Mass and weight

We remind you that mass is a property of an object, while weight depends on the presence of gravity. The mass of a bowling ball is the same on earth and on Mars, but the weights are different. It's easy for those of us who don't engage in interplanetary travel to get confused, because on the surface of the earth, the weight of a kilogram is about 9.8 Newtons, and varies very little from place to place. However, we must use mass units in the Law of Force and Motion.

The Newton force was chosen so that 1 Newton causes a 1 kilogram object to accelerate at 1 m per second per second (that is, in every second the velocity changes by 1 meter per second). Then if we drop a 1 kilogram object, the unbalanced weight of 9.8 Newtons gives

Force = 9.8 Newtons = 1 Kg x acceleration

so that the acceleration is 9.8 meters/sec²; after the object has been falling for 1 second, its speed will be 9.8 meters/second. Changing the mass also changes the weight, so that this conclusion is true for all "falling" objects: baseballs and golf balls and cannon balls, whether moving upwards, downwards, or sideways.

When talking about the strength of the gravitational force, people sometimes refer to the ratio g = 9.8 Newtons/Kg as "the acceleration of gravity." It is indeed the acceleration of a freely falling object, but it has a more general role: there is always a force g x mass acting on any object, even when it is not accelerating at all. So we should call it something else: "the gravitational force constant." (Alternatively, run the words together "theaccelerationofgravity" and deny that you ever mentioned an acceleration).

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Discussion of the laws of motion