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  1. astrorob

    How much work done by friction? (coefficient given)

    Hi Lothar, You first need to calculate the force exerted due to friction, the equation for this can be found here: Have you been supplied with a distance that the crate is pushed?
  2. astrorob

    What color would a green cloth appear in yellow light

    What do you mean "a green cloth"? You mean when illuminated with white light the cloth appears green?
  3. astrorob

    Bats and sound pulses

    Hi m.l. In regards to a), I think you're probably correct. It seems reasonable that the larger the object, the more intense the reflected sound wave. In regards to b), I think you're also correct. If the insect is moving away, the echo returned by the object would have a lower pitch due...
  4. astrorob

    Maximum friction force before slipping occurs

    Hi again electricblue, You're going to have to show some attempt before we can help you. Hint: Use N2L and also work out the maximum frictional force before slipping.
  5. astrorob

    An Easy question about waves

    Careful here. They correspond to points of maximum amplitude but in regards to their physical positions, they must be differentiated (i.e. by the use of maxima/minima).
  6. astrorob

    An Easy question about waves

    Yes it is. Note the 2 on the RHS corresponding to 2 consecutive peaks/troughs of maxima OR minima.
  7. astrorob

    An Easy question about waves

    Hi ZaZu, I'm not 100% of what you're asking so if I'm telling you something you already know - forgive me. I presume the experiment involves counting how many maxima OR minima of a standing wave propagating between two fixed points, the distance between which you know already. In this...
  8. astrorob

    Maximum friction force before slipping occurs

    Hi electricblue, To make you understand this concept, consider a block placed on an angled slope. The block can either; 1. Slide (the frictional force exerted on the slope by the contact surface of the object < the force component due to gravity acting on the block parallel to the...
  9. astrorob

    Sooper easy question

    I have to ask, what are you trying to do? "uhmm, say I have half of a parabolic curve, how do I turn it into a straight line?" That doesn't really make sense.. Are you trying to find the gradient of the curve at a point?
  10. astrorob

    Capacitor Being Discharged

    Firstly, be careful of the equation you use. The equation you state is for charge - but you're interested in current. In this case you're safe since the equations are exactly similar, but you might faulter in the future if you're not careful.. It may be useful to rearrange the equation before...
  11. astrorob

    How far apart are two stars

    Remember, as mgb_phys stated, Rayleigh's Criterion expresses the angular distance in radians. If you're still getting the incorrect answer I suggest you explicitly post how you're calculating the distance.
  12. astrorob

    Critical angle/ last angle of refraction

    If you don't know what angle its entering the medium relative to the normal then how are you supposed to know if it will internally reflect? It could either exit the medium on the second face or bounce around inside for a while... As a best guess, and this seems so entirely unphysical that it...
  13. astrorob

    Critical angle/ last angle of refraction

    I don't understand how you're expected to solve that without the incident angle and also the angle at which the glass is sloped on the left hand side.
  14. astrorob

    Critical angle/ last angle of refraction

    Are we told the angle at which the light is incident on the glass? Is it incident normally (at 90 degrees)?
  15. astrorob

    J shock in a gas for gamma = 7/5

    Hi oro, Apologies but I don't quite understand what you're question is. If you wanted to solve the Rankine-Hugoniot condition for energy flow for something other than a monatomic gas, what's the problem with just plugging in a different value of \gamma into equation 3 in your list?
  16. astrorob

    Parabolic Problem

    Apologies for stepping on your toes lowly, but i'll post anyway seen as how i spent the time to write this out! I'd approach this problem as follows: 1. Consider the vertical component of flight. The speed at which it initially travels vertically (at t=0) will be equal to \nu=35sin\theta...
  17. astrorob

    Reflection and Refraction, much index of refraction

    Yes, that's correct as you've stated. It also gives you the answer to your second question as n_2 represents the refractive index of the medium that the light traveling in medium n_1 is incident on.
  18. astrorob

    Reflection and Refraction, much index of refraction

    Use Snell's law: n_1\sin\theta_1 = n_2\sin\theta_2\ . Note: It's not additive like you suggested. At the critical angle, \theta_2 is 90 degrees (the light refracts along the boundary). That is to say, it's sin is 1. We can therefore rearrange for the critical angle...
  19. astrorob

    Thermodynamics ice cube question

    Consider: and also the URL I gave you in my first post. We separate this problem into stages: Energy is required to raise the temperature of the solid, in this case the ice, from -8 to 0 degrees celcius. This is stage A-B...
  20. astrorob

    Calculate The Surface Temperature of Mercury

    TFM, It's asking you to find the surface temperature. When it references Mercury-Sun distance it's probably referencing the distance between their COMs.
  21. astrorob

    Thermodynamics ice cube question

    Yes, you are. Now you have to also consider the energy required to change phase (ice to water, water to steam).
  22. astrorob

    Thermodynamics ice cube question

    Consider the stages that the ice must go through to become water. Energy is required to raise the temperature, and also to change phases:
  23. astrorob

    Optics D:

    Consider this:!Diffraction_grating_principle.png Q: At what angle, \beta (equal to \vartheta in your equation), would the diffracted light be no longer incident on the screen? Consider the sin of this angle and you...
  24. astrorob

    Intensity-wavelength graph for emission spectra; why is frequency in the x-axis?

