# Search results

1. ### Medical Of Cell Phone radiation

A number of my relatives believe that radiation from cell phones is dangerous. After doing a bit of reading, I've found that the general scientific consensus is that this is not the case, and I myself don't see how low power, low frequency, non-ionising radiation can cause e.g. brain tumours -...
2. ### Is there a difference between initial speed and initial velocity?

You don't need to calculate initial velocity - it is 30 m/s at 60 degrees. But to solve your problems, you will need to calculate the vertical and horizontal components of the initial velocity.
3. ### Will an object decrease in temperature in a perfect vacuum?

How would you explain the mechanism for heating up via radiation (in a vacuum)?
4. ### Potential energy question

The net force you exert in lifting the block is equal to mg=100N up, and net force times distance h is mgh. As previously said, the block doesn't lift itself so it doesn't make sense to take the net force on the block. The work done by the block is ziltch, the work done by you in the lifting the...
5. ### Will an object decrease in temperature in a perfect vacuum?

Temperature is dependent on the kinetic energy of the particles in the object. So if the object loses energy, temperature decreases accordingly.
6. ### Faster than lightspeed

You are using Galilean transformation to add the 40 mph instead of Lorentzian transformation. Here are the wiki pages for both: http://en.wikipedia.org/wiki/Galilean_transformation http://en.wikipedia.org/wiki/Lorentz_transformation Basically (from wikipedia): ". . . Galilean transformation can...
7. ### Potential energy question

The block doesn't lift itself. In other words, it's not doing the work to increase its own potential energy.
8. ### Trying to understand black holes

Large mass gives higher gravity, it doesn't necessarily have to be in a small area.
9. ### Bike wheel question

Oh, whoops, I meant comparing, not equating, the 0.5*m*v^2 (before collision) with 0.5*I*omega^2 (after collision), where I is the rotational inertia of wheel+dart and omega can be worked out via conservation of angular momentum.
10. ### Angular momentum problem

Ok, thank you very much for the help!
11. ### Bike wheel question

What friction is this? Is it due to the fact that the dart stuck to the rim? Wikipedia says that inelastic collision is one where the two objects e.g. carts stick after collision, but I wasn't sure if that would apply here since that dart has a sharp end. Yes, this is usually what I would do as...
12. ### Angular momentum problem

Hm, well the magnitude doesn't change but wouldn't the direction with respect to the second origin be different? But, I gave it a shot (below). Okay, so let ##\vec r_2=\vec r+\vec a## and ##\vec v_2=\vec v## then ##\vec l_2=(\vec r+\vec a)\times m\vec v## and so ##\vec l_2-\vec l=(\vec r+\vec...
13. ### Bike wheel question

Homework Statement Bicycle wheel is at rest, and can rotate frictionlessly about a fixed axle. A dart travels at velocity v (in plane of wheel) parallel to a line that goes through the centre of the wheel. It hits the rim of the wheel and sticks. What kinetic energy, if any, is lost during...
14. ### Angular momentum problem

Homework Statement At time t, particle with mass m has displacement ##\vec r (t)## relative to origin O. Write a formula for its angular momentum about O and discuss whether this depends on choice of origin. The second part is what I'm more unsure of. Homework Equations The Attempt at a...
15. ### Basic Heisenberg uncertainty principle stuff

Okay, thanks for your explanations! And also for the tip for ##\hbar##.
16. ### Basic Heisenberg uncertainty principle stuff

Oh, okay, so I should be finding the y component uncertainty. So it's simple plug and chug with the equation? Is it correct to replace the inequality with the approximately equal sign, or should I be leaving it as an inequality? For question 2, then, is it a matter of finding y component...
17. ### Basic Heisenberg uncertainty principle stuff

Homework Statement A bullet is shot from a rifle. 1) if the position of the centre of mass of the bullet perpendicular to its motion is known to have an accuracy of 0.01 cm, what is the corresponding uncertainty in its momentum? 2) If the accuracy of the bullet were determined only by the...
18. ### Proof of Arccos

You might want to check again.
19. ### Michelson Interferometer

Alright, so I have spent a bit of time looking for stuff and I think I might have got it - if someone can confirm my responses that would be appreciated. So firstly, OPL is 2 mm, and there is an equation that makes part a) trivial, which is OPL = n*##lambda## where n is the number of fringes...
20. ### Michelson Interferometer

Homework Statement My personal question: What does a Michelson interferometer tell us? The actual problem: A Michelson interferometer is used to precisely measure distances of the order of 1 mm using a laser of wavelength 632.8 nm. a) If the motion of one mirror is 1 mm, what is the...

