# Search results

1. ### Classical Mechanics: Work

The work done on the system by gravity is W = -ΔU, whether you are moving the stone up or down a gravitational potential, you just get either a positive or negative answer. When you were performing the calculation of the work you should have noted that F.x is the kinetic energy, thus: F(xf-xi)...
2. ### Classical Mechanics: Work

When considering work, you must be careful whether you are talking about the work done on the system or the work done by the system. Both these meanings are subject to how you wish to define work, both are equally valid but you must stick to the same one. Your first statement describes the...

4. ### Doppler Effect (2nd)

not necessarily, do you have any equations with which to calculate this?
5. ### Gravitational force equation help

oops that didnt work, I mean: F = F mg = GMm/[r][/2]
6. ### Gravitational force equation help

equating: F = F LaTeX Code: mg = G\\frac{Mm}{r^2}
7. ### Gravitational force equation help

those are the force equations!

looks good to me!
9. ### Gravitational force equation help

after equating the two force equations you should have been left with: g=G*M/r^2 you need to rearrange this for r and solve it
10. ### Buoyancy with Water and Oil

If half the volume of the block was below the boundary (5cm below) the block would have a density half way between that of oil and water (i.e 0.8 g/cm^3). From the density you could easily calculate the mass, try using the same argument for the numbers in your question.

Imagine the radiation leaving the sun as the surface of an expanding sphere, calculate the surface area of this sphere when it reaches the earth's surface (i.e a sphere with a radius equal to the distance between the sun and the earth). By considering what proportion of the total surface area...
12. ### Gravitational force equation help

That is the correct answer, if you still don't understand try posting your workings and I'll show you where you have gone wrong.
13. ### Normal mode of string in third harmonic

This is just a thought that might simplify things a lot, the frequency of the wave describes how many times the wave passes a point per second, in this case an antinode. Also, each time the wave passes an antinode it has completed one full cycle from peak to trough and back to peak again.
14. ### Gravitational force equation help

Thats not right, and you still haven't expressed it in units of earth radii.
15. ### Speed physics homework

Re: Speed Got confused there, thought Stratosphere had asked the question originally. ViralRiver already gave the relevent equation (s=d/t) you ( or ViralRiver really) need to rearrange that for t.
16. ### Speed physics homework

Re: Speed You have the correct answer, first is faster, but I think the times you calculated are wrong
17. ### Gravitational force equation help

You already have the answers you need, G is a well known physical constant, get it from a textbook or find it online. And as for the mass of the rocket, equate the equations given by chrisk and you will discover why it is irrelevent.
18. ### Destroying Ourselves

I didn't really know where to post this but i thought it could be a sociological discussion so... I was reading through some of the threads in the general astronomy and cosmology section and there was something in a thread on the likelihood of extra terrestrial existence, something about it...
19. ### Ever seen a full moon in the daytime?

I could swear i have, although when i started thinking about it could happen i got confused. You can probably tell i'm a newbie as there must be some really simple explination to this problem. It might be a bit hard to explain without a diagram, but imagine a metaphorical one with the Sun on...