Red_CCF said:
Hi
When viewing the whole car as one body, friction is the only external force; my main issue is picturing how, as force is a push or pull, friction actually causes the car to move because to me it seems to be just serving as a pivot for the torque provided by the car engine to cause the wheel to turn.
Thanks
Your welcome for the explanation. I feel proud of myself if I can get someone to understand something. I also feel proud of them, too.
I suppose it could be considered as a pivot for leverage. If I were to think of it in a similar way myself, I would probably go with a frog jumping off a log. It's not exactly the same thing, but it illustrates the point for the most part.
Try thinking of a spinning wheel again. Now imagine a treadmill with little to no resistance, so it can simulate the effect of traveling better. If you were to place the spinning wheel on the treadmill, the friction would cause the wheel to grab hold(not really, but you get the idea) and pull the surface in the direction of rotation. If you think of friction as the nanoscopic pits and mountains in the surface of a material(I'm pretty sure this is the correct definition/visual), it becomes a lot more similar to the frog on a log, or in this sense, maybe more like a gear meshing with another gear. I assume you know how a gear transfers power, but for the most part, it's like pushing on a part that sticks out to produce an imbalance of force(thus torque), with the roundness enabling it to continue as long as it continues to spin. On a toy gokart I once had(by toy I don't mean vroom vroom race with gas engine play, it was more like a 4-5inch long model thingie, maybe for playing with), the little steering was fashioned in a similar manner. The steering wheel connected to a rod with a little gear on the end. At the gear, there was a rectangular piece with grooves matching the gear teeth. This rectangular piece was connected to the little tie rods(they connect the tire to the steering mechanism on a car), which enable the wheels to steer. By turning the steering wheel, gear at the end of the rod would be turned. The gear teeth would push the rectangular piece to the side, and the tires would move. A car tire on the ground can be envisioned in the same way, with the friction visualization stated above. The nanoscopic pits and mountains act like the gear teeth. A mountain on the tire's surface can fit into a pit on the ground's surface(it's not exact, but imagine as if this is a spot where this happens) A mountain over a mountain doesn't really do much since the scale is so tiny, but in order to move, the mountains do have to go over the other mountains). The pit is surrounded by a higher elevated surface, so it receives opposition when moving horizontally(like a gear). When the tire has torque applied to it from the engine, it the mountain pushes on the mountains(or simply not pitted part, it will still have the same effect). The tire is still having the torque applied, so the force is kind of moved upward a bit to get over the mountain(like a little ramp). If the mountain can't get out of the pit, resulting from a hook or something, the mountain will just break off, or the hook will break off, thus wear and tear from friction(I assume this is what happens with hooks, at least). If the molecule is bonded weakly enough with the attached surface, then it can break off too. This is why you have to replace brake pads, as they rely on friction to decelerate the car, and it wears the pads down. In the process of going up, they still exert the horizontal force like a gear tooth. With a ramp, if the object is forced up, it still has to exert the horizontal force. If the force wasn't there, there wouldn't be any point in having to keep the ramp in place, as it wouldn't go anywhere. Place a small ramp on a slippery surface(or at least able to slide without too much difficulty), and roll a ball at it. It should move back as a result of the balls horizontal motion. This principle is what enables the friction to take place. So, in effect, it could be argued that the tire drives the ground as well as the car. This comes from Newton's Third Law of Motion. However, the mass of the Earth is HUGE compared to a single automobile(even a fully loaded truck freight truck). So no effect is noticed on the Earth's motion.
In short, a tire can be envisioned as a gear. The gear is meshed with the ground. The ground and tire have the little mountains and pits(causing friction) that enable the transfer of power. If you need another analogy, place a spinning gear on a surface that is grooved to fit the gear's teeth. Notice that the teeth will push against the surface's teeth(remember, it's an analogy of friction). Because of Newton's Third Law of Motion, a force against the surface means the surface exerts an equal force on the gear/tire. As a result, it is the same as an imbalance of forces on a round object, creating rotational motion. A car has axles that can turn in place, so the car doesn't have to roll around itself(That might be a fun amusement park ride, though). The car simply rides along with the wheels, which drag the car with them. Voila! Self-powered(by the object itself or a person on it, at least) transportation!
If you would like to learn about how a car functions itself(like the machinery and such), you should probably obtain a book on the matter, or take a class. Transmissions alone can fill an entire book, so a forum won't be able to completely cover it.
