Why units matter
NASA lost a $125 million Mars orbiter because one team used metric and another used imperial. Units aren't trivia — they're safety.
The big five for ME
| Quantity | SI Unit | Common US | Symbol |
|---|---|---|---|
| Length | meter | foot, inch | m, ft, in |
| Mass | kilogram | pound (mass) | kg, lb |
| Force | newton | pound-force | N, lbf |
| Time | second | second | s |
| Energy | joule | BTU, foot-pound | J |
Conversion without crying
Use dimensional analysis — treat units like algebra:
- 1 inch = 2.54 cm (exact)
- 1 m ≈ 3.28 ft
- 1 kg ≈ 2.2 lb (mass)
- 1 N ≈ 0.22 lbf
Example: 60 mph to m/s
- 60 mi/h × (1609 m/mi) × (1 h/3600 s) ≈ 27 m/s
Force vs. mass (don't mix these up)
- Mass (kg): how much stuff
- Weight/force (N): mass × gravity
- A 50 kg person has mass 50 kg and weighs ~500 N on Earth
Shop rule: Always write units. Always. An answer without units is a guess with extra steps.