shuster@oblio.DEC (ROBERT L. SHUSTER) (10/22/85)
Here's another car/tire question: It is very much easier to turn the wheel (using the steering wheel) of a moving car than that of a stationary car. The surface area of tire on the road does not change as it begins to rotate. The difference in ease of turning, then, is the difference between friction on stationary object and friction on a moving object. What is the mathematical relation, in this case, between the two frictions? Can this relation be generalized for other objects? -R. Shuster
shuster@oblio.DEC (ROBERT L. SHUSTER) (10/22/85)
The car/tire problem: Well, there's more here working than just friction. There's torque created by the spinning wheels, which aids steering of the wheels. Ok, so this was a rather stupid problem...But I'm still interested in a relation, if any, between the two frictions. -R. Shuster
mwg@petrus.UUCP (Mark Garrett) (10/23/85)
++ > Here's another car/tire question: > It is very much easier to turn the wheel (using the steering wheel) of > a moving car than that of a stationary car. The surface area of tire > on the road does not change as it begins to rotate. The difference in > ease of turning, then, is the difference between friction on > stationary object and friction on a moving object. What is the > mathematical relation, in this case, between the two frictions? Can > this relation be generalized for other objects? > -R. Shuster Actually, when the car is stationary, the friction is dynamic, and when it is moving the friction is static! The real issue is the friction involved in the stress of the tire. Suppose the car is stopped. When you turn the wheel slightly, you distort the tire somewhat. The footprint stays in the same place but the rest of the tire is moved. As the car rolls forward, the tire comes back into shape, and the contact patch re-orients itself to the direction of the rest of the tire. If you force the wheel when stationary, then you drag the contact patch across the pavement (very difficult); if you are rolling, then the only friction is that of the sheer forces inside the (distorted) rubber. The faster you are moving, the less the tire is distorted before being able to restore its shape. Therefore, the steering resistance decreses with increasing speed, although this function is very non-linear. -MWG
gwyn@brl-tgr.ARPA (Doug Gwyn <gwyn>) (10/23/85)
> It is very much easier to turn the wheel (using the steering wheel) of > a moving car than that of a stationary car. The surface area of tire > on the road does not change as it begins to rotate. The difference in > ease of turning, then, is the difference between friction on > stationary object and friction on a moving object. What is the > mathematical relation, in this case, between the two frictions? Can > this relation be generalized for other objects? Sure it can: It is very much easier to bank a moving airplane than a stationary one. The surface area of the airplane does not change as it begins to bank. The difference in ease of banking, then, is the difference between air friction on a stationary object and air friction on a moving object. ;-) I mean, if you're going to reason incorrectly about cars, why not go all the way? (Note that, unless a car is skidding or spinning its wheels, the relative speed between the bottom of the tire and the road is 0.)
tino@hou2f.UUCP (A.TINO) (10/23/85)
>It is very much easier to turn the wheel (using the steering wheel) of >a moving car than that of a stationary car. The surface area of tire >on the road does not change as it begins to rotate. The difference in >ease of turning, then, is the difference between friction on >stationary object and friction on a moving object. What is the >mathematical relation, in this case, between the two frictions? Can >this relation be generalized for other objects? ___________________________________________________ Not knowing anything about cars I will assume that the linkage connecting the steering wheel to the car wheels works the same whether the car is moving or at rest. When you turn the wheel of a stationary car the tire rubs against the road forcing you to work against friction. On the other hand, to turn the wheel of a moving car you don't need to overcome friction. As long as the tires don't slip no work is done against friction. (It's possible to turn without slipping because tires are flexible.) This seems to make sense. Al Tino
franka@mmintl.UUCP (Frank Adams) (10/24/85)
[Not food] In article <956@decwrl.UUCP> shuster@oblio.DEC (ROBERT L. SHUSTER) writes: >It is very much easier to turn the wheel (using the steering wheel) of >a moving car than that of a stationary car. The surface area of tire >on the road does not change as it begins to rotate. The difference in >ease of turning, then, is the difference between friction on >stationary object and friction on a moving object. No, something quite different is happening in the moving car. The key is that the tires are not rigid. When you turn the wheel a little, you create stress in the tire near the road. If the car is not moving, turning the wheel further requires the tire to slip against the road, whence friction comes into play. For a moving car, that portion of the tire spins on out of contact with the road, relieving the stress. Thus friction plays little part in the ease of turning the wheel in a moving car (unless it becomes very low, such as on ice). Frank Adams ihpn4!philabs!pwa-b!mmintl!franka Multimate International 52 Oakland Ave North E. Hartford, CT 06108