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Falling from a roof

  1. Jan 12, 2017 #1
    1. The problem statement, all variables and given/known data
    a man slides off a roof that makes an angle of 45 degrees below the horizontal. friction and air resistance neglig. he slides 4m, falls off and lands a certain distance away from the building. another man stands 4m away and is 1.2m tall. how far from the building does the falling man land and does he hit the man below?

    2. Relevant equations

    i don't really know what to do here.

    3. The attempt at a solution

    all i've done so far is found the horizontal component (u) of the roof he fell off of. i don't even think that this is relevant.
    cos45=u/4m
    u=2(sqrt2)



    guidence?
     
  2. jcsd
  3. Jan 12, 2017 #2

    PeroK

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    In general, we can help you solve problems where you are stuck. But, we can't really teach you the physics from the beginning. If you really have no idea, then you should backtrack and try some easier problems.

    What physics do you know?

    The man's motion has two phases here, can you identify them?
     
  4. Jan 12, 2017 #3
    Right... I do know some physics but just need some help with the question...
    Obviously there is an x-component to the slide down the roof, which is a gravity component... if you could help me find that it would be great.

    the slide down the roof being the first part of the motion and the falling off the roof being the second.
     
  5. Jan 12, 2017 #4

    PeroK

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    What do you know about sliding down a roof?
     
  6. Jan 12, 2017 #5
    uhm... not much?

    listen man, i'm guessing that there's an x-component to the gravity that causes continued x-axis momentum/velocity after the man leaves the roof; otherwise it would just be a vertical fall. now if you can help me figure out how to get that x-component, without knowing an initial velocity... i would be grateful.
    and then maybe a tip on getting the correct answer for the second phase of the question.
     
  7. Jan 12, 2017 #6

    PeroK

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    Do you know the term "incline" or "inclined plane"?

    Have you heard of the term "vector"?

    What about Kinetic and Potential energy?
     
  8. Jan 12, 2017 #7

    BvU

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    No need to guess. There isn't.

    [edit] to be a bit more helpful: the advice is to make a drawing, not wait until someone does it for you
     
    Last edited: Jan 12, 2017
  9. Jan 12, 2017 #8

    Honestly, I don't understand. Without an x component to the velocity in the first part of the question, wouldn't the object just fall vertically once it exits the roof? Obviously there is some form of horizontal momentum.
    That's the first thing I did......... bit rude.
     
  10. Jan 12, 2017 #9
    Yes, you are right that there is an x component (and a y component) of the velocity, and if there wasn't an x component, yes it would just fall vertically. But what you originally said was:
    There is no x component to gravity.

    So did you calculate the x and y components of the velocity just as he is falling off of the roof?

    Edit: Was there a height of the roof given, or did I just miss it?
     
  11. Jan 12, 2017 #10
    Yeah the height of the roof was given.. sorry its been a long night.
    The height of the roof is given as 9m.

    I remember doing FBD diagrams where an object was sliding down an incline, and I recall that there was an x-component to the forces when gravity was the only actor. I suppose I should review that.
     
  12. Jan 12, 2017 #11
    It was probably a bad answer on my part to say there is no x component of gravity because it all depends on how you define your axes.

    Edit: I guess I tend to think as the y axis being vertical and x axis being horizontal, but that certainly is not always the case. And as a matter of fact, for the first part of the problem (sliding down the roof), it seems to make more sense to NOT define the axes as vertical and horizontal. I would define the +x axis to be in the direction of the downward slope of the roof.
     
  13. Jan 12, 2017 #12
  14. Jan 12, 2017 #13

    BvU

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    OK, we've dealt with that. Now what about a little sketch of what's happening ?
    Not so hard, or is it ? We do get another bit of input in post #10, but I still have a little something missing to draw the picture for you !
    Is the 9 m the top of the roof ? Where does the 4 m start ?

    upload_2017-1-13_0-15-59.png
     
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