Circular motion - Edexcel Past Question (Jan 2010)

[jsmith613]

Homework Statement

I have attached the question that is troubling me including all relevant previous bits of the question along with their answers.

So the question asks me to show the radius of the track is 100m

Homework Equations

The Attempt at a Solution

Obviously I draw a free body diagram with F_normal vertically upward (as stated) and F_mg vertically downward. The centripetal force acts at a diagonal angle and is hence made by the vertical component force.
to do this calculation I need to know an angle but I don't have one??

The MS calculation is:
4mg - mg (3mg) = mv²/r
hence r = 100m (we have been told the speed is 57 ms^-1)
I don't see though how this is correct
please can someone help me
thanks

 

[darkxponent]

I too think that is correct. I just solved it and got the same answer. Where is the problem?