| Basic Thermodynamics ~ J. Pohl © | www.THERMOspokenhere.com (81-B335) |
The load carried by the six large tires of a fully-loaded strip-mine hauler is 360 tons. Carrying such a great load, we expect that the air pressure in the tires to be great. It is not possible to calculate the pressure of the air in the tires of a given truck. However knowing the hub dimensions (given below), a calculation is possible.
Estimate the least possible air pressure required within each tire to support the truck and its load.
♦ Under normal conditions we would expect each tire to support one-sixth of the load or 60,000 lbm. As our scenario, we sketch a silly-looking, single tire; it looks like a giant unicycle.
Next, for there to be forces (and pressure-forces) there must be a system.
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 of the system. To make that boundary force relevant to the system, we pass the system boundary through the tire and the air it contains. We know the tire walls will help support the load. Since we cannot calculate that; we must assume it is 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.
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Calculations with attention to units yields:
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Were a tire gage applied to our approximated tire, the gage would read 20.8 psi, meaning the pressure within the tire is:
The load carried by the six large tires of a fully-loaded strip-mine hauler is 360 tons. Carrying such a great load, we expect that the air pressure in the tires to be great. It is not possible to calculate the pressure of the air in the tires of a given truck. However knowing the hub dimensions (given below), a calculation is possible.
Estimate the least possible air pressure required within each tire to support the truck and its load.