A little girl slid down a playground slide, decreasing her potential energy by 1000 J while increasing her kinetic energy by only 900 J. Why did she lose 100 J of mexhanical energy

This is because The 100 J of potential energy that doesn't go into increasing her kinetic energy goes into thermal energy-heating her bottom and the slide.

Due to friction, 100 joules were lost in the form of heat energy

Explanation:

The law of conservation of energy states that

"Energy can neither be created or nor destroyed but rather it changes from one form to another"

As the little girl was sliding down the playground slide, her potential energy was decreasing and at the same time her kinetic energy was accumulated. As we know there is friction in the slide and because of this not all of the of potential energy is converted into kinetic energy. Some of the energy will be lost due to friction taking the form of heat. Therefore, the 100 joules of energy was lost in the form of heat due to friction of the slide.