Describe what happens to the molecules that make up was when it is heated

When heat is added to a substance, the molecules and atoms vibrate faster. As atoms vibrate faster, the space between atoms increases.

Explanation:

The motion and spacing of the particles determine the state of matter of the substance. The end result of increased molecular motion is that object expands and takes up more space.

one person pulls 90n one way and the other has 100n pulling the other way.

so the net force = 90 - 100 = -10n,

or the ans can be 10n if the picture shows -90n and 100n instead.