Monster Tires The load carried by the six large tires of a fully-loaded strip-mine hauler is 360 tons. It is not possible to calculate the pressure of the air in the tires of a given truck. However were we to know the hub dimensions (given below), we could estimate the least possible air pressure required within each tire to support the truck and its load.

♦  Under normal conditions we assume each tire would supports one-sixth of the load or 60,000lbm. We sketch a silly-looking scenario of one tire; it looks like a giant unicycle. For there to be forces, there must be a system. A system sketch must be drawn with the body force as one-sixth the weight of the loaded truck. For air pressure in the tire to support the truck, that air pressure (acting over an area to be a force) must be a boundary force. To make that boundary force relevant to the system, we must pass the system boundary through the tire and the air it contains. About the tire, We know it will help support the load - we assume its help to be negligibly small.

Our system sketch is shown to the right. While drawing the system boundary it was noted that atmospheric pressure over the top of the system must be included.

Calculations and attention to units:

Were a tire gage applied to our approximated tire, it would read 20.8 psig, meaning the pressure within the tire is: 14.7psi + 20.8 psi = 35.5 psi.