Question

For part (b), why should be just calculate kinetic energy in x direction (horizontal direction)? i.e. why should we use V0x ( = Vo * cos 55 ) instead of V0?

The problem and its solution is attached below:

http://www.chegg.com/homework-help/University-Physics-with-Modern-Physics-14th-edition-chapter-8-problem-98P-solution-9780133975888

Answer 1

Here we will consider conservation of energy only in horizontal direction as there would not be any impact due to collision in vertical direction.

This means what ever energy released during explosion, it imparted velocity in horizontal direction only. If it would have made an impact in vertical direction also, then one of the pieces will go upward and the other comes downward. As a result one of the masses will land first. But in the question it is mentioned that both of them landed at the same time. It will happen only when there would not be any impact in vertical direction.