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Free Falling Objects - Balls with different masses being throw upward and downward

  1. Sep 24, 2012 #1
    Six balls are thrown from the top of a building. Two balls are dropped from rest, two are thrown downward with a velocity of 10 m/s and two are thrown upward with a velocity of 10 m/s. Each ball has a differing mass as well, such as the two balls thrown from rest, one has a smaller mass than the other and the same goes for the other three categories.

    I'm trying to order the balls from greatest to least velocity and acceleration from when they hit the ground. What I'm confused on is that I thought they would all have the same velocity and acceleration when they hit the ground? When I spoke to my teacher he told me there was a definite order but I remember learning in class that all objects would have the same velocity and acceleration when they hit the ground if they were thrown at the same time, regardless if they were dropped, thrown down or thrown up. Does it have something to do with the differing masses?
     
  2. jcsd
  3. Sep 24, 2012 #2
    Re: Free Falling Objects - Balls with different masses being throw upward and downwar

    Did you take into account air resistance? a larger object will have more air resistance against it than a smaller one.
     
  4. Sep 24, 2012 #3

    berkeman

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    Re: Free Falling Objects - Balls with different masses being throw upward and downwar

    Welcome to the PF.

    All the balls experience the same acceleration (neglecting air resistance) -- the acceleration due to gravity. That does not mean they will hit the ground at the same time, or at the same velocity, or with the same energy.

    What are the relevant equations? Those are supposed to be listed when you use the Homework Help Template that you were provided (and chose not to use...).

    Write the relevant equations, and start filling in values for the initial velocities, etc. Then you should be able to start answering your questions...
     
  5. Sep 24, 2012 #4
    Re: Free Falling Objects - Balls with different masses being throw upward and downwar

    I understood that part but I didn't know how that affects velocity and acceleration?

    We were not given any equations for this assignment.
     
  6. Sep 24, 2012 #5

    berkeman

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    Re: Free Falling Objects - Balls with different masses being throw upward and downwar

    Why not? Have you studied the kinematic equations of motion for constant acceleration (like from gravity)? You can find them on wikipedia.org if you want to see how to use them.

    Your teacher must have given you *some* tools to be able to answer this question...
     
  7. Sep 24, 2012 #6
    Re: Free Falling Objects - Balls with different masses being throw upward and downwar

    The only equations given to us so far are the velocity equals change in distance over change in time and acceleration is change in velocity over change in time. However, the velocity was already given for some of the balls and the acceleration formula can't be applied because we have no time or distance to calculate with. I think the question was meant to be a hypothetical type of question without any formulas.
     
  8. Sep 24, 2012 #7

    berkeman

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    Re: Free Falling Objects - Balls with different masses being throw upward and downwar

    Well, it's pretty hard to answer with confidence without using the (simple) equations of motion for falling objects. You could use your intuition, but that hasn't formed correctly yet, because you haven't learned the equations!

    We are not allowed to give you the answers here on the PF (that's against the rules) -- all we can do is give hints and make sure you are using the right equations.

    So I already reminded you that once the balls are out of the thrower/dropper's hand, the only acceleration they feel is due to the force of gravity. Do you remember the famous experiment where two different weight balls were dropped from the Leaning Tower of Piza? What happened in that experiment? So then what can you say about the two balls that are dropped?

    And then, thinking about the balls that are thrown down instead of dropped... will they hit the ground before or after the balls that are dropped (your intuition will be correct on this part)? And why do they hit the ground at a different time...?
     
  9. Sep 24, 2012 #8
    Re: Free Falling Objects - Balls with different masses being throw upward and downwar

    In the Leaning Tower of Piza experiment, the balls reached the ground at the same time, regardless of mass. The balls will reach the ground at the same time and the balls that are thrown instead of dropped will reach the ground before the balls that are just dropped because they have are thrown down at a velocity of 5 m/s. But how do I calculate the velocity at which they hit the ground? Is there a specific formula for me to use? I assumed that the balls would all have the same acceleration once they all reached the ground since the only force acting on them would be gravity but I was told that there would be a distinct order from greatest to least, which is what is tripping me up.
     
  10. Sep 25, 2012 #9

    berkeman

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    Re: Free Falling Objects - Balls with different masses being throw upward and downwar

    The basic equations are listed here for calculating the final velocity:

    http://www.physicsclassroom.com/class/1dkin/u1l6a.cfm

    .
     
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