Unit 1, lesson 1. About 10 minutes.

Where is it?

How would you tell a friend exactly where your treasure is buried?

Play

Getting the experiment ready…

Explain

Everything has a place.

We can say near or far.

Position tells us where something is. To give a position, you need a starting point, just like the zero on a ruler.

Distance tells us how far apart two places are. We measure it in metres.

Move the flags and watch the distance change. Look: it doesn't matter which flag is on the left.

Position is measured from a chosen reference point, called the origin. The SI unit is the metre (m).

Distance is the size of the gap between two positions, so it can never be negative. We use the symbol x for position.

Worked example: a cyclist starts at 2 m and stops at 8 m. The distance is |8 − 2| = 6 m.

Pick a starting point
Measure along a line
Read the position

Want the full story?

Distance travelled and displacement are different. If you walk 5 m out and 5 m back, you travelled 10 m, but your displacement is zero. Displacement has a direction, so it is a vector. Real positions also need more than one number: a map uses two and the space around you uses three.

The Formula

distance = end position − start position

d = |x_2 - x_1|

$d$: the distance between the points, measured in metres (m)
$x_1$: the start position, measured in metres (m)
$x_2$: the end position, measured in metres (m)

Try the numbers

Start position: 2 mEnd position: 8 m

Distance: 6 m

Try it at home

You need a tape measure or ruler, a small toy, a chalk or paper marker.

You need: a tape measure or ruler, a small toy, a chalk or paper marker.

Hide a toy. Say near or far.

Mark a starting line on the floor.
Put the toy somewhere and measure from the line.
Give a friend the number and see if they can find the spot.

What's happening: a position only makes sense when you both agree on the starting point.

Physics is life

Your body

Your eyes and ears work out where things are.

Your brain works out positions all the time. It uses two eyes and two ears to judge how far away things are.

Daily life

Your home has an address.

Addresses, maps and sat navs all give positions, and they all use a shared starting point.

Nature

Birds find their way home.

Migrating birds can find their way across whole continents using landmarks, the Sun and the stars.

Space

Stars are very, very far away.

Astronomers measure the position of every planet and star. The nearest star after the Sun is about 40 trillion km away.

Quick check

Question 1 of 3

What does position tell you?

Words to know

Position: Where something is, measured from a starting point.

Position: Where something is, measured from a starting point.
Distance: How far apart two places are, or how far something has travelled.
Displacement: How far and in which direction something has ended up from where it started.

Next up

How fast? Speed