homework assignments, but any question where it is preferable to guide the asker to the answer rather than giving it away outright. Please READ THE GUIDANCE IN META before asking homework-like questions.

A detailed explanation of what is and is not a homework (or homework-like) question and how to ask one is available on this meta thread; what follows is a short summary.

What is considered a homework question here?

A "homework question" is any question whose value lies in helping you understand the method by which the question can be solved rather than getting the answer itself. This includes not just questions from actual homework assignments but also self-study problems, puzzles, etc.

Can I ask a homework question here?

Yes, but there are a couple of things you need to make sure of first.

As a general rule, we do not discourage homework questions as long as they are related to physics. But do keep in mind that Physics Stack Exchange is not primarily a homework help site; it's a place to get specific conceptual physics questions answered. Thus, not all homework questions are on topic, and you should check the meta thread to see how to ask an on-topic one.

Additionally, if your question is part of a homework assignment or other such work under an honor code, you should check with your institution and teacher whether it's OK to ask for help online and to post the problem on a public site.

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Why is my hand not burned by the air in an oven at 200 °C?

I have this problem from University Physics with Modern Physics (13th Edition): The inside of an oven is at a temperature of 200 °C (392 °F). You can put your hand in the oven without injury as long as you don't touch anything. But since the air…
InfZero
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Does a gun exert enough gravity on the bullet it fired to stop it?

My question is set in the following situation: You have a completely empty universe without boundaries. In this universe is a single gun which holds one bullet. The gun fires the bullet and the recoil sends both flying in opposite directions. For…
JadaLovelace
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7 answers

Does a thrown ball have kinetic energy at the top of the curve?

I'm going through physics with my 5th grade child. There is a question and answer that indicates that a airborne ball at the top of the trajectory does not have kinetic energy. The diagram below shows the path taken by a ball after it was kicked.…
meow
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How to apply the Faddeev-Popov method to a simple integral

Some time ago I was reviewing my knowledge on QFT and I came across the question of Faddeev-Popov ghosts. At the time I was studying thеse matters, I used the book of Faddeev and Slavnov, but the explanation there is not very transparent, specially…
Alexander Cska
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How to measure the wavelength of a laser pointer?

I'm working on something and I need to know the wavelength of the laser pointer that I'm using. Can you suggest me a way, using some optics formulae, or anything else to calculate the wavelength?
user82920
71
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3 answers

How big would my telescope have to be if I wanted to see the Mars rover from my backyard?

I imagine that with a big enough telescope, I would be able to zoom in and see the Mars rover in enough detail to make out the details (like the wheels, cameras, etc.). How large would the telescope have to be? (or how can I calculate this value?)
Josh
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How do I construct the $SU(2)$ representation of the Lorentz Group using $SU(2)\times SU(2)\sim SO(3,1)$?

This question is based on problem II.3.1 in Anthony Zee's book Quantum Field Theory in a Nutshell Show, by explicit calculation, that $(1/2,1/2)$ is the Lorentz Vector. I see that the generators of $SU(2)$ are the Pauli matrices and the generators…
MadScientist
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8 answers

Ball hits curve of same curvature

I was doing some physics problems for homework and, while procrastinating, I came up with a theoretical scenario that I couldn't figure out the result of. The following is from a side view and in a frictionless, ideal environment: A ball moves…
pzach
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When a balloon pops and lets a brick fall, where does the energy come from?

Let's say a scientist attaches a 1 kg brick to a large helium inflated balloon, lets the balloon go, and then it reaches an altitude of 10 000 meters before it pops, dropping the brick. The brick falls and hits the ground with with a kinetic energy…
CaptainCodeman
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Are we slightly lighter during the day and slightly heavier at night, owing to the force of the Sun's gravity?

Using $g = \frac{Gm}{r^2}$, the force on a point mass located at 1 AU from the Sun ($m = 2 \cdot 10^{30} \text{ kg}$) is about ~0.006 N/kg. Does that mean that, e.g., a 70 kg person is ~42g lighter during the day, and ~42g heavier at night? That…
John Feminella
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How long would it take me to travel to a distant star?

Suppose I wanted to travel to one of the recently discovered potentially Earth-like planets such as Kepler 186f that is 490 light years away. Assuming I had a powerful rocket and enough fuel, how long would it take me?
John Rennie
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54
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7 answers

Solving the helicopter hanging cable problem through differential analysis

The above problem is taken from the 2014 $F=ma$ contest. This question generated a great deal of controversy. At least two test takers challenged the answer, one who even tried to do the experiment. I’m told that several different “Ph.D” physicists…
Neox
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5 answers

Freefall into snow

In the movie Frozen, the following dialogue takes place: Anna: "It's a hundred-foot drop." Kristoff: "It's two hundred." Anna: "Okay, what if we fall?" Kristoff: "There's 20 feet of fresh powder down there. It will be like landing on a pillow...…
user3932000
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Is quantum mechanics applicable to only small things?

There was a fill-in-the-blank question in my university test. It was something like: Quantum mechanics deals with ____ I wrote "everything" and my lecturer gave me no marks. He was expecting something like "small", "nano" or something. I tried to…
Rahul R
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How to get the position operator in the momentum representation from knowing the momentum operator in the position representation?

I know that $$\tag{1}\hat{p}~=~-i\hbar \frac{\partial}{\partial x}~.$$ How can I get $$\tag{2}\hat{x}~=~i\hbar \frac{\partial}{\partial p}~?$$ I think this simple and I'm just over thinking it, but knowing the momentum operator (1), how can I get…
user34032
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