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Homework Help: Ball with maximum velocity on reaching the ground.

  1. Jun 3, 2015 #1
    1. The problem statement, all variables and given/known data
    A ball A is thrown upward to a height H with a velocity of 20m/s, another ball B is thrown down from H with a velocity of 20m/s and a third ball C is just dropped from the same height. Find which ball has the maximum velocity when it hits the ground.

    2. Relevant equations
    v=u+gt , s=ut +1/2gt2

    3. The attempt at a solution
    A - The velocity on hitting the ground must be the same as the projected velocity. So I think it's 20 m/s
    B - Since it starts with 20m/s from top ,and it's going to speed up because of gravity, it's reasonable to assume that it's final velocity will be greater than A's.
    C-This is where i'm confused. Using the formula v=u+gt and u=0 . v=gt .So the final velocity of C is solely dependent on t. Taking the falling part of A's journey, u=0 and v= 20m/s( from first assumption) using v=u+gt. gt=20 . Is the time for the falling of A and C the same and so the answer is A=C<B ?How do i proceed?
  2. jcsd
  3. Jun 3, 2015 #2
    yesterday i told you the solution to this question. i think that you don't get that.
  4. Jun 3, 2015 #3
    clear my point that, A is thrown from H or thrown from ground to H? if it is thrown from ground to H then, final velocity will be 20m/s, using formula we can get that H=20m(u^2/2g=h), by using (2gh)^.5 we can calculate final velocity of C, and v*v-u*u=2gH, you can calculate final velocity of B, no need to find time. please just put values and solve, and it think there should be no confusion
  5. Jun 3, 2015 #4
    There is no confusion at all. Firstly, the answer you said yesterday was that the velocity of both A and B on reaching the ground is 20m/s right? that is what is confusing.
    using the method you yourself just gave.
    A- H=20m v=20m/s
    B- v2-u2=2gh
    v2= 2*10*20 +400
    v= 28.2 m/s
    C- u=0m/s
    v2= 2*10*20
    v= 20m/s
    it's A=C<B right? correct me if I am wrong.
  6. Jun 3, 2015 #5
    and yes A is thrown from the ground to H.
  7. Jun 3, 2015 #6
    i mis under stood the question, i think you told me that A is thrown from height H, but today you said that A is thrown TO height H, there is much difference in between them. if TO is used than this is the correct answer, if From is used than yesterday post is correct answer
  8. Jun 3, 2015 #7
    the question is solved
  9. Jun 4, 2015 #8
    It's a mistake on my part. I never knew the question was not clear.I had this answer in the old question, it was on my mind from the beginning but then you said the opposite and i was completely confused the whole of yesterday. I re posted it today with the edit.
    Well thank you Mr.Sagar and yes it has been solved. CHEERS!!!
  10. Jun 4, 2015 #9
    and btw it's not "brother" like you called me yesterday. :smile: I'm a girl.
  11. Jun 4, 2015 #10
    ohh sorry, *sister*
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