Periodic motions and chaotic attractors of system (5) forα= 0.4. (a) Coexisting stick-slip period-1 solutions forf= 0.2. (b) Coexisting period-2 solutions forf= 0.449. (c) Coexistence of a period-2 solution and a chaotic attractor forf= 0.453 with the largest Lyapunov exponent 0.0475. (d) Chaotic attractor forf= 0.48 with the largest Lyapunov exponent 0.0563. (e) Period-3 solution with three stick-slip motions forf= 0.51. (f) Coexistence of a stick-slip period-2 solution and a chaotic attractor forf= 0.7645 with the largest Lyapunov exponent 0.0538.