If you press a book flat against a vertical wall with your hand, in what direction is the friction force exerted by the wall on the book?.

the bal will not change its velocity.

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.

causes

fossil fuels burn

cities become more industrialized

human population grows worldwide