pretty sure fc is negative also R is negative but W is positive. so... -Fc = -R + W or.. sign conversion or wtv makes it Fc = R - W i think..
You are correct that the net centripetal force at the bottom is 4mg, and the centripetal acceleration at the at the bottom is 4g. Now we already established (given) that v = 720 km/hr = 200m/s. So what's that 800/3.6 for the v value?Fc = R - W
Therefore mv^2r/r = 5mg - mg
So, mv^2/r = 4mg, mass cancles.
v^2/r = 4g, r = v^2/4g
so (800/3.6)^2 / (4*9.8) = r , so r = ~1260m. ??
Now you've got it! I'll leave it up you as an academic exercise if you what to assume something to see what may be happening at the top of the loop. Note that in more typical loop the loop problems, like yoyos in a vertcal circle , or loop the loop roller coasters , the speed is far from constant, so don't take this problem as a typical example of vertcal circular motion problems.Oops. there is an identical question to this, forgot different Velocity. so we would substitute 200ms into it instead of (800/3.6) so we'd end up getting.. ~1020m :)