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Opossing acceleration forces from the same mass, unbalanced?

  1. Dec 12, 2013 #1
    Okay, so this is either really complicated, or really simple. I had this theory about something, that I've never been able to fully test. As a kid, I used to love to sit on a skateboard or a wheeled computer chair and lean to one side, then quickly slide my feet (and the board/chair) underneath me to the other side, then repeat again and again, to move across the floor without touching anything.

    Now, I had a thought recently that perhaps this is a trick on conservation of momentum, which allows an object to create an imbalanced force, and therefore move, using only sources of momentum within its own mass, via a clever manipulation of inertia. However, I couldn't be sure if air friction had anything to do with it or even the rolling friction from the wheels, or well... any kind of friction. I'd basically need to go to space to confirm that theory, so I thought I'd ask some physicists, or at least someone keen on the general concepts of physics like myself.

    Basically, here's the question. If you had a short, high acceleration in one direction, balanced by a long, low acceleration in the other direction, both acting on the same mass, would they cancel each other out, or create a net imbalance? I know that F=MA, but I don't know how that translates to acceleration over varying lengths of time versus varying acceleration rates. any physics majors/geniuses out there who can help me out? This is purely for curiosity reasons.
  2. jcsd
  3. Dec 12, 2013 #2

    Simon Bridge

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    Welcome to PF;
    This is a common question and it is to do with the difference between kinetic and static friction - basically the static friction is higher.

    To make a chair, say, move, you need to provide a force bigger than the friction.
    If you throw yourself about quickly, this is what you are doing: the peak force delivered to the chair is greater than the static friction, but if you move slowly it isn't.

    The details get a bit more sloppy ... try this:

    You are in a box sitting on a frictionless surface - you are at one end and you have a ball . You throw the ball at the far wall, where it bounces back and you catch it.
    What is the motion of the box wrt the ground?

    Now repeat the though experiment, but this time, instead of a bouncy ball you have a blob of goo. When it hits the far wall it sticks. Now what happens.

    (these are conservation of momentum problems)

    Once you've figured that out - you can add friction.
    For that you need to know about specific impulse: the longer the acceleraton period, the lower the peak force.
    Hitting the far wall is a very short acceleration. Throwing the ball is a long acceleration in comparison.
  4. Dec 13, 2013 #3
    Actually once you mentioned static friction I got it. The description sort of got confusing, and I had to read it a few times to understand what you were trying to say, but static friction totally makes sense. How could I have missed that? It was so simple. I knew it. Much Thanks.
  5. Dec 13, 2013 #4

    Simon Bridge

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    Yah - it's one of those things you can get intuitively quite quickly but the details are a bit messier than your intuition.
    Well done.
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