Mass vs. Weight

Mass and weight are not the same thing.  Mass is the amount of matter in an object.  Weight is the force of gravity on an object.  Objects have weight because gravity is accelerating them towards the center of the Earth.  Newton's 2nd Law tells us that F=ma.  That means that an object's weight will be equal to the object's mass multiplied by acceleration due to gravity.  If we write Newton's 2nd Law in terms of the units involved, we see

This is where the definition of a Newton actually comes from!

Some key things to remember about mass and weight:
  • Units for mass are kilograms, and units for weight are Newtons (or sometimes pounds if we're not using SI units).
  • Mass is a scalar quantity, but weight is a vector quantity since it has a downward direction.
  • An object's mass will not change, but its weight may change based on location.  For instance, and object that weighs 150 lbs on Earth will weigh 351 lbs on Jupiter, or 9 lbs on Pluto.  You weight can even change based on your elevation on Earth.  If you weigh 100 lbs at sea level, you will weigh 99.72 lbs at the top of Mt. Everest.

Watch the following video about the difference between mass and weight: