# Homework Help: Unbanked curve problem

1. Oct 22, 2011

### physics_

1. The problem statement, all variables and given/known data

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.

2. Relevant equations

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

Mew = coefficient of static friction.

3. 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.

Last edited: Oct 22, 2011
2. Oct 22, 2011

### 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

3. Oct 22, 2011

### 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.

4. Oct 22, 2011

### physics_

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

Last edited: Oct 22, 2011
5. Oct 22, 2011

### 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

#### Attached Files:

• ###### vanbox.JPG
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6. Oct 22, 2011

### physics_

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

7. Oct 22, 2011

### ehild

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

ehild

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