posted by Anonymous .
A spring has a force constant of 70000 N/m.
How far must it be stretched for its potential energy to be 21 J?
well, let's see. You are only given two pieces of data, so the manipulation must be simple.
energy(N-m) = N/m * m^2
P.E. = 1/2 kx^2
21 = 1/2 (70000)x^2
x^2 = .0006
x = .0245m or 2.45 cm