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I don't understand the difference between static friction and kinetic friction. I understand that the force of frictional force acting on a stationary object is the static frictional force and we use the coefficient of static friction to determine the minimum force needed to start the object moving, but once the object is moving, the frictional force can be static and kinetic and I don't understand how to differentiate between the two cases. For example: When car accelerates gently to a higher speed, the frictional force acting on it is static friction, why ? Why not kinetic ? If a car "floors it" and peels out of a stop light, the friction is kinetic, why ? A car takes a turn gently. Static. Again, how and why ?

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It doesn't matter whether the object itself is moving; what matters is whether the two surfaces involved are sliding past each other. If they are sliding past each other, the friction is kinetic; in contrast, if they are not, then the friction is static.

When a wheel rolls without slipping/skidding, the part of the wheel that touches the ground does not slide along the ground, and so the friction involved is static. In contrast, when a wheel slips/skids, the part of the wheel that touches the ground slides along the ground, and so kinetic friction is involved.

Applying the above, when a car accelerates gently, the wheels do not skid, so static friction is involved. When a car "floors it" and the wheels skid, kinetic friction is involved. When a car turns gently, without skidding/drifting, static friction is involved. When a car turns quickly enough to skid/drift, kinetic friction is involved.

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  • $\begingroup$ Okay thank you so much but if a car is moving at a constant speed, then is the friction kinetic or static ? The car is not sliding it's just moving. $\endgroup$
    – Jaca
    Commented Oct 22, 2019 at 12:08
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    $\begingroup$ @Jaca There is no friction involved at all there :) Similar to a book sitting on a table. $\endgroup$
    – BioPhysicist
    Commented Oct 22, 2019 at 12:12
  • $\begingroup$ @AaronStevens With air resistance etc a car moving at constant speed will have a frictional force acting on it due to the ground. With no slipping/skidding the friction is static otherwise it is kinetic. $\endgroup$
    – Farcher
    Commented Oct 22, 2019 at 12:21
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    $\begingroup$ @Farcher If the car is moving at a constant speed, it's already countering the torque on the wheels from air resistance with an equal and opposite torque from the motor. This means there's no net force at the point of contact. $\endgroup$
    – probably_someone
    Commented Oct 22, 2019 at 12:26
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    $\begingroup$ @probably_someone But this means that there is a friction force acting on the vehicle, which was the clarifying question OP asked (and Aaron said "there is no friction involved"). Your statements seemed to be supporting that there was no friction involved, which doesn't seem correct when the car is moving with a constant velocity and some form of resistance. $\endgroup$
    – JMac
    Commented Oct 22, 2019 at 16:12
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Static friction tries to prevent relative motion between two surfaces when the surfaces that are not moving relative to one another, whereas kinetic friction tries to reduce the relative motion between two surfaces.

If at the point of contact there is no relative movement between the tire and the road then the frictional forces involved are static whereas if there is relative movement between the tyre and the road, due to slipping/sliding/skidding, the frictional forces involved are kinetic/dynamic.

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Think about tire tracks in dirt. If they are smeared it was kinetic friction, if the are "clean" it was static friction.

It's probably more intuitive if you think of the contact surface to be really large and imagine what's going on while accelerating, breaking, moving at constant speed etc.

Let's look at tanks, because they have actually very large contact surfaces.

If a tank drives at constant speed every single track plate in contact with the ground does not move with respect to the ground - static friction. Unless the tank breaks or accelerates hard and the plates slip with respect to the ground. If the track plates move with respect to the ground then it's kinetic friction. Note that every plate of the tank (while driving) that is not in contact with the ground does indeed move with respect to the ground, but that doesn't matter for friction with the ground, because they are not in contact with it.

With car tires it's the same. All the parts which are not in contact with the ground are actually moving with respect to the ground - but don't matter for friction (like in the tank example), but the contact patch is completely stationary with respect to the ground (unless the tire slips).

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Probably someone has already explained but I would like to add few points:-

Static friction opposes impending motion while Kinetic friction opposes relative motion.

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