# 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.

23,870 questions
Filter by
Sorted by
Tagged with
55k views

### 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 ...
• 1,483
12k views

### 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 ...
• 1,419
27k views

### 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 ...
• 977
22k views

### 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?
14k views

### 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 ...
• 1,315
19k views

### 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 ...
• 1,505
5k views

### Ball hits curve of same curvature [closed]

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 ...
• 628
4k views

### 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 ...
• 1,210
13k views

### 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 ...
• 1,298
21k views

### 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 ...
25k views

### 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 ...
• 356k
28k views

### 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: "...
• 1,147
17k views

### 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’...
• 645
6k views

### 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 ...
• 1,017
13k views

### Why do we need to know the shape of the slide to find the time to slide down it?

In my physics book after this solved example: A child of mass $m$ is initially at rest on top of a water slide at height h = 8.5m above the bottom of the slide. Assuming that the slide is ...
• 1,406
6k views

### How far out from the Sun is visible light still sufficient to read a book?

Recent pictures from the New Horizons spacecraft, shown below, seem to indicate that, at Pluto's distance, we are entering a twilight zone, with a distinct lack of colors, although that could be due ...
80k views

### 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, ...
• 555
2k views

### Is 13 really the answer for the "Devil's problem" in physics (a rolling tube with a rod)?

Recently I chewed the fat with a physics student and got intrigued by him mentioning "the Devil's problem," which he described as a simply worded mechanics problem that is extremely ...
• 1,599
16k views

### Which ball touches the ground first?

This is a very well known problem, but I can't find an answer in the specific case I'm looking for. Let's consider two balls : Ball 1 weighs 10 kg Ball 2 weighs 1 kg Balls have identical volumes (so ...
• 503
9k views

### Condensation of Water. Classroom Controversy

In our test there was a question that went like so: Question 4 You have a glass of iced water on an unshaded picnic table and went for a walk for 30 minutes. When you return you noticed the glass ...
10k views

### "Falling upward" - how far you have to be from Earth to start falling to the Moon?

Talking about gravity with my 9 y/o she asked when do we start "falling upward" to the Moon. What is the distance at which the Moon's gravitational attraction is higher than that of the Earth and thus ...
• 525
6k views

### How can a simple physical situation give rise to two different possible outcomes?

An object of mass $5\, \mathrm{kg}$ is projected with a velocity $20\, \mathrm{ms}^{-1}$ at an angle $60^{\circ}$, to the horizontal. At the highest point of its path, the projectile explodes and ...
5k views

### Force of photons from the Sun hitting a football field = weight of 1 dime?

I read, I think, some time ago that the "weight" of photons from the Sun hitting an area the size of a football field at noon on a sunny day would be about the "weight" of a dime? ...
• 447
4k views

### Are the units of energy the same in higher dimensions?

In 3 spatial dimensions, $$[E] = [ML^2 T^{-2}]$$ Would it change in higher dimensions? If yes, then what would be the dimensions for 4 spatial dimensions?
8k views

### Speed of sound at temperatures below 0 °C

How can the speed of sound be calculated for temperatures below 0 °C (down to -40 °C)? Does the calculation $v=331\ \frac{m}{s} + 0.6 \frac{m}{s°C} \times T$ still hold (where T's unit is °...
• 269
9k views

### Can a gym be built to supply electricity to homes? [closed]

A moving magnet induces a current in a conductor, then shouldn't we be able to generate electricity through manual labour? I was thinking about building a gym that used magnets as weights. People ...
• 1,323
34k views

### Why can't a single photon produce an electron-positron pair?

In reading through old course material, I found the assignment (my translation): Show that a single photon cannot produce an electron-positron pair, but needs additional matter or light quanta. My ...
• 293
7k views

### How many X-rays does a light bulb emit?

I read somewhere that most things1 emits all kinds of radiation, just very few of some kinds. So that made me wondering whether there is a formula to calculate how many X-rays an 100W incandescent ...
• 621
6k views

### When should you jump off a falling ladder?

If you stand on the top of a falling ladder you will hit the ground at a higher speed (and therefore presumedly sustain more injury) if you hold on to the ladder than if you jump off it. This was ...
• 535
15k views

