# [Vwdiesel] mileage

Erik Lane eriklane at gmail.com
Sat Nov 28 18:44:41 PST 2009

```On Sat, Nov 28, 2009 at 5:26 PM, Val Christian <val at mongo.mongobird.com> wrote:
> Apparently the physics classes didn't work.
>
> Assume that you have a human sized sheet of sandpaper.  Put a feather on
> the sandpaper sheet and try to pull it.  No problem, just a little
> resistance, as you scuff the finish on the wood floor.
>
> Now on that human sized sheet of sandpaper, put the biggest, meanest
> and heaviest person you can think of.  Now pull it.  Can't move it?
>
> That's the physics part.
>
>
> F(f) = uN, where N is the normal force.  Weight and N work together,
> and the big guy on the sandpaper increases the N.  Therefore the
> frictional force increases linearly with an increase on the weight
> (hence the normal force).
>
> Seriously, you took university physics in college, and engineering
> courses?  Sorry I'm being hard, but it's pretty elementary.

You're right, it's elementary. You completely forgot about inertia.
The frictional force increase is directly in proportion to the
increase in weight, but the inertia is also a direct proportion. The
feather takes almost nothing to stop, while the heavy person takes a
*lot* to stop. The equations for the distance that something will
travel don't have weight in them, because it cancels out. (Well,
actually mass, but since we're doing all of this on Earth, that's not
an important distinction.)

If you want to play this game some more I can go find equations to back it up.

Erik
```