difference between kinetic and static friction

[ibeatupnerds]

what's the difference between the two?

if a car goes constant velocity (say 10 m/s), this is static?
if a car accelerates (say a=5m/s^2), this is still static?

is kinetic just when there is "slippage" i.e. glide on ice?
is there any other examples?

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[salim271]

Static friction opposes any motion that causes sliding, as does kinetic. The only difference is that kinetic has two surfaces sliding against each other, and static doesnt.

A cars tire's experience static friction whilst they're accelerating or moving at a constant velocity, unless they turn too much too fast, like when you jam the accelerator or its icy/wet concrete and the coefficient of static friction is lower. So when a question asks you what direction does the static frictional force point with a car's tires when said car is moving forward, it points in the direction of the car's motion.

I find it best to think of it this way, imagine your car is driving up a ramp, if there was no friction, your car would slide backwards no matter how hard you press on the gas, thats because the friction between the car and the road gives it purchase and stops it from sliding backwards, because friction opposes any motion that causes sliding of the two surfaces against each other.

 

[Futurecalipt]

Car tires are tricky with static and kinetic friction. The way I think of it is like a shoe. When a tire rolls, it never leaves contact with the ground. For that instant, the ground and tire match like a shoe and the ground. It is stationary. So when a tire rolls, its like thousands of stationary moments. So its static friction. When a car begins to skid, it becomes kinetic because there is sliding between the two surfaces. Like those incline plane problems we did in class. When it begins to slide down the ramp, its kinetic. So when a tire begins to slide without grip, its kinetic. hope that ramble helps.