[Phys-L] Re: getting tired

[Scott Goelzer <s.goelzer@COMCAST.NET>]

We do this with my student's cars by first taking a ink print of the
tire tread on graph paper and then working out a percent coverage.
Then, the contact patch rectangle is measured by sliding sheets of
paper under the tires as far as they will go. Patch corrected for %
tread gives the actual amount of contact with the ground. This is
much better than trying to have them drive or lower a tire over graph
paper - too scary for HS.

We multiply by the tire gauge pressure to get the force on that tire.
Sum of the tires is usually close to the registration weight.
Correcting for gas and junk in the car, results within 100 lbs have
been obtained. I still believe most of the error is in the cheap tire
gauge. I have never had time to test one.

Extra fun: if one kid has a pickup (this is NH - so always), have
1000 lb worth of kids climb into the bed and remeasure the tire
pressure - most are amazed that it does not change.

Scott




**********************************
Scott Goelzer
Physics Teacher
Coe-Brown Northwood Academy
Northwood NH 03261
603-942-5531e218
sgoelzer@coebrownacademy.com
**********************************


On Feb 7, 2006, at 3:49 PM, Bernard Cleyet wrote:

> Don't need to assume equality;   assume contact force is normal.
>
> It's the difference in pressure that holds up the bike.
>
> Here's a "thought" soln.:   With no air, the rim and tire rubber
> hold it
> up (gauge p=zero), as one adds air, the bike lifts; more air less
> contact area (once it's riding on air and not rim)
>
> bc, prays he's got it.
>
> Anthony Lapinski wrote:
>
> > Suppose you know the pressure inside your bike tires and the
> > contact area
> > between each tire and the ground. Assuming the bike is supported
> > equally
> > by the two tires, you can determine the weight of the bike.
> >
> > Here's my question. In using the formula for pressure (P = F/A),
> > do you
> > use the GAUGE or ABSOLUTE pressure in the tire for this
> > calculation (Pgau
> > = Pabs - Patm)? I'm thinking gauge since the atmosphere is pushing
> > from
> > the outside on ALL parts of the bike. And this give rise to a buoyant
> > force on the bike? Can anyone assist me through this subtle
> > distinction as
> > it relates to the bike problem?
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