How can a one-ton car drive over a person’s foot and not cause any significant damages?
In: Physics
Force per unit area; basically the weight of the car is evenly distributed on each of the four tyres so each tyre “only” has 1/4 of a ton and that weight is distributed over the whole area of the tyre which is in contact with your foot or road. This means that each square inch of your foot gets far less weight push on it than if something like a stiletto heel was to stand on the foot with a far smaller surface area.
Well, 1 ton divided by four tires, so 1/4 ton per tire. Now the contact patch of the tire (the part in contact with the ground) is key to how much pressure the foot would feel. You would take the 1/4 ton and divide it by the surface of the contact patch to find the mass per unit of surface of the contact patch. Turns out that it’s a much lower number than you would think, hence the foot feels little to no pain!
The car has four tires, so each one only holds up 500 pounds. If that weight is spread out over an area, the pressure could be low. This is how the air in the tire holds up the car at only a couple of atmospheres of pressure. In the right geometry, with the right support under it, a person’s foot might be able to withstand a couple atmospheres of extra pressure.
1) each tire carries 1/4 the weight so 500lbs per tire
2) it’s not a point weight. It’s 500 lbs on a patch that’s about half a foot square.
3) generally, your foot isn’t the only thing holding up that tire. The tire is partly resting on the ground too, so the spread out 500 lbs is not close to putting the full weigh on your foot.
Pressure is spread out over area. This is the same reason why people have been able to survive falls from the sky without parachutes. If you can find away to spread out the impact over enough space and/or time, you can minimize the effect.
Also, the weight of a car isn’t distributed evenly. The front tires (usually) carry more weight, because that’s (usually) where the engine is.
A knife can cut so easily because the force that’s applied (which is not very large, you don’t need a lot of force to cut vegetables) is focused over a very sharp edge, a very very small area of contact. For stabbing, the entire force is focused down to a point; compare that with trying to stab someone with, say, a hammer, same force, a lot bigger area of contact.
So the tires of a car are basically “spreading the weight of the car” over the surface that makes contact with the ground. It’s why bigger cars, trucks, etc., have bigger or more tires, to “spread out the weight.”
So when a 1 ton car runs over your foot, it has 4 tires so any one tire only feels 1/4 of a ton, 500 lbs, and then, given the surface of that tire vs. the part that’s over your foot, you may only feel 200 – 250 lbs of that actually pressing on your foot, the rest of the tire presses on the ground around your foot.
So 250 lbs is the weight of a heavy person standing on your foot. It may hurt, but it’s not *that* bad.
1 ton, 4 tires = 250 lbs if the weight is distributed evenly. Can a 250 lb person stand on your foot?
But the weight is not distributed evenly: tires are relatively soft and stretchy, so you don’t get all that weight at one point, and the shocks and struts on your car mean that most of the weight actually transfers to the other tire.
You also have extra force from the torque of the spinning tire that’s not to do with the weight of the car, but the direction of that force through your foot is at an angle, so it’s not adding that much extra stress to any given point in your foot.
Latest Answers