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An electrically neutral model airplane is flying in a horizontal circle on a 2.5-m guideline, which is nearly parallel to the ground. The line breaks when the kinetic energy of the plane is 50 J. Reconsider the same situation, except that now there is a point charge of +q on the plane and a point charge of -q at the other end of the guideline. In this case, the line breaks when the kinetic energy of the plane is 52.6 J. Find the magnitude of the charges.

q=_____________C
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Added Sun, 28 Jun '15
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Suppose the centripetal force in case 1 is Fcmax

F_cmax = mv_1^2 / 2    , KEmax = mv_1^2 / r

The centripetal force in case 2 is F_c' = F_c + F_q where F_q is the extra force by the charges.

Now, at the breaking point, F_cmax + F_q = mv_1'^2 / r

Hence, KE'_max - KE_max = m(v_1'^2 - v_1^2) / 2 = 2.6 J

and F_q = m(v_1'^2 - v_1^2) / r

=> k_q^2 / r^2 = m(v_1'^2 - v_1^2) / r

=> q^2 = mr(v_1'^2 - v_1^2) / k = 2(KE' - KE)r / k = 5.2(2.5)/(9*10^9) = 14.44*10^-8

=> q = 3.8 * 10^-4C
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