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Homework Help: Gradient / Velocity

  1. Aug 8, 2005 #1
    Ok, this may sound a little basic but I can’t think of a way of explaining it other than common sense. I should also say that this is for a Geography assignment so not really physics but hope you guys can give me an insight.

    I’m trying to explain why the steeper the gradient of a river, the faster the water flows.

    I was thinking that it must be something to do with friction but then wondered whether an object on a ramp angled at 45 degrees in a frictionless environment would accelerate at gravity and I’m not too sure. :confused:

    Any help in explaining it would be great
    Thanks in advance
  2. jcsd
  3. Aug 8, 2005 #2
    yes it would, though the acceleration will not be equal to g but a bit less. It really depends on the angle of the incline

    Also, rigid body mechanics will teach you that the acceleration at which an object rolls of does not depend on the magntude of the object, also not on the mass but on the way the mass is distributed throughout the object's volume. This is expressed by the socalled mass tensor or the socalled rotational inertia (I-tensor). For example, two equally big and heavy spheres will roll down at different accelerations if their mass is distributed differently when compared to each other.

  4. Aug 8, 2005 #3
    Try think of the river as a singel massive point. In this way you can think of it s a vlock on a slope insted of a liquid. You can than simmpaly excamen the problem with forces. There are 2 forces the force of gravity and the force of the ground (it may help to draw this). The force of gravity can than be divided into 2 others. One parallel to the slope and onathe having a -90 deggres angel to it. The one parralel to the slope is the one responsable for the movment of the block (river). Friction does reduce the speed by some factor. But in this very basic explination we don't really need to think about it.

    I hope this helps.
  5. Aug 8, 2005 #4
    If want, i have written a little tutorial on Newtonian Physics . It also covers problems related to inclines. Also, look at the sample problems


  6. Aug 8, 2005 #5
    Thank you both very much for your replies. I'll try to get my head aroud it as I'm probably still a couple of years off learning this in physics. Though if I combine your two replies I'm sure my geography teacher will be impressed :smile:
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