# How fast is this roller coaster?

1. Mar 9, 2009

### Bottomsouth

1. The problem statement, all variables and given/known data
A 350kg roller coaster starts from rest at point A and slides down the frictionless loop-the-loop shown in the accompanying figure.

http://session.masteringphysics.com/problemAsset/1027487/5/1027487.jpg
1. How fast is this roller coaster moving at point B?
2. How hard does it press against the track at point B?

2. Relevant equations
1. it has potential energy at A = mgh =350*9.8*25= 85750 joules
at B it has PE = 350*9.8*12=41160 joules

the difference has been changed to kinetic energy = 1/2 mv^2

1/2 mv^2 =
1/2 (350)v^2 =85750 - 41160
=16 m/s this is good.

3. The attempt at a solution

2. F = mv^2/R = 350*16^2/6 = 14933.3

It says this isnt the answer.

2. Mar 9, 2009

### rock.freak667

There is your problem, the centripetal force,mv2/r is the resultant force. Normal reaction acts upwards,the weight acts downwards, the resultant of these two is the centripetal force (which you can calculate and you can also calculate the weight ,mg)

3. Mar 10, 2009

### Bottomsouth

How do I find normal force?

n-mg = C_f?

n - 350(9.81) = C_f

what is n?

4. Mar 10, 2009

### rock.freak667

Cf is the centripetal force mv^2/r, which you can easily find since you have v at point B, mass,m and the radius r.