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Free Fall Problem

  1. Feb 12, 2016 #1
    1. The problem statement, all variables and given/known data
    Calculate the length of time it would take for an object to fall 10.0m if g were one sixth the value of earth's g.

    g=1/6(-9.8)= -1.633333333 m/s^2

    a=-1.633333333m/s^2
    displacement=10.0m[down]

    2. Relevant equations
    Maybe one of the big 5 kinematics equations? Or maybe a=change in velocity/time.

    3. The attempt at a solution
    I didn't have enough variables to use any of the listed equations. This is because the question does not give any velocity values and I don't know how to solve for them. How would you do this problem. Note: According to my book, the correct answer is 3.5s.
     
  2. jcsd
  3. Feb 12, 2016 #2

    ehild

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    The object falls - it means its initial velocity is zero. What relation exist between constant acceleration, time, and displacement?
     
  4. Feb 14, 2016 #3
    I'm not sure. what do you mean?
     
  5. Feb 14, 2016 #4

    SteamKing

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    Knowing that the initial velocity is zero, which one of the five SUVAT equations relates constant acceleration, time, distance, and initial velocity?

    Scan the list of these equations and use a process of elimination to narrow your choices to the correct equation.
     
  6. Feb 14, 2016 #5

    Don't objects have a non-zero initial velocity when dropped?
     
  7. Feb 15, 2016 #6

    ehild

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    The problem meant that the object was hold and then released and let to fall. What is the initial velocity?
     
    Last edited: Feb 15, 2016
  8. Feb 20, 2016 #7
    I thought that the initial velocity meant the velocity right after the motion began, just like final velocity is the velocity right before motion ends, not the velocity when the motion is over and the object is at rest.
     
  9. Feb 20, 2016 #8

    CWatters

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    The "final Velocity" isn't always the Velocity just before the object stops moving. It depends on the problem. For example there is a common problem that involves an object falling past a window - after passing the window it keeps falling. When ,you break the problem down the final Velocity you plug into one of the suvat equations is typically the Velocity as it passes the bottom of the window.

    The initial velocity when an object is dropped is usually zero but there are situations where it isn't. For example when it's dropped from a helicopter that is ascending or descending.
     
  10. Feb 20, 2016 #9
    Why is the final velocity in the window problem the velocity as it passes the bottom of the window?
     
  11. Feb 20, 2016 #10

    ehild

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    What is the velocity of the object right after the accelerating motion began if it was in rest before? You keep a pebble in your hand. What is the velocity of the pebble? Then you open your hand and let the pebble fall out. What is the velocity of the pebble at the instant when you release it?
    "Initial" and "final" are defined by the problem. If you observe a pot falling in front of a window, you observe it from the room during the time it moves from the top of the window down the bottom.
     
  12. Feb 21, 2016 #11

    CWatters

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    Why not? It depends on the actual version of the question but the most common version of the problem requires you to solve it in two phases, in the first phase the final velocity is the top of the window. That value is then used as the initial velocity for the second phase of the solution. In the second phase it seems reasonable to label the velocity at the bottom of the window the final velocity although you can solve the problem without calculating it.
     
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