Physics Question

[HopefulOncoDoc]

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Which of the following reasons explains why a skidding car takes longer to stop than one that is not skidding?

A. A skidding tire transfers momentum more effectively than a rolling tire
B. The coefficient of static friction between the car's tires and the road surface is greater than the coefficient of kinetic friction
C. The skidding car loses some of the mass of its tires on the road surface
D. The force of kinetic friction is greater than the force of static friction between the car's tires and the road surface

I understand why and A & C are incorrect but I'm having trouble conceptualizing the reasoning behind why answer B is correct. Could someone please chime on this? Thanks a lot!

 

[JohnnyBravo]

Which of the following reasons explains why a skidding car takes longer to stop than one that is not skidding?

A. A skidding tire transfers momentum more effectively than a rolling tire
B. The coefficient of static friction between the car’s tires and the road surface is greater than the coefficient of kinetic friction
C. The skidding car loses some of the mass of its tires on the road surface
D. The force of kinetic friction is greater than the force of static friction between the car’s tires and the road surface

I understand why and A & C are incorrect but I'm having trouble conceptualizing the reasoning behind why answer B is correct. Could someone please chime on this? Thanks a lot!

This is a brilliant question and a typical one of the test. The answer is backwards. In order to take longer to stop the car, the kinetic frictional force must be weaker than the static frictional force. This is why it takes longer to stop when you slide. Now, with this in mind, this means that coefficient of static friction is greater than coefficient of kinetic friction.

You were confused because in order to take longer to stop, that means it has a lower force (mu kinetic) and in order to stop faster it has a greater force (mu static). I hope this helps.

 

[Geekchick921]

As the PP said, the coefficient of static friction is typically greater than the coefficient of kinetic friction for the same surface.

And just to add, if it's not clear, the wheels of a car (or bike, or wheelbarrow, whatever) actually utilize the static friction of the road, not the kinetic, when they are rolling. It is the static friction is what allows the tires to roll and the car to move forward instead of the tires just skidding.

 

[Excelsius]

Which of the following reasons explains why a skidding car takes longer to stop than one that is not skidding?

A. A skidding tire transfers momentum more effectively than a rolling tire
B. The coefficient of static friction between the car's tires and the road surface is greater than the coefficient of kinetic friction
C. The skidding car loses some of the mass of its tires on the road surface
D. The force of kinetic friction is greater than the force of static friction between the car's tires and the road surface

I understand why and A & C are incorrect but I'm having trouble conceptualizing the reasoning behind why answer B is correct. Could someone please chime on this? Thanks a lot!

For all practical purposes, you can always assume that the static frictions is always stronger than kinetic friction. In some cases both coefficients come very close, but the case of kinetic friction being stronger than static friction is a pretty complicated problem beyond the scope of any MCAT physics question.

 

[fizzgig]

And just to add, if it's not clear, the wheels of a car (or bike, or wheelbarrow, whatever) actually utilize the static friction of the road, not the kinetic, when they are rolling. It is the static friction is what allows the tires to roll and the car to move forward instead of the tires just skidding.

+1

a condition of rolling without slipping is that the velocity of the point where the wheel touches the ground = 0, so you have static friction

i remember being very weirded out by that idea initially though it does make sense..