    Just remember how frequency and wavelength are linked... The choice is relatively arbitrary.
  25. astrorob

    Spectra Y-axis

    Different designs of spectrograph will record different intensities for the same wavelength band for the same source - this is due to the sensitivities of each instrument (through CCD efficiency etc..) and also the quality of observation time (spectroscopic, twilight, night etc..) unless you're...
  26. astrorob

    Wave and measurement question.

    Sometimes it's best not to go searching for the answers after handing in an exam paper but if you still want to; [Broken] I'm thinking any number of rational answers. Maybe inherent errors in the measuring apparatus (equal to one unit) or the...
  27. astrorob

    Superwind phase in stellar evolution

    I think it just means that the bound system becomes energetically unfavourable in favour of the unbound? That is to say, the envelope would prefer to be ejected..
  28. astrorob

    Superwind phase in stellar evolution

    I'm not sure this mechanism is fully known, however during the AGB phase we observe that some stars begin to pulsate (Mira variables). The systematic compression and expansion causes material to "bunch up" making the formation of molecules (and dust etc.) easier. These trap the outgoing flux and...
  29. astrorob

    Angle deflection from gravitational lenses

    I'm thinking it's probably the latter..
  30. astrorob

    Convective cores - why?

    Hydrogen burning processes in large stars, as you've pointed out, is dominated by the CNO cycle. This is confined to the central regions of the star and so there's a large energy flux which naturally favours a convective central region. The resulting steep radiative gradient towards the...
  31. astrorob

    Galactic Dynamics (spherical potential)

    Aha, sorry! I don't post on there, I've just read a few threads like
  32. astrorob

    Galactic Dynamics (spherical potential)

    Interesting question, I'm wondering if you'd need to know how isolated the planet is since the potential will be the cumulative effect of all bodies in proximity.. That's to say, the mass distribution mightn't be symmetric, but by some random chance the potential is due to the presence of...
  33. astrorob

    Coherence light

    Yes, it's the only law you'll need to solve this. How?
  34. astrorob

    Coherence light

    As sx70 said, you really should show your working. For further reference, look here:'s_law
  35. astrorob

    Question about physics of light / light travel

    The brightness of a bulb at distance D from the source is equal to: B=\frac{L}{4\pi{D^2}} The distance is indeed ~52 foot, B = 5.8x10^-4 W/square foot. It may bode you well to convert to metres from feet first though!
  36. astrorob

    Help needed please

    No problem! Memorising those five equations of motion (listed under classic version) will enable you to solve any further questions like this, I strongly advise it!
  37. astrorob

    Help needed please

    Correct! It may be nicer to convert it into minutes (but this is not necessary!) that is, 780/60 = 13 minutes. Either way is right so long as you state the units.
  38. astrorob

    Help needed please

    Yes, The 419.91 figure is in seconds, and the 6 is in minutes. These need to be the same unit.
  39. astrorob

    Help needed please

    correct, now the question asks for how long it takes for the train to travel 10km (10000m) We know it takes 6 minutes to accelerate traveling 3000.24m in the process. So all that's left is to travel ~7000m at a constant speed. We use: distance / speed = time we know the distance...
  40. astrorob

    Help needed please

    Correct, now use this in the equation: s = 0.5at^2 You now know a (0.046) and t (360). This will give you the distance traveled in the accelerating leg of the journey.
  41. astrorob

    Help needed please

    Compare these two. It should be 16.67 / 360.
  42. astrorob

    Help needed please

    60km/h = 60'000/3600m/s ~= 16.67m/s - you actually stated this in your initial post :) Then divide through by the 360 to get the acceleration.
  43. astrorob

    Help me….my hard disk is damaged

    Assuming it is spinning, i'd just check your connections if you're getting a disk boot failure error. Sometimes they can become loose due to vibrations etc inside the case (I've had to refix my SATA cables back to my hdd and motherboard a couple of times in the past few years!) Failing that...
  44. astrorob

    Help needed please

    Yes, it's always wise to convert units you're given into SI units.
  45. astrorob

    Resistance from I vs. V graph problem

    Since the gradient's defined as \frac{\delta{y}}{\delta{x}}, if your y-axis represents voltage and your x-axis represents current then the gradient will simply be the resistance. If your axes are reversed, then indeed it will be the reciprocal of the gradient. In terms of non-linear plots...
  46. astrorob

    Celestial Mechanics This may help you. N.B. You don't need any of those equations apart from Einstein's.
  47. astrorob

    Beta minus decay?

    You're welcome.
  48. astrorob

    Beta minus decay?

    hasnain, You are correct. There would be only a small range of energies the electron could carry away and this would manifest itself (essentially) as a straight line at x=0.78.
  49. astrorob

    Beta minus decay?

    hasnain, Beta decay involves the transition of a parent nucleus into a daughter nucleus with two sideproducts; an electron and an antineutrino. Since the nucleus is so massive, it carries little kinetic energy away from the reaction (it recoils very little), the vast majority of the...
  50. astrorob

    Adiabatic Hot-Air Balloon Rising Question

    Hi again, What exactly is the question?