Moving or Not?

Why it matters

You're 'sitting still' reading this — while spinning at 1,600 km/h with the Earth. Rest and motion are always RELATIVE to something. Then physics splits every quantity into two tribes.

The idea in plain words

The two tribes. Tap them.

scalar: magnitude · vector: magnitude + direction

Walk 3 m east then 4 m north. Distance vs displacement?

7 m of walking, 5 m of displacement, direction north-east-ish

• tribe 1: scalars — distance, speed, mass, time, energy — add like ordinary numbers
• tribe 2: vectors — displacement, velocity, acceleration, force — direction matters, they add geometrically
• the fine print: relative rest — a body is at rest or in motion only WITH RESPECT TO an observer
• distance = 3 + 4 = 7 m (scalar sum)
• displacement = √(3² + 4²) (vector sum)

Predict first

Which pair is scalar–vector in that order?

Speed is just 'how fast' (scalar). Velocity is 'how fast AND which way' (vector) — the direction is not decoration, it changes the physics.

• speed – velocity — correct
• velocity – speed
• both are scalars

What you do

Sort the six quantities into scalars and vectors — the tribe decides how they add.

Check yourself

A passenger in a moving bus is at rest relative to…

Relative to co-passengers his position doesn't change; relative to the road it does. Both true at once.

Which quantity NEEDS a direction to be fully stated?

'A 10 N force' is incomplete — 10 N WHICH WAY decides whether the door opens or slams.

Two people walk 5 km each; one ends north, one south of the start. They have equal…

5 km of legwork each — but as vectors, north ≠ south.

• the other passengers — correct
• the road
• nothing — he is moving, full stop
• force — correct
• mass
• time
• distances but different displacements — correct
• displacements
• velocities