Roller Coaster Physics: Calculating Energy, Speed, and Force

[Schaus]

Homework Statement

Jessie (m=56.2 kg) is traveling at a speed of 12.8 m/s at the top of a 19.5-m high roller coaster loop.

a) Assuming negligible losses of energy due to friction and air resistance, determine Jessie's total mechanical energy at the bottom of the loop (h=0 m). (ans: 1.53 x 104 J)
b) Determine Jessie's speed at the bottom of the loop. (ans: 23.4 m/s)
c)What force does Jessie feel from her seat at the bottom of the loop? (ans: 3707N)

Homework Equations

F=ma
a_c=v²/r
mgh
1/2mv²

The Attempt at a Solution

I figured out a) with mgh+1/2mv²=Efinal.
I figured out b) with using energy from a) 1.53 x 10⁴= 1/2mv²
But I can't figure out c).
I tried using a_c=23.4²/19.5m to find my acceleration then multiply the mass but the number isn't close to the weight I need.

 

[TomHart]

It looks like you used the diameter, as opposed to the radius.

 

[TSny]

2nd law: F_net = ma. Note the "net" here.

 

[gneill]

Think again about what the radius of the loop is.

 

[Schaus]

Thanks! I totally missed that one and now my answer works!