Angular Momentum Conservation and Conservation of Energy

Question

A heavy solid sphere is thrown on a horizontal rough surface with initial velocity $u$ without rolling. What will be its speed, when it starts pure rolling motion?

So using conservation of energy, I get the answer to be $\sqrt{5u}/\sqrt{7}$. But everywhere I checked the answer is marked as $5u/7$. They solved it using conservation of angular momentum. Why can't conservation of energy be applied here?

Answer 1

The sphere is initially slipping. Only when its speed drops to $5u/7$ does it start rolling. The sphere's speed drops because it is moving on a rough surface. The friction between the slipping sphere and the rough floor results in a loss of energy in the form of heat. Thus, the sphere's energy is not conserved. On the other hand, its angular momentum is conserved.