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Relative velocities of balls on reaching the ground.

  1. Jun 3, 2015 #1
    Here's a physics question that's been giving me trouble. At a height H ,a ball A is thrown upward with a velocity of 20m/s and another B one is thrown down with a velocity of 20m/s and a third ball C is just dropped. Note that it's the same height for all three.I need to find the relation between their velocities when they hit the ground.
    attempt at solving:
    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 it's definitely going to speed up, so 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? I'm stumped.
  2. jcsd
  3. Jun 3, 2015 #2
    Are the sing of ##u## the same?
  4. Jun 3, 2015 #3
    i can easily answer this question, but i can not understand that what kind of relation you want? can you please post the original question statement?
  5. Jun 3, 2015 #4
    The last part of the question says " Find the relation between their velocities when they hit the ground"
    there are options as well like A=B>C , A>B>C, A=C<B. Meaning of the course the velocity of A,B,C.
  6. Jun 3, 2015 #5


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    Try energy conservation.
  7. Jun 3, 2015 #6
    Brother this question not for solving it can easily be solved with just common sense , A is going upward with 20 m/s so after reaching H it will be 20m/s downward, so final velocity of A and B will be same, c is just dropped, and relative acceleration is 0, so difference between velocities V(a or b)- V(c)=20m/s
  8. Jun 3, 2015 #7
    how can the final velocity of B be 20m/s? it's starting with 20m/s from the top and it will speed up right? it's accelerating under gravity so it should increase from 20m/s ? please correct me if i am wrong.
  9. Jun 3, 2015 #8
    i give up. i don't know how energy conservation comes into this. Please help.
  10. Jun 3, 2015 #9
    Write the energy conservation condition for the three cases.
  11. Jun 3, 2015 #10
    i said difference between velocities, not actual velocities, we don't know time, we don't know height, so we cannot find actual velocity.
  12. Jun 3, 2015 #11
    "so final velocity of A and B will be same"
    you said the final velocity of A and B will be the same. A's final velocity is 20m/s, but B starts with 20m/s and speeds up right? so how can their final velocities be the same?
    or does the velocity of B just be constant throughout the entire downward journey?
  13. Jun 3, 2015 #12
    A will go upward, reach a certain height, calculating, h=u*u/2g= 20m
    ar an height of 20+H it will stop and start falling, when it falls 20. its velocity will be =20m/s, so both A and B will fall rom 20m/s downward.
  14. Jun 3, 2015 #13


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    Takando12, this should have been posted in the introductory physics homework section, per the rules. If you still require help, please start a new thread there.

    For the rest of you, please do not answer homework or homework-style questions that have been posted outside of the homework forums. Use the report function to bring them to the attention of the mentors.
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