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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.

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149 votes
10 answers
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 ...
InfZero's user avatar
  • 1,483
93 votes
5 answers
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 ...
JadaLovelace's user avatar
  • 1,419
77 votes
8 answers
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 ...
meow's user avatar
  • 977
72 votes
2 answers
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?
user avatar
71 votes
3 answers
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 ...
Josh's user avatar
  • 1,315
65 votes
2 answers
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 ...
MadScientist's user avatar
  • 1,505
63 votes
8 answers
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 ...
pzach's user avatar
  • 628
63 votes
0 answers
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 ...
Alexander Cska's user avatar
58 votes
15 answers
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 ...
CaptainCodeman's user avatar
57 votes
9 answers
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 ...
John Feminella's user avatar
55 votes
2 answers
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 ...
John Rennie's user avatar
52 votes
5 answers
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: "...
user3932000's user avatar
  • 1,147
52 votes
7 answers
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’...
Neox's user avatar
  • 645
44 votes
12 answers
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 ...
Rahul R's user avatar
  • 1,017
42 votes
7 answers
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 ...
pcforgeek's user avatar
  • 1,406
42 votes
4 answers
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 ...
user avatar
42 votes
6 answers
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, ...
user34032's user avatar
  • 555
41 votes
3 answers
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 ...
Mitsuko's user avatar
  • 1,599
40 votes
6 answers
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 ...
FlipFlapFlop's user avatar
40 votes
10 answers
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 ...
class_question's user avatar
40 votes
8 answers
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 ...
rafb3's user avatar
  • 525
37 votes
9 answers
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 ...
Tanisha Daharwal's user avatar
33 votes
3 answers
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? ...
lee herfel's user avatar
30 votes
2 answers
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?
Shubham Gothwal's user avatar
30 votes
4 answers
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 °...
user40343's user avatar
  • 269
29 votes
6 answers
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 ...
masterwarrior123's user avatar
29 votes
8 answers
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 ...
Andrey's user avatar
  • 293
29 votes
3 answers
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 ...
wythagoras's user avatar
29 votes
4 answers
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 ...
cvr's user avatar
  • 535
28 votes
4 answers
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 ...
rb612's user avatar
  • 1,177
27 votes
4 answers
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 ...
user202944's user avatar
27 votes
1 answer
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 ...
rainman's user avatar
  • 2,993
27 votes
3 answers
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 ...
Deven Ware's user avatar
26 votes
3 answers
8k views

Why is the ideal gas law only valid for hydrogen?

I got this question in school: Explain, based on the properties of an ideal gas, why the ideal gas law only gives good results for hydrogen. We know that the ideal gas law is $$P\cdot V=n\cdot R\...
jng224's user avatar
  • 3,688
26 votes
5 answers
21k views

Is it possible to shoot bullets in space or would the recoil of the gun be too strong?

I've read a few articles that say that astronauts have already brought guns in space and that shooting bullets in space is possible. But won't the recoil of the gun be too strong? Law of ...
Quantum Force's user avatar
26 votes
7 answers
13k views

Why does gravity act at the center of mass?

Sorry if this is a trivial question. Why does gravity act at the center of mass? If we have a solid $E$, shouldn't gravity act on all the points $(x,y,z)$ in $E$? Why then when we do problems we ...
Ahmed S. Attaalla's user avatar
26 votes
2 answers
5k views

Quantization of a particle on a spherical surface

Suppose we have a particle of mass $m$ confined to the surface of a sphere of radius $R$. The classical Lagrangian of the system is $$L = \frac{1}{2}mR^2 \dot{\theta}^2 + \frac{1}{2}m R^2 \sin^2 \...
Jiang-min Zhang's user avatar
26 votes
3 answers
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 ...
Justin L.'s user avatar
  • 5,980
25 votes
2 answers
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 ...
Bard's user avatar
  • 504
25 votes
4 answers
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 ...
Sigma's user avatar
  • 604
25 votes
1 answer
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 ...
Spoilt Milk's user avatar
  • 1,349
24 votes
8 answers
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 ...
ramsay's user avatar
  • 470
24 votes
6 answers
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 ...
Rations's user avatar
  • 642
24 votes
3 answers
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 ...
Yossarian's user avatar
  • 6,017
24 votes
3 answers
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 ...
user avatar
23 votes
5 answers
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 ...
Aasim Azam's user avatar
23 votes
4 answers
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?
user70808's user avatar
  • 343
23 votes
2 answers
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 ...
user avatar
22 votes
18 answers
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...
Chris's user avatar
  • 355
22 votes
2 answers
9k views

How to calculate how many photons are in the universe?

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}{}} \...
physnolimits's user avatar

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