Learning goals

  • Convert among km/h, mph, m/s, and knots using reliable factors
  • Relate average speed to distance and time
  • Avoid mixing hours and seconds in the same calculation

Speed is distance over time

Average speed is distance traveled divided by time taken. The unit you report depends on which length and time units you choose: meters per second, kilometers per hour, miles per hour, or knots (nautical miles per hour).

Everyday driving uses km/h or mph. Physics problems often prefer m/s. Marine and aviation contexts frequently use knots.

Because speed already embeds two dimensions, converting is still a factor problem — but keep track of both the distance part and the time part when deriving from first principles.

Average speed

speed = distance ÷ time

Rearrange as distance = speed × time or time = distance ÷ speed. Keep units consistent (e.g. km with hours for km/h).

Common speed factors

1 m/s = 3.6 km/h and 1 mph ≈ 1.60934 km/h

Also: 1 kn ≈ 1.852 km/h ≈ 1.15078 mph. The 3.6 factor comes from (3600 s/h) ÷ (1000 m/km).

Example 1 — km/h to mph

Given

A limit is 100 km/h. What is that in mph?

Steps

  1. Divide by 1.60934 (km per mile): 100 ÷ 1.60934 ≈ 62.14.
  2. Round to a posted-style value: about 62 mph.

Answer

100 km/h ≈ 62 mph.

Tip: Rough mental check: km/h × 0.62 ≈ mph.

Example 2 — m/s to km/h

Given

A runner’s pace is 5 m/s. Convert to km/h.

Steps

  1. Use the factor 1 m/s = 3.6 km/h.
  2. Multiply: 5 × 3.6 = 18.

Answer

5 m/s = 18 km/h.

Example 3 — Distance from speed and time

Given

Cruise at 90 km/h for 2.5 hours. How far do you travel?

Steps

  1. distance = speed × time = 90 × 2.5.
  2. 90 × 2.5 = 225 km.

Answer

225 km.

Forgetting the 3.6 when moving m/s ↔ km/h

Common mistake

Treating 10 m/s as 10 km/h.

Better approach

10 m/s × 3.6 = 36 km/h. Leaving out 3.6 understates highway speeds badly.

Mixing minutes with per-hour speeds

Common mistake

Computing time as distance ÷ km/h but entering minutes as if they were hours.

Better approach

Convert minutes to hours (÷ 60) before using t = d ÷ v with km/h, or convert speed to km/min.

Check your understanding

  1. 1.Convert 60 mph to km/h (× 1.60934).

    Answer: ≈ 96.6 km/h.

  2. 2.Convert 72 km/h to m/s (÷ 3.6).

    Answer: 20 m/s.

  3. 3.At 15 kn, about how many km/h? (× 1.852)

    Answer: ≈ 27.8 km/h.

Key takeaways

  • Speed = distance ÷ time with matching units.
  • m/s ↔ km/h uses the factor 3.6.
  • mph and km/h differ by about 1.609; knots tie to nautical miles.
  • Never mix minutes and hours without converting first.