Mass

Mass

Newton's first law says that a body would remain at rest, or move at the same velocity, unless a force acts on it. Suppose a body is at rest. Consider what happens when a force acts on it.

To make things simple, suppose that the body is on a perfectly smooth floor. When a force acts on it, it starts to move. How fast does it move? That depends on how big is the force, and how long the force acts. Here, we are interested in comparing the effect of the force on different bodies. Suppose we have a brick and a book. If we apply the same force to each for the same time, which obe moves faster?

In everyday language, we know that the book is lighter than the brick, so we would expect the book to move faster. However, lighter just means that there is less weight, which is a downward force of gravity. What has that got to do with pushing the book horizontally along the floor? The answer is, there is no obvious connection.

For some reason, the book would move faster and the brick slower. We can describe this by saying that the brick resists a change to its motion, more than the book does. Likewise, two books would resist change more than one book does, meaning that two books would move slower than one book if you apply the same force for the same time to both.

There is something in the book or the brick which resists a change to its motion. Some body resists change more than others. To tell us which body resists change more, we can associate a number to the body - a number that is bigger if the body resists change more. This number is called "mass."

It looks like I have taken a roundabout way to explain what mass is. Why can't I just say that it is the weight? The reason, as I have emphasised above, is that weight is a downward force, but mass is about how much the body resists a change to its motion. They are completely different ideas. It is a coincidence that a greater mass always means a greater weight.

The property that a body would resist a change to its motion is called "inertia."

We can summarise the above descripion of mass by saying that: mass is a measure of the inertia.

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