(2) When Xiaoyu replaces the weight in the right board with the weight of 40g, the block tends to move to the left, and a friction force to the right is generated between the block and the desktop, but the resultant force of the pulling force to the left and the friction force is equal to the pulling force to the right, so the block is still subjected to a balancing force; To avoid this phenomenon, we should reduce the friction between the wooden block and the desktop, replace the wooden block with a trolley, and replace the desktop with a smooth horizontal surface.
(3) Let F 1=F2, twist the car horizontally, and observe whether it is balanced when letting go.
(4) Under the action of the balancing force, the mass, velocity and kinetic energy of the object remain unchanged, and the object can rise in a straight line at a uniform speed, increase in height, increase in gravitational potential energy and increase in mechanical energy. This statement is correct.
So the answer is: (1) moving in a straight line at a uniform speed.
(2) The block is subjected to friction to the right; Balance; Replace the wooden block with a trolley (or reduce the friction with the desktop).
(3) Set F 1=F2, horizontally twist the trolley (block), and observe whether it is balanced when letting go.
(4) correct.