The diagram shows a swinging pendulum.

A pendulum hanging from a bar is shown in 3 positions. 1 is swung to the right, 1 to the left, and 1 in the middle. Solid lines connect the bob to he support in the right and left positions. A dotted line connects the bob to the support in the center position. Dotted lines connect right, left, and center along the arc of the pendulum swing.

Which best explains what happens to the motion of the pendulum due to the law of conservation of energy?

The mechanical energy transforms to thermal energy as the pendulum slows and eventually stops moving.
The mechanical energy disappears as the pendulum slows and eventually stops moving.
The pendulum will continue moving at the same speed until someone stops it because energy cannot be destroyed.
The pendulum will slow down but will never stop moving because energy cannot be destroyed.