### Why doesn't a spinning object in the air fall?

Let's say I have a ball attached to a string and I'm spinning it above my head. If it's going fast enough, it doesn't fall. I know there's centripetal acceleration that's causing the ball to stay in a ...
• 1,177
5k views

### Does bonfire night in the UK increase temperature of the country / local area?

I am not sure if this is the right forum for the question but failing to have any better location to ask it, i have come here. In the UK we have a tradition of lighting bonfires on the 5th of ...
• 281
16k views

### Lorentz Invariant Integration Measure [closed]

When we canonically quantize the scalar field in QFT, we use a Lorentz invariant integration measure given by $$\widetilde{dk} \equiv \frac{d^3k}{(2\pi)^3 2\omega(\textbf{k})}.$$ How can I show that ...
• 2,993
27k views

### "Find the net force the southern hemisphere of a uniformly charged sphere exerts on the northern hemisphere"

This is Griffiths, Introduction to Electrodynamics, 2.43, if you have the book. The problem states Find the net force that the southern hemisphere of a uniformly charged sphere exerts on the northern ...
• 430
8k views

• 4,120
9k views

### Deriving the speed of the propagation of a change in the Electromagnetic Field from Maxwell's Equations

I've been told that, from Maxwell's equations, one can find that the propagation of change in the Electromagnetic Field travels at a speed $\frac{1}{\sqrt{\mu_0 \epsilon_0}}$ (the values of which can ...
• 5,980
4k views

### Estimate the Sun's temperature, based on the duration of sunrise

The full problem statement is: Measured from the time when the first rays of sunshine appear above the horizon until the moment when the sun is fully visible, sunrise lasts 2.1 minutes. Based on this ...
• 504
49k views

### What is the force between two charged objects when the space between them is partially filled by a dielectric medium?

I am given two charged particles of same charge at a distance of $r$. They initially apply force $F$. Now an infinite dielectric (of dielectric constant $4$) of width $\frac{r}{2}$ is introduced ...
• 604
950 views

### Aether existance in alternate universe made of Bose-Einstein condensate

I came across an interesting question which was shown to me by my professor, it is as follows: Investigation of an alternative Universe: This Universe contains three spatial and one time ...
• 1,349
5k views

### Will the ball come out of the well or not?

A small ball moves at a constant velocity $v$ along a horizontal surface and at point $A$ falls into a vertical well of depth $H$ and radius $r$. the velocity of the ball forms an angle $\theta$ with ...
• 470
5k views

### Conical train wheels

I've been reading about how the conical shape of train wheels helps trains round turns without a differential. For those who are unfamiliar with the idea, the conical shape allows the wheels to shift ...
• 642
7k views

### Problem understanding the symmetry factor in a Feynman diagram

I am trying to understand a $1/2$ in the symmetry factor of the "cactus" diagram that appears in the bottom of page 92 In Peskin's book. This is the diagram in question (notice that we are ...
• 6,017
22k views

### How long does it take to travel 36 light years with tolerable acceleration and deceleration?

The recent discovery of HD85512b only 36 light years from Earth has promising attributes to harbor life. Assuming we want to travel there, we cannot instantaneously jump to light speed, (StarTrek ...
15k views

### Is this scene from the A Team explainable by Physics?

This scene (youtube link) from the movie The A-team, the four members are in the tank and its falling from the air, they fire the canon and it slows the tank from falling for a moment before falling ...
• 349
106k views

### How much energy in form of heat does a human body emit?

How much energy in form of heat does a human body emit at rest level?
• 343
595 views

### The dual of a surface element in 4-space

In reading the classic text, "The Classical Theory of Fields", Third Edition, by Landau and Lifschitz, I found an "obvious" statement not so obvious to me. It is on p.19, the statement of the ...
16k views

### Doesn't a box holding a vacuum weigh the same as a box full of air?

This was recently brought up, and I haven't been able to conclude a solid answer. Let's say we have two identical boxes (A and B...
• 355
The "universe" is a sphere with a radius of $10^{25}$ m the medium temperature is 3K, how many photons there are in the universe? n_\gamma = \int_{0}^{\infty} \frac{8h\pi\nu^3} {{c^3}{}} \...