As shown in the figure, the left end of a light spring is fixed on the left end of the long wooden board m2, and the right end is connected with the small wooden block M1, and the contact surface between M1 and M2, M2 and the ground is smooth. At the beginning, M1 and M2 are stationary. Now, the horizontal constant forces F1 and F2 are applied to M1 and m2 at the same time. In the whole process after the two objects start to move, the system composed of M1, M2 and the spring (spring deformation in the whole process) is analyzed No more than its elastic limit) A. Because of the reverse direction of F1 and F2, the mechanical energy of the system is conserved. B. because F1 and F2 do positive work for M1 and M2 respectively, the kinetic energy of the system increases continuously. C. Because F1 and F2 do positive work for M1 and M2 respectively, the mechanical energy of the system increases continuously. D. when the spring force is equal to F1 and F2, the kinetic energy of M1 and M2 is the largest

As shown in the figure, the left end of a light spring is fixed on the left end of the long wooden board m2, and the right end is connected with the small wooden block M1, and the contact surface between M1 and M2, M2 and the ground is smooth. At the beginning, M1 and M2 are stationary. Now, the horizontal constant forces F1 and F2 are applied to M1 and m2 at the same time. In the whole process after the two objects start to move, the system composed of M1, M2 and the spring (spring deformation in the whole process) is analyzed No more than its elastic limit) A. Because of the reverse direction of F1 and F2, the mechanical energy of the system is conserved. B. because F1 and F2 do positive work for M1 and M2 respectively, the kinetic energy of the system increases continuously. C. Because F1 and F2 do positive work for M1 and M2 respectively, the mechanical energy of the system increases continuously. D. when the spring force is equal to F1 and F2, the kinetic energy of M1 and M2 is the largest

A. Because the sum of the work done by F1 and F2 on the system is not zero, the mechanical energy of the system is not conserved, so a is wrong; B. at the beginning, the spring force is less than F1 and F2, and the kinetic energy of the two blocks accelerates, and when the spring force is equal to F1 and F2, the force of M and M is balanced, and the acceleration decreases to zero, and then the velocity decreases and the kinetic energy decreases, so B is wrong; C. Because F1 and F2 correct the system first When the speed of the two objects decreases to zero, the spring force is greater than F1 and F2, and then the two objects accelerate to move in opposite directions. F1 and F2 do negative work on the system, and the mechanical energy of the system begins to decrease, so C is wrong. D. when the spring force is equal to F1 and F2, the force of M and M is balanced, and the acceleration decreases to zero. At this time, the speed reaches the maximum, so their kinetic energy is the maximum, so D is correct So D