It's not a dumb question, I am also not far out of high school and can relate. Hopefully my answer can help a little. The relationships can be seen by using equations. For example, using the question you posed: You can use the equation v = u + a*t rearranged to t = (v-u) / a You know that the...
22. ### Just signed up for steam - game recommendations please!

Path of Exile, or RIFT maybe. PoE is ARPG but has got some great reviews.
23. ### Acceleration And Velocity Problem

v = u + a*t gives INSTANTANEOUS velocity at time t. But, if you use v = s/t you get the AVERAGE velocity.
24. ### How do string waves pass through each other without interfereing?

Think about it this way. You and a friend are pushing against each other with equal force (as hard as both of you can). So overall, neither of you are moving - acceleration, velocity and displacement are all zero. Now both of you suddenly slide to the right (i.e. in opposite directions). Each...
25. ### Centripetal Acceleration along a curve

You have found the magnitude of average velocity, but your answer is incomplete because velocity is a vector. a = v^2/R applies for circular motion. Is the overall motion in your question circular?
26. ### Wave frequency and velocity!

Think of a wave passing from one medium into another, and the speed of the wave changes across the boundary. Incident wavefronts hit the boundary at a fixed number per units time - so the number of wavefronts 'produced' in the second medium per unit time is also the same (because you need one...
27. ### Specific heat capacity

I googled it, as you suggested. This is the first link: http://en.wikipedia.org/wiki/Heat_capacity Taken from the second paragraph on that page: '. . . and the specific heat capacity, often simply called specific heat, is the heat capacity per unit mass of a material.'
28. ### How do string waves pass through each other without interfereing?

The wave is not reflected in your scenario - reflection occurs when the wave reaches the end of the rope, bounces back, and starts coming towards you. You are right that the waves can cancel each other out so that you'll end up with no displacement in the rope. See diagram for destructive...
29. ### Angular momentum free particle problem

Oh... derp. I feel silly for not knowing that. That makes the question so trivial now. Thanks for pointing that out to me :)
30. ### Angular momentum free particle problem

Homework Statement 'Consider an inertial frame in which a free particle travels past the origin O but does not go through it. Show by direct calculation that the particle's angular momentum about O is constant.' Homework Equations \frac{d\vec{l}}{dt}=∑\vec{\tau}...
31. ### Question in dynamics projectile motion

Hint: 1. The ball travels on a parabolic path. 2. If you know the max height of the ball, you can calculate the vertical and horizontal components of initial velocity. Or, at least, that's how I managed to do it but there may be a simpler method/formula..
32. ### Abuse of Notations?

Change in y with respect to x is a fraction Δy/Δx dx and dy respectively represent infinitesimally small x and the y value of that infinitesimally small x, so dy/dx is a fraction as well (it's the same as Δy/Δx with Δx→0 but written as dy/dx instead). That's how I think of it.
33. ### What is mass?

Bathroom scales are calibrated to Earth's gravitational field and does indeed show mass - on Earth. If you take it onto another planet it will no longer be correctly calibrated and won't show the same value until it is calibrated to the new planet's field strength. (Sorry if I used incorrect...
34. ### LaTeX for High School Students?

I recently installed The program for myself (just two days ago actually!) and to be honest I was quite lost at the beginning as I had no experience with anything like it. But this page really helped me get started on typing some maths: http://www.mecmath.net/latex-tutorial.pdf
35. ### Is pressure constant when you heat a balloon?

T\propto VV\propto \frac{1}{P}hence\frac{P_1V_1}{T_1}=\frac{P_2V_2}{T_2}So perhaps if the ratio of \frac{V_1}{T_1} and \frac{V_2}{T_2} were equal, then P_1=P_2? Like, say V_1=a, T_1=b, V_2=c, T_2=\frac{bc}{a} In words, maybe it could be explained along the lines of; as temperature...
36. ### What is mass?

As with defining a meter or a second, mass was also arbitrarily chosen. The kilogram is a block of metal that sits somewhere in France (I think). That block of metal was defined to the 'the kilogram'.
37. ### Where Physics currently stands

Thanks for all the replies! Sorry if the thread was too general - I was trying to get a bit of an idea of where studying physics might lead me, although I guess asking about 'physics' in general wasn't very helpful considering all the different branches of physics out there. Nevertheless your...
38. ### Where Physics currently stands

I am fairly new to Physics and it seems to me that there are a lot of things we know to be true. I'm interested to know the sort of concepts that are still being debated nowadays. If you as a physicist held a conversation with another physicist, would there be anything at which where you might...