Rollercoaster homework problem

1. May 10, 2012

arrax

1. The problem statement, all variables and given/known data
A roller-coaster car has a mass of 1150 kg when fully loaded with passengers. As the car passes over the top of a circular hill of radius 16 m, its speed is not changing. (a) At the top of the hill, what is the normal force (using the negative sign for the downward direction) FN on the car from the track if the car's speed is v = 8.5 m/s? (b) What is FN if v = 20 m/s?

m=1150kg
R=16m
v= 8.5 or 20

2. Relevant equations

a=v^2/R
F=m*[(v^2)/R]
-FN-mg=m[(-v^2)/R]

3. The attempt at a solution

Using the above equation I got b) correct with -17468.5 N. However, using the same equation with a) I get -6088.53 and it's incorrect. I don't know what I'm doing wrong.

Also, this is from Wiley Plus.

2. May 10, 2012

ehild

Re: Rollercoaster

Check the sign of the force.

ehild

3. May 10, 2012

arrax

Re: Rollercoaster

aside from a possible wrong sign, did you get a different number?

4. May 10, 2012

ehild

Re: Rollercoaster

Think: can the track pull the roller coaster? What happens if upward force is needed to keep it on circular path?

ehild