Force is an interaction between objects that causes them to change motion. So, when you opened your locker door, your hand applied a force to the door, causing it to change its motion. We measure force in newtons (N), the scientific unit for measuring weight. One newton is equal to about 0.22 pounds. So, a girl weighing only 100 lbs. would be equal to approximately 445 newtons, or her force due to gravity (F g).