Minimum static friction problem

1. Oct 10, 2004

AwesomeMan

A banked circular highway curve is designed for traffic moving
at 60 km/h. The radius of the curve is 200 m. Traffic is moving
along the highway at 40 km/h on a rainy day. What is the
minimum coefficient of friction between tires and road that will
allow cars to take the turn without sliding off the road?
(Assume the cars do not have negative lift.)

I managed to solve for the angle of the bank through the equation
angle = tan^-1((v^2)/gR). Where v=velocity, g=the acceleration of gravity,
and R=the radius of the curve.

My trouble is that after I solve for the angle I cannot think of anyway to solve for static friction without the weight or mass of the car. Am I thinking in the wrong direction to believe that i need the mass of the car?

It would be very appreciated if someone could point me in the right direction on this problem. Thanks.

2. Oct 10, 2004

Staff: Mentor

While it may seem that you need the mass of the car (for example, how else can you find the weight = mg?), if you actually work it out the mass of the car will drop out of the final solution. Give it a try.

Don't rush to "plug in numbers" right away. Work it out algebraically, using symbols. Once you've done the algebra, then plug in numbers and calculate.