OMG Step 1 was soooo hard

[PlasmaBomb]

Uhh....?

Wheels not locked: Dynamic friction between brake pad and brake rotor.
Wheels locked: Dynamic friction between tire and pavement.

.....

Another way to look at it would be that the dynamic friction of the brake and rotor is not sufficient to overcome the static friction of the tire and road, however the static friction of the pad and rotor is...

 

[Farmer]

Key word - tire

Wheels rotating - coefficient of static friction between tire and road.

Wheels locked - coefficient of dynamic friction between tire and road.

Ok, but I don't see how the static friction in rolling without slipping has anything to do with slowing down your car. Wheelslip or locked wheels occur when your braking torque exceeds the torque available from tire-pavement static friction ("grip"), and hence you can no longer roll without slip. As far as I see that's the only part where static friction comes in.

If you want to compare how fast your car slows down, wheels unlocked versus locked, you're still comparing two dynamic frictions.

The dynamic friction of the pads/rotors is designed to be high - when the brakes are applied the pads transfer small amounts of friction material to the rotor (majority of this occurs during bedding in). The friction material on the pad and rotor then sticks to each other, slowing the vehicle.

Well, the pads are stationary, the rotors are moving. There is relative motion between the two surfaces. That's still dynamic friction.

The other thing to remember is that comparing static and dynamic friction only works within the same system i.e., changing between rotors and tires changes systems and you can't draw comparisons between static and dynamic friction as easily.

Well I thought you were comparing how long it took to stop a car using brakes (wheels unlocked) versus rubbing tired against pavement (wheels locked). In both cases, I believe it is dynamic friction.

 

[Farmer]

Another way to look at it would be that the dynamic friction of the brake and rotor is not sufficient to overcome the static friction of the tire and road, however the static friction of the pad and rotor is...

I see what you mean.

But the pads are at a "different level of engagement" in the two cases, so the situation is not exactly the same as, say, a box sitting on the ground still versus sliding.

 

[zebano]

Is it really that hard to cut a hole in a box?

damnit I came here to post that.

 

[PlasmaBomb]

Is the OP actually talking about this?

http://en.wikipedia.org/wiki/USMLE_Step_1

Well, the pads are stationary, the rotors are moving. There is relative motion between the two surfaces. That's still dynamic friction.

You will note that the sentence never implies that it is not dynamic...

 

[OCGuy]

Is it really that hard to cut a hole in a box?

Haha

 

[alien42]

Is it really that hard to cut a hole in a box?

winner