# Using friction in circular motion

1. Sep 29, 2015

### PAstudent

1. The problem statement, all variables and given/known data

2. Relevant equations
Fnet=m(v^2/R)

3. The attempt at a solution
I understood the FBD and how to find the bank angle. I just don't fully understand what part a and b are giving me in terms of friction. So for a, since it is skidding would only kinetic friction be used? Then for b, would I have to find the fs max because the tires don't skid?

2. Sep 29, 2015

### Student100

That sounds correct.

3. Sep 29, 2015

### SammyS

Staff Emeritus
Yes to: "So for a, since it is skidding would only kinetic friction be used? ".

Yes to: "Then for b, would I have to find the fs max because the tires don't skid?".

4. Sep 29, 2015

### PAstudent

Thank you for the replies

5. Sep 30, 2015

### haruspex

I'm not quite sure what you are asking there. Are you asking whether the only difference in the two solutions is which coefficient you should use?
If so, the answer is no. You also need to consider which direction the frictional force acts in.

The problem interpretation is a bit awkward in (a). What does it mean to ask whether the car will safely negotiate the bend while skidding, given that its speed will not be constant? To answer correctly, one would need to know how much margin there is, i.e. how far the car starts from the outer edge of the road. So presumably we must interpret it as how fast can the car go without its radius of travel increasing, i.e. as though it is already at the edge of the road.

6. Sep 30, 2015

### PAstudent

Is it correct for (a) to solve for the mass and plug that in to the other Newton's 2nd law to cancel the mass and normal force

7. Sep 30, 2015

### haruspex

As I indicated, you need to think about the direction in which the frictional force acts. The car is moving tangentially at some speed v, wheels locked, and skidding. If two surfaces are in sliding contact, which way is the force of friction?

8. Sep 30, 2015

### PAstudent

The friction force would oppose the direction of the velocity. So it's wrong to have my friction force down the slope of the track?

9. Sep 30, 2015

### haruspex

In case (a), yes, that is wrong.