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When a car's engine injects fuel into the cylinder chambers, the reaction creates a force that generates rotational momentum to the shaft and over the transmission, it translates that power to the wheels, right?

But, something's bothering me, how does the car actually move, is it dependent on the force of friction? Wheels have a contact with the road in one exact point and when the drive gets to rotate them, a force is exerted on the road in the direction of movement and there is friction that is counteracting it.

Is the key to get a car properly moving to have the drive force equal or less than the force of friction to keep that point locked to the ground and use the rolling of the wheel for translational displacement ie. moving forward? If it's bigger than the force of friction, that would cause the wheels to spin in place?

Am I completely off or am I getting something right? If there's a knowledgeable person on this topic, I'd greatly appreciate some insight, perhaps even a bigger response with some basic vehicle physics.

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Hi Scienation, and welcome to Physics Stack Exchange! We already have a few questions about the role of friction in driving that you might find useful. Also, I edited your post to make the title in the form of a question, which is the recommended way to post on this site (since it is a Q&A site). –  David Z Dec 17 '11 at 5:40
It's not right to say that "Wheels have a contact with the road in one exact point". I think you're imagining that the wheel is perfectly round and the ground is perfectly flat. In that case there would be a line of contact. In fact the tyres are soft (and the ground is slightly soft too), and they get squashed together under the weight of the car, so there is some area in contact. Even a trains wheels have some small area in contact with the track, not just a point. –  bdsl Nov 16 '13 at 20:31

3 Answers 3

Yes , friction is the driving force for the car to run, consider simple example where you put grease on road, is the car able to move properly..? no..because friction becomes very less..hope you got the point...

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And even for trains with "leaves on the line" en.wikipedia.org/wiki/Slippery_rail –  Martin Beckett Dec 17 '11 at 16:44

Friction does help in movement of a wheel aka rolling motion.

enter image description here

See the image, the point touching the surface actually stays at rest which causes the rest of the wheel to move forward. This is caused due to friction. Now, if you don't have friction, nothing would be there to stop the wheel from spinning around its axis.

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Yes I think it is propulsion due to rolling, also the tires put a reaction on the road. The engine spins the wheels through transmission as you push the gas pedal and the engine accelerates, then you shift to higher gear and spin the wheels faster with less engine acceleration as you go to higher speed.

Hope that answers the question.


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