MCAT practice - qualitative friction questions

[cbrons]

Homework Statement

224. When a car is towed at constant velocity, the frictional force between the road and its tires is:

A: static and in the direction of the motion of the car
B: static and in the opposite direction to the motion of the car
C: kinetic and in the direction of the motion of the car
D: kinetic and in the direction opposite to the motion of the car

225. 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:

A: static and in the direction of the motion of the car
B: static and in the opposite direction to the motion of the car
C: kinetic and in the direction of the motion of the car
D: kinetic and in the direction opposite to the motion of the car

Homework Equations

none

The Attempt at a Solution

The answers given at the back of the book is B for 224 and A for 225.

I selected C for both, since I figured if the car is moving it would involve kinetic friction and my understanding was that it would be in the direction of the car, since the tires would, at least in my mind, be "pushing the road" backward so therefore friction would be in the direction of the car.

So though the answers were given, I do not at all understand why this is? Perhaps someone might be able to offer me a good explanation of this? Thank you for your time..

 

[JaWiB]

It's static in both cases because there is no slipping occurring. Contrast with either wheels being locked while stopping (the car is skidding) or "burning rubber" while speeding up, which both involve kinetic friction.

For the direction of the forces, I'm not sure what the simplest explanation is. One way to think about it is that when the car starts to move, the wheels have to have an angular acceleration contributing to rolling forward. A friction force in the direction that the car ends up moving would cause a torque in the opposite direction.

For the second question, the wheel has a net torque on it such that the bottom of the wheel wants to move in a direction opposite the car's motion. Think about trying to push a block in that direction: friction has to be opposite the direction that you are pushing.

 

[cbrons]

thank you, makes more sense now