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Homework Help: Tennis Ball Drop

  1. Oct 13, 2008 #1
    1. The problem statement, all variables and given/known data

    A 0.5 kg tennis ball is dropped from rest at a height of 5.6 m onto a hard floor.

    2. Relevant equations

    a) What is the speed of the ball at the instant of contact with the floor?

    v = m/s

    3. The attempt at a solution

    I need acceleration and I have attempted using the equation:

    v^2 = u^2 +2as to solve for a. I set the variables as follows:

    v = 0
    u = 9.81
    s = 5.6

    I got a = 8.6 as my answer.

    I either have the wrong equation or I am making a mistake on how I set it up.

    I know this is a one-dimension kinematic problem but I have been having trouble setting these up, any help in the right direction is much appreciated!
  2. jcsd
  3. Oct 13, 2008 #2


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    It seems you have some confusion. You used v, the final velocity, as 0. This doesn't make any sense. You used u, the initial velocity, as 9.81, which also doesn't make any sense.

    The usual idea for these types of equations is to use a=g=9.81m/s^2, the acceleration due to gravity. Another keyword is "dropped", which means initial velocity is zero. I don't know why you set the final velocity to be 0, but if you read the equation that's what you're trying to solve for!

    Let's try this again.

    v^2 = u^2 + 2*a*s

    u = 0 since the ball is being dropped
    a = 9.81m/s^2 since we are on earth
    s = 5.6m
    v = what we want to solve for
  4. Oct 13, 2008 #3


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    I think you have your variables screwed up. You know the acceleration a:


    This is the acceleration due to gravity. It is the same for all free falling bodies close to the Earth's surface.

    The final speed v, is not 0, I do not know why you set it as such. This is what your looking for.

    The initial speed, u is not 9.81 either. It is zero since the ball starts at rest.

    It seems you have your variables screwed up. Does this help clear things up?

    (Perfect timing, Nick!)
  5. Oct 13, 2008 #4
    Ok, I understand better what I did with the variables, I had the right numbers but not in the right places!

    After calculating using:

    a = 9.81
    u = 0
    s = 5.6
    v= ?

    I get 2(9.81)(5.6) = 109.872 but this is not the final answer, I am unsure where to take this from here but I think it has something to do with the m/s^2.
  6. Oct 13, 2008 #5


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    Check your equation. If you fill in the numbers you get:


    You have v squared. Take the square root to get the answer.
  7. Oct 13, 2008 #6
    Thank you very much!! These are the little mistakes I continually make and it is VERY helpful having a forum like this to help me get it all straight :redface:

    You rock!
  8. Oct 13, 2008 #7


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    No problem. Be sure to tell your friends to stop by if they need any help (or just want to chat about physics). :smile:
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