# Homework Help: Coeffincient of static friction

1. Oct 9, 2007

### pinkyjoshi65

Kim has her coffee cup on her car's dash when she takes a corner with radius 4m and 20km/hr. What is the minimum coefficient of static friction which would aloow the coffee cup to stay there without slipping?

this is what i did, but i think it is wrong

MU= friction force/normal force= ma/mg= a/g= v^2/rg
so mu= v^2/rg= 30.80/16*9.8=0.19

2. Oct 9, 2007

### Staff: Mentor

You equation is correct but your arithmetic is not. Did you square the r by mistake?

3. Oct 9, 2007

### pinkyjoshi65

so mu should be = v^2/rg= 30.80/4*9.8=0.78
that is the minimum coefficient of satic friction which would allow the cup to stay in postition without slipping?

4. Oct 9, 2007

### Staff: Mentor

Yep. (If you're dealing with one of those online systems, you may want to double check the accuracy of your calculations lest you be off a bit. Redo v^2.)

5. Oct 9, 2007

### pinkyjoshi65

how do you know that it is the minimum mu and tht it will make the cup stay in position?

6. Oct 9, 2007