Difference between kinetic and static friction?

[Sean1218]

Homework Statement

A car of mass 1.1 x 103 kg negotiates a level curve at a constant speed of 22 m/s, The curve has a radius of 85 m. Determine the coefficient of static friction that allows the car to make the turn.

Homework Equations

mv^2/r = uFg and not sure what for static friction

The Attempt at a Solution

I know the difference between kinetic friction and static friction, but I can't see a difference in how to calculate them. How do I calculate static coefficient of friction?

I figured kinetic coefficient of friction (u) would be this:

mv^2/r = umg
v^2/r = ug
v^2/rg = u

 

[Delphi51]

I figured kinetic coefficient of friction (u) would be this:
mv^2/r = umg

All that looks good, Sean. But the μ here is the static coefficient, not the kinetic one. You would only use the kinetic value when the rubber is slipping on the road. The kinetic μ is smaller so once it starts to slip, it will really slip.

I recall when I was 16 and starting to drive my Mom's car, I couldn't get the darn thing to go up a little hill into our driveway. The wheels just spun on the spot. The μk was too small to get a grip. My cousin took it over, put the car in second (manual transmission) and slowly drove it up the driveway. He was very careful on the gas pedal and the car had very little torque in 2nd gear so he didn't slip and the μs was adequate.

 

[Sean1218]

How would I calculate coefficient of kinetic friction then?

 

[Delphi51]

You can't calculate the coefficient of kinetic friction with the information given. You would need information about a slipping car, not one whose wheels are rolling on the road. Note that the question asks for the coefficient of static friction.

 

[Paulie323]

The only type of friction that you have enough given information to solve for is μs there is no way unless there is more information to solve for μk