when any block slides down a smooth incline which is unfixed then the incline moves back.WHY??
This will only be true if there is low friction between the inclined ramp and the ground surface. (For example, the ramp might be on rollers or tue surface below might be ice.)
The motion is due to the reaction force on the ramp.
Conservation of momentum. The ramp will move back with equal but opposite momentum from the block. In any case, even a fixed ramp, momentum is conserved but it becomes a much larger system, the earth ramp block system.
When a block slides down a smooth inclined plane, there are two main forces at play: gravity and friction. Gravity pulls the block downward, trying to make it slide down the incline. Friction, on the other hand, opposes the motion of the block, attempting to keep it in place.
When the block starts moving down the incline, friction acts in the opposite direction, trying to prevent its motion. However, since the incline is smooth, the friction force is minimal. As a result, the block can slide down the inclined plane.
Now, according to Newton's third law of motion, every action has an equal and opposite reaction. As the block moves down the inclined plane, it exerts a force on the incline in the downward direction. In response, the incline exerts an equal and opposite force on the block, pushing it back. This force is known as the reactive force.
The reactive force causes the incline to move back or recoil. The incline moves backward because it needs to exert an equal force on the block in the opposite direction to maintain equilibrium. This recoil motion of the incline is a direct consequence of the interaction between the block and the incline.
In summary, when a block slides down a smooth incline, the incline moves back due to the reactive force it exerts on the block to maintain equilibrium, as dictated by Newton's third law of motion.