# Static and kinetic friction in a car halting

1. Dec 24, 2013

### alingy1

In my textbook, there is a problem that made me think a bit more (this is not what the textbook was asking).

In the problem, there is a 1500 kg car travelling at 30 m/s on a 10 degree sloped hill (the car is driving uphill). The driver slams on the brakes and skids to a halt. We have to determine the stopping distance.

Okay. I get the answer to the problem.
What is intriguing me is how we classify the type of friction the car is going through at different steps.

It seems clear that, as the car is going to speed 0m/s, the friction is kinetic friction because of the word "skid".

Now, what happens when the car stops?
The problem says the car halts.
How is that possible? Won't the car immediately start falling down? The only thing that would be holding it back is the friction. However, which friction is it? Static or kinetic?

The car has velocity zero at a specific moment. Static friction only applies to objects "at rest". Does a car that reaches 0m/s then has the possibility to slide back considered "at rest"? Does having 0m/s velocity underlie being at rest?

I hope I have been clear. :(

2. Dec 24, 2013

### SteamKing

Staff Emeritus
The same brakes on the car which stop it will also keep the stopped car from sliding back down the hill. Brakes work both ways.

3. Dec 24, 2013

### alingy1

Yes, but, the brakes don't really work because the brakes are "locked". The friction is kinetic. Skidding happens and the car basically becomes a block of mass without any way of stopping no?

4. Dec 24, 2013

### alingy1

And what about the type of friction at 0m/s? Does the car suddenly switch to static and back to kinetic?

5. Dec 24, 2013

### SteamKing

Staff Emeritus
Obviously, if the car has stopped on the hill, there is no kinetic friction any longer. There must be friction between the tires and the road for the car to be able to move before the brakes were applied. If the car is stopped, it must be at rest.

6. Dec 24, 2013

### alingy1

How does the car stop if the brakes are locked? I don't really understand how a car with locked brakes can permanently halt on a hill.

7. Dec 24, 2013

### SteamKing

Staff Emeritus
If the brakes are locked, the car can't roll. If the static friction is greater than the car's weight component parallel to the incline, the car can't slide down the hill, either. Analyze and discuss.

8. Dec 25, 2013

### alingy1

The problem states that the car is skidding. If the car is skidding, the moment it reaches 0m/s velocity on the hill, it will either fall back or stay put. The brakes don't work as they usually do. I want to know how we can calculate if the car is falling or staying put. We have to know if the friction overcompensates the x-component force of gravity. Which coefficient of friction should we use, static, kinetic or rolling?

9. Dec 25, 2013

### Staff: Mentor

Presumably the wheels are locked. Once the car is at zero speed, you'll use static friction to see if it will slide back of course.