Difference Between Weight and Mass?

Objects in free fall have no weight but still have mass. However, we measure both weight and mass in kilograms. This is very confusing since weight is the push an object exerces on the ground while mass, according to E=mc2, is the amount of energy the object has. Why do we use the same unit for these 2 very different concepts? For example, the car has a weight of 1,350 kg but if one were to find the kinectic energy while the car is moving, one would have to use the car's mass in kilogram in the formula, which would be the same as its weight here on Earth. But the same car's weight will be different on the moon or in free fall while still having the same mass. While this works quite ok here on Earth, how one would find out an object's mass in free fall since it will have no weight? Why not using kilograms for weight and some other unit, such as Joules, for mass? Thanks
- Anonymous

Well, it's always been confusing to people.   Mass and weight are certainly related since the more mass you have the more you weigh.  They are different, however, since you would weigh less than on the moon than what you weigh on the earth even though you have the same mass in both places.   Mass and weight have different units of measurement, as you have pointed out.  Mass has dimensions of kilograms in the SI system.  Weight has dimensions of force or Newtons in SI units.  They are related because in the earth's gravitational field the 'weight' or force exerted is proportional to the mass times g, the acceleration of gravity at the earth's surface. 
The confusion exists because people factor out the constant g in weighing things on a bathroom scale.  In essence they arbitrarily set the scale factor g equal to 1, which is not true it's g  =  9.8 meters per second².
In sum: weight is a force,  mass is an inertial resistance to force.

If I have contributed to your confusion you might look at  for enlightenment.

LeeH

(published on 08/27/2010)