What Is the Normal Force on a Rollercoaster Car at the Top of a Hill?

[arrax]

Homework Statement

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

Homework Equations

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

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.

 

[ehild]

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.

Check the sign of the force.

ehild

 

[arrax]

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

 

[ehild]

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

ehild