Understanding speed
Distance over time.
One length, one time, four common ways to write the answer.
One quantity, many habits.
Speed is just distance divided by time. The metre per second is the SI unit and the one that physics uses; everyone else has adopted whichever combination of length and time fits their domain. Cars use kilometres or miles per hour. Ships and aircraft use knots. Athletes count metres per second; runners and cyclists translate to a pace.
1 m/s = 3.6 km/h ≈ 2.23694 mph ≈ 1.94384 kn
The everyday bridges.
The two conversions worth committing to memory are ×3.6 for metres per second to kilometres per hour, and ≈ 1.609 for miles per hour to kilometres per hour — the same factor as the mile-to-kilometre length bridge.
Practical conversions
- 1 km/h = 0.27778 m/s
- 1 mph = 1.609344 km/h
- 1 mph = 0.44704 m/s
- 1 knot = 1.852 km/h
- 1 ft/s = 0.3048 m/s
Three worked conversions.
100 km/h to mph
1 mph = 1.609344 km/h
An mph is bigger than a km/h, so the count shrinks — divide.
100 ÷ 1.609344 ≈ 62.137
= 62.137 mph
60 mph to m/s
1 mph = 0.44704 m/s
Multiply by the mph-to-m/s factor.
60 × 0.44704 = 26.8224
= 26.8224 m/s
20 knots to km/h
1 knot = 1.852 km/h
Multiply — knots are bigger than km/h.
20 × 1.852 = 37.04
= 37.04 km/h
A note on Mach.
Mach number is a ratio, not a unit — Mach 1 is whatever the local speed of sound happens to be, which depends on air temperature and density. The converter uses the standard sea-level value of 343 m/s; in cold thin air at altitude, Mach 1 is closer to 295 m/s. For headline numbers it's fine; for an aerospace engineer, context matters.
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