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## Homework Statement

As test of strength, a diabolical trainer sets up the following apparatus. The trainee must maintain a variable pulling force which is always tangent to a nearly frictionless, semicircular surface . By slowly varying the force, a block with mass 23.0 kg is moved (at a very slow constant speed), and the spring to which it is attached is stretched from position 1 to position 2 (through an angle of 36 degrees). The spring has negligible mass and force constant 450 N/m. The end of the spring moves in an arc of radius a~=~ 49.0 cm.

http://session.masteringphysics.com/problemAsset/1000054849/9/YF-07-41.jpg

**What is the necessary size of the trainee's variable pulling force at position 2?**

## Homework Equations

F=-kx

## The Attempt at a Solution

I found the length of the arc first

36/180 = .2

C = 2(pi)R(1/2)

C = 0.49(pi)

Arc = .2(.49*pi)

Arc = 0.307876 m

then i multiplied that by the spring force constant

F = 0.3079(450)

F = 138.5 N

I think this is wrong cause i am not taking the mass into consideration but i wasnt sure where or how to include the mass of the block.

Any suggestions?

Thank you