# Homework Help: Physics homework help -- roller coaster car is going over the top of a 18m radius circular rise

1. Oct 9, 2014

### SilverAu

1. The problem statement, all variables and given/known data
A roller coaster car is going over the top of a 18-m- radius circular rise. At the top of the hill, the passengers "feel light," with an apparent weight only 60% of their true weight. How fast is the coaster moving?

2. Relevant equations
I think F=mv^2/ (r)
F=ma

3. The attempt at a solution
Attempt:
F=mv^2/r
(.6)g=v^2/r
(9.8*.6)r=v^2
5.88*18=v^2
v=10.3 m/s My answer is said to be wrong

2. Oct 9, 2014

### Staff: Mentor

This is not advanced physics, so moving to the intro physics section.

3. Oct 9, 2014

### nasu

You did not calculate what the question asks.
It's not the centripetal force which is 60% of the weight.
Draw a diagram with all the forces.

4. Oct 10, 2014

### SilverAu

OK well if I am solving for the velocity of the moving coaster, how would I do that because I've drawn the diagram with forces and I don't see it.

5. Oct 10, 2014

### nasu

Write Newton's second law for circular motion. The acceleration is the centripetal acceleration.
What is the expression for the net force at the top of the hill?

6. Oct 10, 2014

### HallsofIvy

If they only felt 0.6 of their weight then the centripetal force on them is 1- 0,6= 0.4 of their weight.

7. Oct 10, 2014

### SilverAu

Yeah I actually just figured it out, that is the only thing I messed up... Should be
F=mv^2/r
(.4)g=v^2/r
(9.8*.4)r=v^2
3.92*18=v^2
v=8.4m/s
Thanks for helping you guys