Measuring the world: metres, kilograms and seconds
Scientists use numbers and units to describe things. Meet the three units you will use all the time.
Imagine you tell a friend: "I threw the ball really far." How far is that? Far for you? Far for a giant? Words like far, heavy and fast are not much use to a scientist, because they mean different things to different people.
Numbers mean the same to everybody. "I threw the ball 30 metres" is a sentence anyone in the world can understand. That is why physics is full of measuring.
Numbers need units
A number on its own is not enough. If someone says "the table is 2 long", you will ask: 2 what? Two steps? Two hands? Two metres?
The word that goes with the number is called the unit. Scientists around the world agree on the same units, so everyone's measurements can be compared.
The big three
Almost everything in physics is built from three measurements:
- Length is how long, tall or far something is. The unit is the metre (m). A door is about 2 metres tall.
- Mass is how much stuff something is made of. The unit is the kilogram (kg). A bag of sugar is about 1 kilogram.
- Time is how long something takes. The unit is the second (s). One heartbeat takes about 1 second.
You can see these in the games. Launch Lab shows the distance in metres and the flight time in seconds. Marble Machine shows heights in metres.
Mass is not weight
People often mix up mass and weight.
- Mass is how much stuff you are made of. It stays the same wherever you go.
- Weight is how hard gravity pulls on you. It changes from planet to planet.
On the Moon, gravity is weaker, so you would weigh less. But you would have exactly the same mass. You would not be made of less stuff, you would just be pulled less. (Everyday speech uses "weight" for both, but the difference matters in physics.)
Big and small: the prefixes
What if a distance is much bigger than a metre, like a road trip? Or much smaller, like a pencil lead? We add a short word in front of the unit:
| Prefix | Means | Example |
|---|---|---|
| kilo- | 1,000 times | 1 kilometre = 1,000 metres |
| centi- | one hundredth | 100 centimetres = 1 metre |
| milli- | one thousandth | 1,000 millimetres = 1 metre |
So 1 kilogram is 1,000 grams. And there are 60 seconds in a minute, and 60 minutes in an hour.
Units can be combined
Some measurements use two units at once. For example, speed tells you how many metres you travel in each second, so it is written metres per second (m/s). The slash means "per". We will use this a lot in the next lesson.
Tips for measuring well
- Measure more than once. Repeat three times and find the average. This cancels out small mistakes.
- Start at zero. Line up the end of your ruler with the start of the thing you are measuring.
- Look straight on. Reading a scale from the side gives a wrong answer.
- Write the unit down. Always.
How many of your own steps are 10 metres? Measure it with a tape measure and count. Now you can estimate distances just by walking. Next, count how many heartbeats you have in 10 seconds. Scientists did exactly this kind of thing with simple tools before there were clocks and computers.
Words to know
- Unit
- The name that goes with a number, like metres or seconds.
- Metre (m)
- The unit of length.
- Kilogram (kg)
- The unit of mass.
- Second (s)
- The unit of time.
- Mass
- How much stuff something is made of.
- Weight
- How hard gravity pulls on something.
Check yourself
Why is "I threw it 30 metres" better than "I threw it far"?
Show the answer
It uses a number and a unit, so everyone understands exactly how far, even people who were not there.
How many metres are in 3 kilometres?
Show the answer
3,000 metres, because kilo- means 1,000.
You travel to the Moon. What happens to your mass, and what happens to your weight?
Show the answer
Your mass stays the same, because you are made of the same stuff. Your weight gets smaller, because the Moon pulls less than Earth.