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Mass

We can think about mass in several ways. First, the mass of an object is a measure of the total amount of material (or stuff) in the object. The amount of stuff in an object is a fundamental property of the object. It doesn't change if you move the object to a new place, like a mountaintop or to Mars.

There is another way to think of mass. The mass of an object is a measure of how difficult it is to get it moving at a certain velocity if it starts from rest.
For example, if John wants to set a car, initially at rest, to moving at 1 m/s, he has to push hard for a little while. We are assuming the car's motion has no friction. If John and the car were on the Moon, his task would be equally difficult. The fundamental concept here is the mass of the car, not the astronomical body the car is on. (See Figure 2-1.)


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Figure 2-1

 

Saying this another way, the mass of an object is a measure of how much it hurts if your stub you toe on it. Stubbing your toe on a bowling ball is a painful proposition, even on the Moon.
 
There is a wrong way to think about mass. Many people think the mass of an object is a measure of how difficult it is to pick it up. But that definition depends on where you are. It is easy to pick up a bowling ball on the Moon, but nearly impossible on the surface of Jupiter. The difficulty in picking up an object is a matter of weight, which is a force. And weight does depend on the astronomical body near by.





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