Periodic motions and chaotic attractor of system (5) forα= 0. (a) Coexisting stick-slip period-1 solutions forf= 0.2. (b) Period-8 solution forf= 0.38. (c) Period-3 solution forf= 0.7. (d) Chaotic attractor forf= 0.75 with the largest Lyapunov exponent 0.0218.