# Homework Help: Normal force at the top of a loop the loop

1. Feb 25, 2010

### BikeSmoth

1. The problem statement, all variables and given/known data
A car drives over a hill that is shaped as a circular arc with radius 65.0 m. The car has a constant speed of 14.0 m/s and a mass of 423 kg. What is the magnitude of (a) the centripetal force on the car at the top of the hill and (b) the normal force exerted on the car by the road at this point?

2. Relevant equations
F=(mv^2)/r
(mv^2)/r=mg+N

3. The attempt at a solution
so i have answered (a) correctly with F=1275.5N, but I am not sure if I have the correct equation for (b) with the equation above I got N=-2869.9N

2. Feb 25, 2010

### Mindscrape

You're algebra is good, so your numbers should be good too.

3. Feb 25, 2010

### jpas

No,

$$m \frac{v^2}{r} = N - mg$$

Try drawing a picture and the equation becomes clear.

4. Feb 26, 2010

### Mindscrape

Good catch, I didn't notice the wrong sign, my bad.