An unbanked circular highway curve on level ground make a turn of 90 degree. The highway carries traffic at 60 mph, and the centripetal force on a vehicle is not to exceed 1/10 of its weight. What is the minimum length of the curve in miles? I am working towards the arc contained within the angle theta which is 90 degrees. I have easily determined that of course, that arc is 25% of the circumference. I am trying then to find the radius. I have worked backwards here, fiddling with Ar=V^2/R as well as MA = mv^2/r. I am stuck though. Not really looking for the answer, just a shove in the right direction as that I would REALLY like to get this one on my own. I just dont have the physics brain.