In my physics textbook it says cars move because the wheels exert a backward force on the ground and the ground exerts an equal and opposite forward force on the car. If the wheel spins clockwise, then there is a resultant clockwise moment on the wheel due to the force from the engine. My question is, why doesn't the equal and opposite force from the ground create an anticlockwise moment on the wheel which cancels out the clockwise moment and hence stop the wheel from spinning, instead of making the car move forward?
[Physics] Why do wheels spin
conservation-lawseveryday-lifeforcesnewtonian-mechanics
Related Solutions
Leverage
Often, a limiting factor in tasks required from a tractor is the amount of pulling force a tractor can apply without tipping over (the front rising up) - the engine is strong enough to do so.
Having the driving axle be high from the ground helps by simple lever action - see the illustration; twice the height means twice the maximum pulling force allowed before the tractor tips over. The other thing for the same problem that's commonly done is to attach a large heavy object far to the front - https://www.google.lv/search?q=tractor+front+weight&tbm=isch
[edit] - a more accurate but more complex illustration of the actual leverages.
In terms of how you find this out, the direction is given by the right hand rule. Hold your right hand out in front of you with your thumb pointing left. (Your thumb sort of acts as the rotational axis of the wheel). If you think of the wheel as your guide, in which was does the wheel turn in order to move the car forward?
A.) Anti-clockwise.
Does that make sense? Let me know if it doesn't, I will try an explain a bit better.
Best Answer
The engine provides a force to turn the wheel ( in physics lingo torque ) this causes the wheel to turn counter clockwise. Now look at the point of the tire that touches the ground. This point on the ground experiences one force as the wheel tries to turn - static friction (not kinetic, but that's an entire discussion in itself). This force applies a push that is proportional to the weight of the car (assuming the car is on a flat surface). This pull on the wheel creates traction (a difference in forces) which allows the wheel to turn and the car to move. Using Newton's third law which you cited isn't the correct line of thought as it means that the force friction exerts on the wheel will be equal to the force that the earth experiences from the wheel. I hope my explanation helps clarify this subject.