How do you calculate this?? Physics Help! Friction?

Question

We have had to build a roller coaster. Now we have to calculate force of friction but I do not know how. Here is the data that I have:

the acceleration of the ball on the initial drop= 6.4 m/s

the average speed of the drop= 1.67m/s

the average theoretical speed of drop = (speed without friction) = 1.73 m/s

theoretical instantaneous speed after drop( without friction)= 3.46 m/s

height of drop = 0.61 m

length (distance) of drop= 0.9 m

mass of marble = 5 g

Answer 1

The way to get at the answer is to look at the loss of energy due to friction:
The theoretical kinetic energy less the starting
.5*5*3.46^2/1000
0.0299 J
or use the loss in PE
5*9.81*0.61/1000
0.0299 J

The actual KE is
.5*5*3.34^2/1000
0.0279 J
the loss is due to friction

using
work=F*d
0.0299-0.0279=F*0.9
solve for F, that is friction
F=0.00222 N

j