# Help with centripital motion questions.

1. Nov 17, 2004

### DLxX

At what minimum speed must a roller coaster be traveling when upside down at the top of a circle if the passengers are not to fall out? Assume a radius of curvature of 8.6.

2. A 1000kg sports car moving at 20m/s crosses the rounded top of a hill(radius = 100m). Determine (a) the normal force on the car, (b) the normal force on the 70kg driver, and (c) the car speed at which the normal force equals zero.

Thanks

2. Nov 17, 2004

### Zlex

Well, I would solve the question for you, but looking around the forums that seems to be the thing not to do.

I'm not awesome at physics so perhaps I wll be corrected. But, consider the following for the first question:

1] F = m*a
2] For centripetal motion a = v^2 / r
3] Centripetal force will have to conteract that Force of gravity. ie Fg = Fc

2 would also seem to be pretty simple. Again, F = m*a, a =mv^2 / R

Mostly looks like plug and chug to me

Someone correct me if I'm wrong.

3. Nov 17, 2004

### Sirus

Centripetal force is not counteracting the force of gravity; they both point in the same direction.

4. Nov 19, 2004

### rhia

Thats right...if you solve the problem in the frame of the roller-coaster then centrifugal force will counteract gravity.

From the earth's frame the gravity and reaction force of the seat will provide the centripetal force.
So R+mg=mv^2 /r
For min v, R=0 i.e. the passengers are not in contact with their seats.
So vmin = sqroot(gr)

5. Nov 19, 2004

### Sirus

Actually centrifugal force does not exist.