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Free Falling Object's Initial Height

  1. Jan 28, 2007 #1
    :surprised The problem reads: A freeley falling object requires 1.50s to travel the last 30.0 m before it hits the ground. From what height above the ground did it fall?

    If anyone can please help me with this problem, I would be very grateful. Thank you in advance for your time and effort helping me with this problem!
     
  2. jcsd
  3. Jan 28, 2007 #2

    arildno

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    What equations might come in handy here? :smile:
     
  4. Jan 28, 2007 #3
    Re: What equation could be useful

    I am thinking that the Kinematic equations might be useful, but I'm not sure if they could be used, or which ones.
     
  5. Jan 28, 2007 #4

    arildno

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    Well, how is, in general, initial height, initial velocity, time, acceleration and present height related?

    Under the assumption of CONSTANT acceleration, that is..
     
  6. Jan 28, 2007 #5
    Maybe using:

    Distance (final) = Distance (initial) + [Velocity(initial)] (time) + .5(Acceleration)(time)^2 to get original velocity, and then subtract (9.8 m/s2)(1s) till hitting zero, n adding that time to the 1.5 s, and then try solving for distance?
     
  7. Jan 28, 2007 #6

    arildno

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    First of all:

    Make a list of the KNOWN quantities in that equation, and what is UNKOWN.
     
  8. Jan 28, 2007 #7
    a = 9.8m/s2
    xm = 30.0 m
    tm = 1.50s
    xf = 0.00
    xi = ?

    where i = initial, f = final, and m = somewhere in between
     
  9. Jan 28, 2007 #8

    arildno

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    So, consider now the interval from the intermediate values to the final values, regarding the intermediate values as "initial" for this particular time, then the only unkown would be the velocity the object had 1.5 seconds prior to hitting the ground.

    Agreed?
     
  10. Jan 28, 2007 #9
    Yes, that makes sense.

    So now that I know how to get the velocity (intermediate), Do I use that as the new Final velocity, put zero as the initial velocity, solve for time and then the distance (using 30.0 as the final distance) and then add the two distances together?
     
  11. Jan 28, 2007 #10
    :tongue: I think I know what to do now! Thank you for helping me!
     
  12. Jan 28, 2007 #11

    arildno

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    Just one issue:
    Remember in your second part of the problem (after you've found the velocity had at 30m height above the ground), you have TWO unkowns:

    The initial height from which the object fell, and the time that has elapsed until you've reached a height of 30meters (and having achieved the solved for velocity).

    Final note:
    Instead of adding one equation, you might solve the second part by using the relation that relates distance&velocities&time, ignoring to solve explicitly for time.

    Therefore, you'll need another equation as well, it is simplest to use the kinematic relation relating velocity&acceleration&time.
     
  13. Jan 28, 2007 #12
    :rofl: Thank you for your help! It worked!
     
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