Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Initial velocity of ball B hit a free falling ball A

  1. Jan 1, 2018 #1
    1. The problem statement, all variables and given/known data
    A boy release ball A from height 5m from ground at the same time other boy throw ball B vertically upward to exactly strike ball A. When ball B released it was 1m from ground find minimum initial velocity of ball b. Given g=10m/s^2
    a) 2m/s b) 3m/s c) 4m/s d) 5m/s

    2. Relevant equations
    x (t) = v0t+1/2at^2

    3. The attempt at a solution
    From the problem distance between each ball is 4m when it was released and both was released at same time so when it strike each other time must be equal
    Then i let x be the distance ball A travel and 4-x be distance ball B travel so i got.
    x=1/2gt^2 = 5t^2 since ball A was released it has no initial velocity
    and 4-x=v0t-1/2gt^2=v0t-5t^2
    Solve for both I got ut =4 then I stuck here then I substitude value from choice and got 5 (the exact is about 4.4) so is there any real way to solve this
    P.S. I try using energy conservation and end up with 0=0 all time.

    Sorry if the problem look weird since I translate it from my own language
  2. jcsd
  3. Jan 1, 2018 #2


    User Avatar
    Science Advisor
    Homework Helper
    2017 Award

    Hello Bond, :welcome:

    yes, your exercise text puzzles me: what if boy B just holds te ball ? Then ball A will also strike ball B exactly ?

    Or does it say that the balls hit each other exactly half way ?
  4. Jan 1, 2018 #3
    It doesn't say where they hit but I'm very sure that the ball B has to be thrown and hit ball A mid air because they were released at the same time
  5. Jan 1, 2018 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    I reckon they have to collide before ball B hits the ground. That's where the minimum velocity comes in.
  6. Jan 1, 2018 #5


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Some of your equations have ##g## in them and some don't. Is that a typo?

    You need to think about displacements rather than distances here. Also, I would set the ground as ##x = 0## and use that as my reference frame.

    That said, ##ut = 4m## is correct. Now, you need an idea about how to calculate the minimum ##u## from that. Hint: if ##u## is a minimum, what is ##t##?
  7. Jan 1, 2018 #6
    So it mean that t should be maximum which is the time ball A take to hit the ground is that correct?
  8. Jan 1, 2018 #7


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    That's right.
  9. Jan 1, 2018 #8
    Thanks a lot
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted