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: Free fall height question - true/false

  1. Jan 15, 2008 #1
    Hey guys, I'm new here and need some help with a physics homework question.

    Two students stand on top of a building of height (h). One throws a ball straight upward at the same speed as the other student who simultaneously throws a ball straight downward toward the ground. Which of the following must be true?

    Here is what I think is right so far, but I would like to double check to make sure.

    a) The balls hit the ground at the same time. - False
    b) The ball thrown downward hits the ground first. - True
    c) The balls hit the ground at the same speed. - True
    d) After both balls leave the hand, the ball thrown downward has a larger acceleration than that thrown upward. - False
    e) The ball thrown downward hits the ground before the ball thrown upward reaches its peak. - Ans. Unsure, because I'm confused on this. Doesn't it depend on the height and initial speed? Which isn't given in the problem.
    f) None of the above. - False

    Again, I'm not sure if any of my answers are correct and I need some help to double check.
    Thanks for any help in advance.
  2. jcsd
  3. Jan 15, 2008 #2


    User Avatar
    Science Advisor
    Homework Helper

    If we assume that the speed thrown upward and downward are the same and we ignore air resistance then C is true because the ball you threw up comes back past your hand at the same speed, so is equivalent to throwing it downward with that speed.

    e, It would depend on the height of the building / initial speed.
    Imagine you throw the ball up very gently so it only goes 1m above you but you are stadning on top of the Empire State and the ball you throw down falls for 10s of seconds!
  4. Jan 15, 2008 #3
    That's exactly what I was thinking. Therefor the answer to e) would be false. My other question is for part d. Does the ball thrown downward have a larger acceleration than that thrown upward?

    Because the ball thrown upward has gravity acting against it from the start, slowing it down. Where as the ball thrown downward has both the initial acceleration and gravity moving down. So couldn't part d be true also?
  5. Jan 15, 2008 #4
    as soon as they leave your hand they have exactly the same accn - g from gravity
    (come to think of it they also have same acceleration when still in your hand, but in opposite directions in that case...)

    all your answers look good to me
  6. Jan 15, 2008 #5
    Hey, Thanks a lot YellowTaxi and mgb_phys.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook