A student makes a simple pendulum by attaching a mass to the free end of a l.50-meter length of string suspended from the ceiling of her physics classroom. She pulls the mass up to her chin and releases it from rest, allowing the pendulum to swing in its curved path. Her classmates are surprised that the mass doesn't reach her chin on the return swing, even though she does not move. Why does the mass not have enough energy to return to its starting position and hit the girl on the chin?

The answer to the question are

Explanation:

This is due to the change of a portion of the kinetic energy of the swinging pendulum to other forms of energy to overcome the resistance to its motion . This is known as losses of energy

The energy losses  of the incurred by the swinging pendulum include

1. Drag from air resistance - This creates turbulence in the in the air surrounding the moving pendulum

2. Frictional resistance from the pendulum attachments -  this creates heat and sound at the point of attachment of the string

3. Conversion of kinetic energy to other forms of internal energy.