# Physics -- URGENT

Estimate the force a person must exert on a string attached to a 0.200 kg ball to make the ball revolve in a circle when the length of the string is 0.600 m. The ball makes 1.40 revolutions per second. Do not ignore the weight of the ball. In particular, find the magnitude of FT, and the angle phi it makes with the horizontal. [Hint: Set the horizontal component of FT equal to maR; also, since there is no vertical motion, what can you say about the vertical component of FT?]

FT = ? N
Angle = ? degrees
_______________________________
MY WORK:

FTy = Fg = (9.8)(.2) = 1.96 N

v = 2 rps * 2π(0.6) = 1.68π m/s

FTx = Fc = (mv^2)/r
FTx = [(.2)(1.68π)^2]/(0.6)
FTx = 9.28532 N

FT^2 = FTx^2 + FTy^2
FT = root(90.0587675)

FTy = FTsin(angle)
sin^-1(1.96)/(9.4899) = angle
Angle = 11.9 degrees [FINAL ANSWER]

Both of those answers were marked as incorrect by Webassign, but I've worked this problem over dozens of times and it's always the same. Am I doing something wrong?

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1. v = 2 rps * 2π(0.6) = 1.68π m/s

BUT it says 1.4 rps, not 2

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posted by Damon
2. Oops, that's a typo. With 1.4, it works out to the 1.68π m/s velocity I use in the next step. Did you see anything else?

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