Maybe I am missing the point here, but I don't think either pressure is
useful in answering the problem posed. I immediately think of the usual
example of three glasses of different shape filled to the same level with
water. The pressure is the same at bottom inside surface of the glasses.
But, the weight of each depends on the shape (volume) of the glass. If all
three glasses have the same diameter circular bottom, then multiplying the
water pressure by the area does NOT give the weight of the glass and
contained water for each - it actually gives the same value for all three.
What differs is the pressure of the filled glass against the table. The
heavier glass (given identical bottoms) will have the higher pressure
against the table - regardless of the water pressure inside.
Bob at PC
-----Original Message-----
From: Forum for Physics Educators [mailto:PHYS-L@list1.ucc.nau.edu] On
Behalf Of Anthony Lapinski
Sent: Tuesday, February 07, 2006 12:31 PM
To: PHYS-L@LISTS.NAU.EDU
Subject: getting tired
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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