Does Weight on a Tire Affect Pressure?
Most recent answer: 05/08/2013
- Al Cella (age 54)
Roseburg, OR USA
Let's begin with your friend's example. If you have 50 psi air pressure in a truck tire, and then you put a massive weight in the truck bed, the tires will be squished downward. This will decrease the volume of the air in the tire, and the pressure will increase (by Boyle's law).
This effect might be small, since the tire can squish out sideways, and almost maintain its volume. Depending on how rigid the rubber is, the volume will still decrease somewhat. If you want to test how large the effect is, get your pressure gauge and forklift, do the experiment, and let us know!
Now, for your question. Because the pressure increases as you put weight on the tire, the tire becomes more difficult to pump up. Somehow when you take into account that increased air pressure, the extra work required must be enough to lift the truck.
So, your intuition is correct.
p.s. Actually, I thought of a much simpler way to prove my argument. If you pump up the tires on a vehicle which is suspended, then the center of mass of the vehicle doesn't move, so all the work you have to do is to force the air into the tire. If the vehicle isn't suspended, then as the tires expand, the vehicle is lifted slightly higher into the air, raising its center of mass against the force of gravity. This increase in potential energy could only have come from the work you did in pumping the tire, so you clearly had to do additional work.
(published on 05/08/2013)