Physics
posted by Elisa on .
The four tires of an automobile are inflated to a gauge pressure of 1.8E10^5 Pa. Each tire has an area of 0.025 m2 in contact with the ground. Determine the weight of the automobile.
I was helped with this before, but the answer was wrong. Can someone please help me, I keep doing it, but it's still wrong.

The absolute pressure inside the tire, multiplied by the area in contact with the ground, equals the force of the ground upon the tire. Four times that force equals the weight of the car, if each tire supports the same weight.
The absolute pressure in this case is 2.8*10^5 Pa. It is higher than the gauge pressure by an amount equal to atmospheric pressure, 1.0*10^5 Pa.
I calculate
Wt. = 4 *(0.025 m^2)*(2.8*10^5 N/m^2) =
2.8*10^4 N = 6300 lb
That seems pretty heavy for a car. They may expect you to use gauge pressure instead of absolute pressure in the formula, but I don't agree with that assumption. There is no atmospheric air under the tire at the point of contact.