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According to Newton's laws, if net force is zero on a particle then the particle is at rest or in uniform motion, and if it is not zero then it is accelerating. So when a car moves on the road, friction helps it to accelerate. If the acceleration is zero, (i.e) it is moving with constant velocity, the net external force must be zero, so friction is zero. Then with friction equal to zero how is the car moving on the road?

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3 Answers 3

You conclusion does not follow. You wrote:

it is moving with constant velocity, so net external force must be zero so friction is zero

But it does not follow, from the fact that net external force is zero, that frictional force is zero.

The net external force is the vector sum of all external forces on the object. The frictional force is but one of those forces.

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The friction force is clearly not the only force that is working on the car in this scenario. It is, for instance, not the force that is driving the car forward in the first place. Why else would we need engines, if friction did all the work for us? So, even when we ignore the vertical gravitational and (equal and opposite) normal forces, friction is not the only force at work here. Hence $$F_{friction}+F_{motor}=\sum F_x = 0 \rightarrow F_{friction}=-F_{motor}$$ when the car's velocity is constant.

This does NOT mean that $$F_{friction}=0$$

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I think you are not still familiar to this level of physics

So this is what happening there in a laymen's way

In a car the engine creates a force as you decide by pressing the accelerator, this force help's the car to move forward. Now there is a total frictional force acting on the car's wheels which constantly tries to slow down the car which is countered by the force crated by the engine which overcomes the frictional force acting on the vehicle

Now you are confused at the time when

it is moving with constant velocity, so net external force must be zero so friction is zero

actually here is bit different thing happening

Even if the vehicle is moving at an constant velocity still there is friction acting between the wheels of the vehicle and the road which constantly tries to slow down the vehicle but the force produced by the engine balance's it and keep's the vehicle moving forward, now as soon as you will release the accelerator the engine will stop producing any force and the force of friction will overcome it and the vehicle will gradually slow down.

And in the scientific language as said by @Alfred Centauri

The net external force is the vector sum of all external forces on the object. The frictional force is but one of those forces.

and hence there is still friction which keep's your vehicle from slipping from the road

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