Scientists usually use a metric unit called a Newton to measure force. One Newton is about .225 pounds—a bit less than the force we'd need to exert to lift a stick of butter off a table. The Newton is named after Sir Isaac Newton.

How do we know how strong or weak a particular force is? Let's think about a force we encounter all the time: gravity. When we weigh ourselves on the scale, we're really measuring how strongly the force of gravity is pulling us down onto the scale. So, if a person weighs 130 pounds, she can calculate her weight in Newtons to be 29.23 N.

130lbs / 4.448 lbs/N = 29.23 N