Weight and mass

The force with which the Earth pulls an object is called its weight. Weight tells us how strongly the Earth pulls on something. Because weight is a force, it is measured in newtons (N), like any force.

Does the Earth pull everything equally? (Activity 5.9) Hang a spring from a nail and hang different things from it, such as a pencil box, a tiffin box and a small stone. The spring stretches by different amounts. So the Earth pulls different objects with different forces: their weights are different. And a spring can be used to measure weight.

A spring balance does exactly that. A spring is fixed at one end and has a hook at the other. Hang an object on the hook; the more the spring stretches, the greater the weight. Its scale is marked in newtons, usually with a second scale in grams showing the matching mass, worked out for use on the Earth. (A bathroom weighing machine, on the other hand, shows your mass, in kg.)

Read your instrument first (Activities 5.10 and 5.11).

Every spring balance is different, so always check the one you are given. And never hang anything heavier than its maximum (Activity 5.12), or you may damage it.

Now the difference that muddles many people:

Gravity varies very slightly from place to place on the Earth, and a lot on other planets. So weight changes but mass does not. On the Moon, gravity is about one-sixth of the Earth's, so you would weigh about one-sixth as much, but your mass would be exactly the same. The book's table shows one object's weight on the Earth, the Moon, Mars, Venus and Jupiter.

In everyday talk we say the wheat bag weighs 10 kg. In science that's not correct: 10 kg is its mass. Use the right word with the right unit, even if daily speech is more casual.

Floating and sinking. Push a closed empty bottle into a bucket of water (Activity 5.13). You feel the water pushing it up, and when you let go it bounces back to the top. A mug feels lighter inside the water for the same reason. All liquids push up on objects in them. This upward force is called upthrust, or the buoyant force.

So two forces act on anything in a liquid: gravity pulls it down and upthrust pushes it up.

The Greek scientist Archimedes found that the upthrust equals the weight of the liquid the object pushes aside (Archimedes' Principle). Upthrust also depends on the density of the liquid, which you'll learn about later.

Most rocks sink, but pumice floats. It forms in volcanic eruptions, when lava full of gas cools so fast that tiny bubbles are trapped inside. It is full of air pockets, which make it less dense than water.