24. What is the difference between mass and weight?
Mass is the measure of an object's inertia. You have more mass than a book, meaning that you are harder to accelerate than a book. If you and the book were each inside boxes, mounted on wheels, I could quickly determine which box you were in. I would simply push on both boxes and see which one accelerated most easily. That box would contain the book and you would be in the box that's hard to accelerate. Weight, on the other hand, is the amount of force that gravity (usually the earth's gravity) exerts on an object. You weigh more than a book, meaning that the earth pulls downward on you harder than it does on the book. Again, I could figure out which box you were in by weighing the two boxes. You'd be in the heavier box. So mass and weight refer to very different characteristics of objects. They don't even have the same units (mass is measured in kilograms, while weight is measured in newtons. But fortunately, there is a wonderful relationship between mass and weight: an object's weight is exactly proportional to its mass. Because of this relationship, all objects fall at the same rate. Also, you can use a measurement of weight to determine an object's mass. That's what you do when you weigh yourself on a bathroom spring scale; you are trying to determine how much of you there is-your mass-but you are doing it by measuring how hard gravity is pulling on you—your weight.