What is the Angle of Swing for a Block in a Van on an Unbanked Curve?

[physics_]

Homework Statement

A block is hung by a string from the inside roof of a van. When the van goes straight ahead at a speed of 28m/s, the block hands vertically down. But when the van maintains this same speed around an unbanked curve (radius=150m), the block swings toward the outside of the curve. Then the stringmakes an angle A with the vertical. Find angle A.

Homework Equations

F=mv^2/r?
FsMax = (mew)s Fn ?

Mew = coefficient of static friction.

The Attempt at a Solution

Managed to find coefficient of static friction by equating the two equations above and substituting w=mg=Fn. Coefficient based on my calculatinos was 0.53. Didn't help me answer the question.
Then I reasoned that based on a drawing, which may be flawed, that tanA = Fn/r. Fn= normal force which would lead to equating FsMax = Mv^2/r. Couldn't find the mass though. If I had found the mass, I could find one of sides of hte triangle the angle is in and since radius is given, it would be easy to determine angle A.

 

[ehild]

When the block is stationary with respect to the van it moves around the same circle and with the same speed as the van. Where does the centripetal force come from?

ehild

 

[HallsofIvy]

And what does "static friction" have to do with this? I see no mention of friction in the problem. The block is not sitting on anything to have friction with.

 

[physics_]

Anyone have a solution? The answer in the book is 28 degrees. I was merely guessing with teh work I posted above.

 

[ehild]

Make a drawing like the one attached, and find out what forces act on the hanging box. The resultant has to be the centripetal force.

ehild

 

[physics_]

I seem to have made the drawing fairly accurately already. The problem is the reasoning to solving the problem.

 

[ehild]

You see the right triangle with sides mg and Fcp? What is tan(theta)?ehild