As shown in the figure, the inclined belt at a constant speed of 2 V upward movement, a small block with initial velocity v 1 from bottom up thrust belt, and more than 1 v v 2, the small block from the bottom end of the drive belt to the top in the process of doing it (slow motion.)
So A is correct; when the block rate is less than V2, the friction block do do positive work, mechanical energy increases, so C error; if the belt sliding friction on the block is greater than the downward force of the conveyor belt, when the block rate reduced to V2, V2 to do uniform motion,
Therefore, the speed at the top may be equal to V2, so B error; because the mass has been slowing down, according to kinetic energy theorem, you can get the external force to do negative work, so D is correct.
By you know, when the conveyor belt on the sliding friction block is smaller than the downward force of the conveyor belt, the small block may have been slow to reach the top speed by just minus zero
