Let's say we are talking about an inflated car tire, installed on a car. Is work being done to hold up the weight of the car by the tire? And if so, where is the energy coming from to hold up the weight of the car? I inderstand 2 things already; as the weight of the car tries to deflate the tire and thus decrease the volume of the container, the pressure will increase, and I know that there is some amount of energy being pumped into the tire from the outside air's temperature via conduction through the tire's rubber walls. If there is work being done, it has to be the gas doing it right? Through collisions by molecules on the inner wall of the container to keep the tire inflated. If the molecules are not imparting energy on the wall and losing energy themselves as a result, then it feels like a proverbial free lunch to me. Work for nothing! But if they are imparting energy and losing it for themselves as a result, the energy being lost must be being replenished somehow. The two things I mentioned that I knew at the beginning of my question do not seem to me like they can account for replenishing the tremendous energy being spent to hold up the car. Help me understand this one guys. Thx!
-
Work is force times displacement along the line of action of the force.. You can have a force acting with no displacement of the force resulting in the force doing no work. – Farcher May 20 '17 at 08:10
-
Your absolutely right. I forgot that work is only being done when something is moving. However, the crux of my question is really the part about the free lunch. Maybe no work is being done, but isn't there force acting on the inside of the tire to keep it inflated. Does that force require energy, and if so where is it coning from? I've read about the basic assumptions of gas molecules being elastic in their collisions etc. or maybe it was nonelastic. Can't recall. Point is that the collisions of these molecules results in no net loss of energy. But if they don't impart energy... – James Stovall May 21 '17 at 18:29
-
But if they don't impart energy to the inside of the container, then how can the gas inflate the tire to begin with. The idea were given is that the tire is being kept inflated by countless impacts be the gas molecules. All that energy being spent during those collisions has to come from somewhere right? I can see how some energy would leak into the tire from outside temperature of the air. But this doesn't seem sufficient. What am I missing? Thank you for youranswer btw. – James Stovall May 21 '17 at 18:35
-
Work is done inflating the tyre. Once that work is done that's it, no more work is done – Farcher May 21 '17 at 19:38
1 Answers
There is no work being done to keep your tire inflated. Work is only done when a system feels a net force, and your car feels no net force toward the center of the earth because the ground pushed back on it exactly enough to cancel gravity. This is true whether it's in motion or stationary.
However, if the car had flat tires, then the tires were inflated, there would be work done on the car and you would need to provide the energy to raise it up as the tires are inflated. In that case the energy would need to come from the pump.
Perhaps the reason your intuition tells you there should be work done is that human beings certainly feel exertion and burn off extra energy when we try to hold an object above the ground. But this is specific to the way that our muscles work via repeated firings, even when we try to keep our limbs stationary. If your muscles instead truly locked into place, we would not need to spend any energy by just holding up an object at a constant height.
- 2,027