# Homework Help: Problems with problems

1. Oct 6, 2004

### Scotty

Hey everyone, I'm pretty new here, I've stopped by a couple times before to browse the other problems.

In my physics class, my teacher gave out about 10 problems, and most of them I was able to get by. I've looked at em...tried em out...decided to go on with other problems, and now i'm down to these 2 darn problems lol.

If anyone can post some hints or pointers on how i might go about solving these, it would be greatly appreciated.

thanks

3.) In an amusement-park ride, riders stand with their backs against the wall of a spinning vertical cylinder. The floor falls away and the riders are held up by friction. If the radius of the cylinder is 5.0 m, find the minimum number of revolutions per minute to prevent riders from dropping when the coefficient of static friction between a rider and the wall is 0.45.

5.) A civil engineer is asked to design a curved section of roadway that meets the following conditions: With ice on the road, when the coefficient of static friction between the road and rubber is 0.07, a car at rest must not slide into the ditch and a car traveling less than 55 km/h must not skid to the outside of the curve. What is the minimum radius of curvature of the curve and at what angle should the road be banked?

I really appreciate any help that people have. I'm sorry if this has already been posted, i've been at these problems for a while, and my eye's are strained worse than any clear eye's commercial could ever portray.

thanks again,

-Scott

2. Oct 6, 2004

### HallsofIvy

"Coefficient of static friction" is the number multiplied by the "normal" force to get the friction force. In this problem, you need the friction force to overcome the person's weight: 9.8*mass. with 9.8*mass= 0.45*force of wall, you get
force of wall= 9.8*mass/0.45.

Now, what formula do you know for calculating centrifugal force (or its twin centripetal force)? That is the "force of wall" above.

For the second problem, its much the same thing. Here you are given the speed (55 km/hr) and asked to find the smallest radius that will not cause too much centrifugal force.