Car on an incline, kinetic/static friction when break applied confussion.

[alba_ei]

I've been doing some problems relative to car on an incline (and also boxes on an incline).
Let's suppose we are given a problem with kinetic or static coefficient.
I'm confusing when should or should not apply kinetic friction to a car, for example in some problems if brakes are applied then friction is involved (as if it were a box), however if brakes are not applied friction is not taken into account(Am I right? I'm not sure, if it is true then why kinetic friction is not taken into account despite the car is moving).
Does this mean that the maximum brake force is (kinetic friction)*(normal)?

What really confused me was the other day a problem in which impending slipping was involved.

Homework Statement

Data is not important, the problem is concepts not the problem itself.
A car of weigh W is on an incline it has certain initial velocity then brakes are applied, knowing that slipping is impending find static coefficient.

The Attempt at a Solution

I'm not looking for an anwser to this problem, I just want to understand. why is it that when brakes are applied static friction coefficient is involved. Shouldn't kinetic friction be involved as the car is moving with respect to the ground? Why in other problems with horizontal surface surface kinetic friction was involvved and not static friction as in this problem? Does this happen only when impending slipping is involved?

 

[LowlyPion]

You have 2 basic kinds of friction represented by the static and kinetic coefficients.

When you are applying the brakes, presumably they aren't locked and you aren't skidding, for if you were you would be dealing with kinetic as opposed to static friction considerations.

When you step on the brakes of a car the tire - even though rolling is not slipping - the crucial phrase. So the maximum force the driver can apply will be based on static friction coefficient. Usually the static coefficient is larger than than the kinetic and offers the better braking opportunity.

 

[nlightner]

As a scientist, it is important to understand the concepts behind friction and how it applies to different situations. In the case of a car on an incline, there are two types of friction that may be involved - kinetic and static.

Kinetic friction is the force that opposes the motion of an object when it is already in motion. This type of friction is present when the car is moving down the incline and the brakes are applied. In this situation, the kinetic friction force acts in the opposite direction of the car's motion, slowing it down and eventually bringing it to a stop.

Static friction, on the other hand, is the force that prevents an object from moving when a force is applied to it. This type of friction is present when the car is at rest on the incline and the brakes are not applied. In this situation, the static friction force acts in the same direction as the applied force (gravity), preventing the car from sliding down the incline.

Now, to address your question about when to apply kinetic or static friction in a problem - it depends on the specific situation and the forces involved. In the case of a car on an incline, if the brakes are applied, then kinetic friction will be involved because the car is already in motion. However, if the brakes are not applied and the car is at rest, then static friction will be involved to prevent the car from sliding down the incline.

In the specific problem you mentioned, where impending slipping is involved, static friction is used because the car is at rest and the brakes are not applied. The maximum brake force in this case would indeed be equal to the product of the static friction coefficient and the normal force acting on the car.

In conclusion, the type of friction involved (kinetic or static) depends on the situation and the forces at play. It is important to carefully analyze the problem and determine which type of friction is applicable in each case.