v = 72km/h = 72000m/3600s = 20m/s
向心力F = mv^2/r = 1 * 20^2/400 = 1N;
重力G = mg = 10N;
此外弹簧弹力提供向心力
根据力的合成原理
(F^2 + G^2)^(1/2) = 根号101