On 10/28/2003 01:23 PM, Hugh Haskell wrote:
>
> So does that mean that the rain that hits the outside of the front
> wall of the car contributes to its change of speed, even though it
> doesn't contribute to its final mass (since it mostly runs off the
> side onto the ground). If this is true then just calculating the
> change due to the increased mass and conservation of the momentum of
> that new mass would not account for all of the change in the speed of
> the car, right?
>
> On the other hand this implies that that part of the falling rain
> that does not come in contact with either the front or the rear wall
> of the car doesn't contribute to the final speed of the car. So how
> do the two countering effects work out?
As I hinted in my previous msg, at some point this
becomes a fluid-dynamics problem. That provides us
some good ways to "name that force".
What we have here is a slightly weird _drag_ force. Drag
can be subdivided in various ways. One useful division
goes like this:
-- There will be pressure drag on typical vertical surfaces,
including the front wall and the rear wall.
-- There will also be skin-friction drag on typical
horizontal surfaces. Consider for example a thin
flat-car with no vertical surfaces at all. There will
still be drag.
The general analysis seems pretty straightforward: draw
a boundary around the boxcar. Account for
-- mass coming in
-- momentum coming in
-- mass going out
-- momentum going out.