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Homework Help: Kinetic Theory!

  1. Oct 26, 2011 #1
    I need help on question 1b here,

    1. The problem statement, all variables and given/known data

    1. In a one-minute interval, 1000 hailstones, each of mass 0.5 g and speed 10ms-1, strike a
    window pane of area 1m2 at an angle of 45°.
    (a) If the collisions are elastic, calculate :
    (i) the average force and
    (ii) the average pressure on the window pane.

    (b) How would your answer di ffer if, in Q1, instead of striking a window pane, the
    hailstones strike soft putty and are embedded in it?

    2. Relevant equations

    Δρ = (mv) - (-mv) = 2mv ??? now it is not elastic = mv?
    P = F/A = N/V (mv^2) = N/v ΔT A (mv^2) = N/ΔT (mv)
    V = v ΔT A

    3. The attempt at a solution

    I worked out that the average force is 0.0833N and the average pressure is 0.0833Pa. I don't think the force changes at all, but the change of momentum obviously does as there is no rebound. I'm stuck with this and am not sure how to find out how it would change in ai) and aii)

    Any help is very much appreciated! Thank you
  2. jcsd
  3. Oct 26, 2011 #2
    Yes if collision is perfectly elastic.
  4. Oct 26, 2011 #3
    Anybody help please? :)
  5. Oct 26, 2011 #4
    For av F try to use Newton's 2nd law.
  6. Oct 26, 2011 #5
    You already have the change in momentum of one particle.
  7. Oct 27, 2011 #6
    I need help for question 1b, nothing else.

    Thanks anyway!
  8. Oct 27, 2011 #7
    If the stones get embedded in the putty one can assume that all the initial momentum becomes zero.

    Hence change in momentum = mv for one stone, as you yourself said.
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