Physics textbooks say that the car's engine spins the wheels which put a force on the ground, which pushes the car (somehow) by Newtons 3rd law. But how does the ground actually make the car move? If the ground put a reaction force on the wheels, then the only place where it can occur is at the contact point, linear force from the wheel and the linear force from the ground would cancel and the wheel wouldn't spin. So how does the ground transfer the engine force to the rest of the car?
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If I was asking this question, I would start by asking about a bicycle. The reason being, it's easier to draw a diagram showing all of the components of the bicycle's drive train and all of the forces acting on those components than it is to do for a car's drive train. – Solomon Slow Jan 14 '22 at 13:55
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Seems like a duplicate of https://physics.stackexchange.com/questions/45653/given-newtons-third-law-why-are-things-capable-of-moving – d_b Jan 14 '22 at 16:33
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. . . . linear force from the wheel on the ground and the linear force from the ground on the wheel would not cancel as the two forces act on separate bodies and the wheel will spin.
Farcher
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But how does the ground actually make the car move?
By applying a static friction force forward on the car.
The ground's static friction force acting forward on the wheel is equal and opposite to the rearward force the wheel applies to the ground, per Newton's third law. Since the static friction force acting forward on the wheel, and thus the car, is the only horizontal external force acting on the car (neglecting air resistance) that force is responsible for accelerating the car, per Newton's second law.
If the ground put a reaction force on the wheels, then the only place where it can occur is at the contact point, linear force from the wheel and the linear force from the ground would cancel and the wheel wouldn't spin.
You have the common misconception that Newton's third law means the forces two objects exert on one another cancel. They don't because the force acts on on a different objects.
The static friction force the ground applies to the car causes it to accelerate forward per Newton's second law. The equal and opposite force the wheel exerts on the ground technically causes the Earth to accelerate backwards, also according to Newtons second law. But because the mass of the Earth is so great, its acceleration would be infinitesimal since acceleration equals force divided by mass.
Regarding the wheel "spinning", if by spinning you mean slipping on the road, the static friction force prevents that from happening, otherwise the car would not be able to accelerate.
Hope this helps.
Bob D
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I guess I should have phrased the question more clearly, but I was thinking about how the static friction force from the ground transates from the wheels to the chassis of the car and making it move as a whole. – Kefas Jan 14 '22 at 14:54
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from the free body diagram you see that the car move due to the constraint force $~F~$ between the wheel axel and the car suspension, for a static case this force is equal to the force between the wheel and the road $F_T$
- $~\tau~$ Engine torque (transmitted to the wheel)
- $~F~$ constraint force
- $~F_T~$ Tire force or constraint force (in case of no slip between the wheel and the road)
- $~I_W~$ wheel inertia
the equation of motion for the car acceleration $~\dot v_C~$ and with "no slip condition" is:
$$~\frac{M}{4}\dot v_C=F\\ F=\frac{M\,R\,\tau}{R^2(4\,m+M)+4\,I_W}$$
Eli
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