The shared language of science

At home you might say, "I've done so much work today" or "I threw it with force." In science, words like force, work, cell and reaction have one exact meaning each. You will meet force and work properly in Chapters 6 and 7. Exact words mean two scientists in different countries understand each other perfectly.

Science also gives quantities short symbols and fixed units:

• mass: m
• velocity: v
• force: F
• electric current: I

"Is maths in science just for calculating?" No. Think of it as a language. An equation is a short sentence that says how things are linked. A road sign says 80 km/h. If you drive 240 km in 4 hours, your average speed is 240 ÷ 4 = 60 km/h, so you were within the limit. The equation speed = distance ÷ time tells you how distance, time and speed belong together.

So, the right order is: first understand the situation, then pick out the quantities, and only then use the equation to reason.

Why units matter: a true story. An aircraft needed 22,300 kg of fuel. The crew worked out how many litres to load using the fuel's density in pounds per litre instead of kilograms per litre. A pound is much lighter than a kilogram, so they loaded far too little: about 15,000 litres short. The aircraft ran out of fuel in the air and had to glide to an emergency landing. Using one system of units everywhere, the SI units, avoids mistakes like this.

When you buy 1 kg of rice in Delhi and 1 kg in Kolkata, you expect the same amount. A kilogram means the same everywhere because measurements are based on agreed international standards, not on local weights or anyone's opinion. That keeps trade fair.

Many symbols come from history. c, the symbol for the speed of light, comes from the Latin word celeritas, meaning speed. Today the speed of light is defined to be exactly 299792458 m/s.