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Projectile Motion Problem. Very HARD

  1. Sep 24, 2005 #1
    Well... for me.

    A Basketball Star covers 2.80m horizontally in a jump to dunk the ball. His motion through space can be modeled as that of a particle at a point called his center of mass. His center of mass is at elevation 1.02 m when he leaves the floor. It reaches a maximun height of 1.85 above the floor and is at elevation .900 when he touches again. Determine his time of flight.

    Okay... well i usually get these kinds of problems. But I can't seem to get this one. You're (supposedly) not supposed to know anything about the center of mass. So i used the three center of mass heights as my Y component, but I still can't solve it because I dont have the initial velocity and i need that in order to use the different equations...

    HELP ME PLEASE!!! :confused: :confused: :confused: :confused:
  2. jcsd
  3. Sep 24, 2005 #2
    I'm pretty sure that the center of mass is just supposed to be your reference point. So what information is given to you from this problem? What information do you need in two dimensional motion?
  4. Sep 24, 2005 #3
    Well... I'm pretty sure that I dont need to know the center of mass yet. So I started to guess that the starting Y direction is 1.02 m and the final Y is .900 m so im guessing it's like a parabolic 2-D motion problem but the guy ends up a little further than normal. If I were given an initial velocity I think I can solve this problem, but I can't seem to get the problem going. I've tried the X = Xinitial + Vinitial * t + .5a(t)^2 but that won't work. I've been working on this problem for like 45 mins and I still Don't know how to get kinda started on it.
  5. Sep 24, 2005 #4
    I was trying to tell you that the problem is just stating .9m as the lowest point that a reference point on the player reaches, and that 1.85m is the highest point. They did that do you know that the problem wasn't starting by counting distance from his shoe and then ending up with his head.

    There is another section of physics that deals with centers of mass, in 2-d and 3-d, but I this isn't one of those problems.

    To your question.... at half the total time, what is true about the y-distance? What is true about the x-distance? What equations can you use to model two dimensional motion?
  6. Sep 24, 2005 #5


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    Homework Helper

    If two balls are shot horizontally with speeds [itex]v_1[/itex] and [itex]v_2[/itex] from the same height [itex]h[/itex], which ball will stay in the air longer (let's forget about the curvature of earth here).
  7. Sep 24, 2005 #6
    they'd be the same

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