## Essential University Physics: Volume 1 (4th Edition)

a) $-81.33 \ J$ b) $-221.34 \ J$
We know that the energy stored is equal to the negative integral of the force with respect to distance. Thus, we find: a) $U=-\int_0^{7.5} (.43x - .033x^2+.00086x^3)dx$ Evaluating this integral gives: $U = -8.133 = -81.33 \ J$ b) $U=-\int_0^{15} (.43x - .033x^2+.00086x^3)dx$ Evaluating this integral gives: $U = -22.134 = -221.34 \ J$