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PE lost from P to Q is converted to KE. Thus the 50J at P is converted to KE, that is 50J. It also has initial KE of 5J, so total becomes 55J. 10J is lost in work against friction, so the rest that is left is 45J.
The answer should be B
At the greatest height, the velocity of the ball would be 0. Thus this option can be ruled out.
For B, as we are neglecting air resistance, no energy is lost while doing work against friction. Thus, all the initial KE is converted to PE, and back to KE as it falls. This should be correct.
For C, the principle of conservation of momentum applies to a system, not just the ball. We would have considered this if it had stated that the momentum of the ball and the earth remained constant.
For D, we know that the ball is decelerating. Thus, it does not cover the same amount of distance every second. As PE depends on height, it therefore can not increase uniformly