Density-dependent symmetry energy is a hot topic in nuclear physics.Many laboratories across the world are planning to perform related experiments to probe symmetry energy. Based on the semiclassical BoltzmannUehling-Uhlenbeck (BUU) transport model,we study the effects of nuclear symmetry energy in the central reaction 40Ca+124Sn at 140 MeV/nucleon in the laboratory system.It is found that the rapidity distribution of the free nucleons neutron-to-proton ratio is sensitive to the symmetry energy,especially at large rapidities.Free neutron-to-proton ratios at small or large rapidities may reflect high or low density behavior of nuclear symmetry energy.To probe the density dependence of nuclear symmetry energy,it is better to give the kinetic distribution and the rapidity distribution of emitted nucleons at the same time.