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Homework Help: Question about gravity

  1. Oct 11, 2005 #1


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    I was doing some momentum questions. There some that goes like "a man dives into water, and comes to a stop x seconds after reaching water, what force did the water act on him" or "stuntman jumps onto a mattress, matress compressed down x meters before he is brought to rest, what force did the matress apply on him?"
    Basically, answer= (guys mass)(acc. of medium) + 9.8(guys mass)

    Everytime I do those questions, I keep forgetting account for gravity, and I don't know why you are suppose to add 9.8*(guys mass). I mean, isn't gravity pulling the guy down, while the water/mattress's force is pushing the guy upwards, to stop him? So why do you add the 2 forces, if anything, wouldn't you subtract? (someone please clear this up for me :confused: )
  2. jcsd
  3. Oct 17, 2005 #2

    Tom Mattson

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    Sorry for the late reply. In case you're still interested, here are my comments on this.

    The matress supplies an upward force that has to do 2 things:

    1.) Decelerate the man.
    2.) Support his weight.

    So if we apply Newton's second law to the man + mattress system we have:

    [tex]\Sigma F=F_{mattress}-mg=ma[/tex]

    So you are subtracting the two forces. And as you can see, upon solving for [itex]F{mattress}[/itex] you get [itex]mg+ma[/itex].
  4. Dec 1, 2007 #3
    here's another way of looking at it.....

    downward force= (mg+ma)....this is applied onto the surface by the guy

    for every force applied to the surface(mattress/water).....there will be an equal and opposite reaction by the surface in the upward direction....
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