Question about friction on moving tires

[Raigon]

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I'm confused - the car is moving forward, so why is the force of friction on its wheels pointing in the same direction as the car? And why is it static instead of kinetic friction, because the car is definitely moving.

Help is appreciated.

 

[sleepy425]

I'm confused - the car is moving forward, so why is the force of friction on its wheels pointing in the same direction as the car? And why is it static instead of kinetic friction, because the car is definitely moving.

Help is appreciated.

look at it this way. imagine you hooked the car to a wall so it couldn't move, and then gunned the engine so that the tires would spin out. if you were looking at the portion of tread that is contacting the road, you would see it moving backwards from the direction that the car would be moving if it were not tied back. so the movement is in the backwards direction so the frictional force is in the forward direction. now in this case, with the tires spinning out, it's kinetic friction, because the tread is moving with respect to the ground. under normal conditions, it's static friction because the velocity of the tread with respect to the ground is zero (the velocity of any translating and rotating object at the point of contact with the ground is zero). but nevertheless, that's why the car moves forward, because the tread of the tire is trying to go backwards on the ground and friction is pushing forwards on it, thus pushing the car forward. like I said, it's static because at the point of contact with the ground, the velocity of the tread with respect to the ground is zero.

 

[Raigon]

Ah - I get the part about the direction of the friction pointing forward, because the tire is trying to push back, and it's the friction that's keeping the entire car from sliding back.

Now the part I don't understand is why the point of contact with the ground doesn't move. =/ Is it because the wheels are only rotating and have no translational movement that it's static? Or am I totally getting it wrong?

 

[sleepy425]

Ah - I get the part about the direction of the friction pointing forward, because the tire is trying to push back, and it's the friction that's keeping the entire car from sliding back.

Now the part I don't understand is why the point of contact with the ground doesn't move. =/ Is it because the wheels are only rotating and have no translational movement that it's static? Or am I totally getting it wrong?

It's only at the instant of contact. Imagine sitting on a skateboard (with your legs off the ground and everything). Now you put your palms on the concrete and push backwards to get yourself sliding forward. You'll notice that your hands don't move until you pick them up off the ground, but that your shoulders pivot and you roll forward. It's the same thing with a car. You hit the gas and the car starts to roll forward, but the piece of tread that was touching the ground doesn't start to actually move until the wheel has rotated so that it's no longer on the ground (another piece of tread will move in to take its place).

 

[Raigon]

...but the piece of tread that was touching the ground doesn't start to actually move until the wheel has rotated so that it's no longer on the ground (another piece of tread will move in to take its place).

Oh - I get it now. It was so un-intuitive. Bleh. Thanks for your help sleepy! =D I'm going back to work on my work and force problems now.

 

[RPedigo]

Now the part I don't understand is why the point of contact with the ground doesn't move. =/ Is it because the wheels are only rotating and have no translational movement that it's static? Or am I totally getting it wrong?

Yes, this is correct. Imagine slamming on your brakes in a car that doesn't have ABS. The tires lock up and you start skidding. The contact patch of the tire is constant because the wheels are locked, so it would be kinetic friction. However, each piece of tire is only touching the pavement for a fraction of a second, so it's not the same piece as you move along, so it's not dragging. Technically it's what's called rolling friction, but for the MCAT you can consider it static friction.

Edit: I'm too slow!

 

[sleepy425]

Oh - I get it now. It was so un-intuitive. Bleh. Thanks for your help sleepy! =D I'm going back to work on my work and force problems now.

no problem. and I'm actually sleepy now, so I'm gonna go sleep.

 

[Raigon]

Aight cool! Thanks guys! I finally get it!