Kinetic vs Static Friction - The basic idea

[betterfuture]

I have always gotten confused between the idea of when kinetic friction is applicable vs static friction. I've gotten the bulk of Force and friction and how to solve when needed except when both show up. I know that kinetic friction is always smaller than static friction because it takes more force to get a stationary object moving that one that is already moving. But, I have read up on something that really bothers me.

When a car is towed at constant velocity, the frictional force between the road and its tires is static and in the opposite direction to the motion of the car.

Yet, when a car moves under its own power at constant velocity, the frictional force between the road and the tires that propel the car is static and in the direction of the motion of the car.

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[aldol16]

Friction always opposes the direction of motion. Where you're confused is that you're drawing the motion of the car whereas the friction is opposing the movement of the wheel. The wheel is rotating to the left in both cases.

 

[betterfuture]

Okay. So if the wheel is moving to the left, then friction is opposite, meaning pointing towards the right.

But how is it - friction - opposite the motion of the car in the first scenario but in the direction of the motion of the car in the second scenario. That I seem to not understand.

 

[aldol16]

But how is it - friction - opposite the motion of the car in the first scenario but in the direction of the motion of the car in the second scenario. That I seem to not understand.

I'm not sure where you're reading all this then. Are the wheels in the first case rotating or locked? That is, is the car being dragged with the wheels locked or no? If not, then I don't understand why you would draw the friction force in the direction you did in the first case. The wheel is rotating towards the left, meaning the frictional force will act towards the right, opposing the motion of the wheel.

 

[betterfuture]

Ignore the messed up drawing lol. I just wasn't sure what they meant. Read this from EK physics book and was trying to figure out what they meant.