13. A system of two pulleys lifting a weight.

Solution:

This image shows a movable pulley system with two pulleys. The weight is attached to the lower pulley, and the rope is attached to a fixed point above, then goes around the lower pulley, then around the upper pulley, and is pulled down. This arrangement provides a mechanical advantage, meaning less force is required to lift the weight compared to a single fixed pulley.

Answer: A system of two pulleys is lifting a weight.