The difference between mass and weight is commonly misunderstood in the world of physics and engineering. Mass is the traditional metric (SI) term for gram or kilogram. The English (Imperial) unit of mass is called a slug. A pound is actually a force (F=ma), or weight when a is acceleration due to gravity (downward). (m is mass in the equation for force, also referred to as Newton's 2nd Law) Weight can be a complicated term, confusing the issue in my opinion, so I prefer to use the term force when dealing with physics or engineering problems. If you weight 150 pounds, that is the same as 150 pounds of force downward. Remember, a pound is a unit of force, not mass.