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Physics Foundations

Physics is a science of measurement. Master the SI units and prefixes, convert between them, and learn to estimate quantities the way a physicist does.

⏱️ 15 min 🎯 13 activities
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What you'll cover

A science of measurement

Physics describes the world with numbers and units. To compare results everyone uses the same SI system of units. Master the units, the prefixes and the art of a sensible estimate, and the numbers stop being scary.

SI base units

Every measurement builds on a few base units:

Match each quantity to its SI unit

  • Length
  • Mass
  • Time
  • Electric current
  • metre (m)
  • kilogram (kg)
  • second (s)
  • ampere (A)

A derived unit

What is the SI unit of force?

  • the newton (N)
  • the joule (J)
  • the watt (W)
  • the metre (m)

Prefixes

Prefixes scale a unit up or down:

Match each prefix to its multiplier

  • kilo (k)
  • milli (m)
  • mega (M)
  • centi (c)
  • 1000 (a thousand)
  • 0.001 (a thousandth)
  • 1,000,000 (a million)
  • 0.01 (a hundredth)

Convert it

A distance is 3 km. How many metres is that? (1 km = 1000 m)

Convert again

A length is 250 cm. How many metres is that? (100 cm = 1 m)

Sensible estimates

Physicists often need a quick estimate with the right order of magnitude. An adult is about 2 m tall (not 20 m or 0.2 m); a family car has a mass of about 1000 kg; a person walks at about 1.5 m/s. Knowing typical sizes lets you spot when an answer is clearly wrong.

Base units

Select the TWO correct SI base units.

  • the metre (m) for length
  • the kilogram (kg) for mass
  • the newton (N) for length
  • the litre (L) for mass

Estimate it

Pick the sensible estimate each time, checking the order of magnitude.

  • Estimate the height of an adult person.
  • Estimate the mass of a family car.
  • Estimate a person's walking speed.

Foundations frame

The SI unit of length is the _____ and of mass the kilogram. The prefix kilo means a thousand, while _____ means a thousandth. To convert 3 km to metres you multiply by _____. A good estimate has the right order of _____.

metre milli 1000 magnitude newton mega 100 colour

The prefix

What does the prefix "milli" mean?

  • one thousandth
  • one thousand
  • one million
  • one hundred