There are three ways to express speed: “kilometers per hour (the distance traveled in 1 hour),” “meters per minute (the distance traveled in 1 minute),” and “meters per second (the distance traveled in 1 second).”
The unit to use in the answer depends on the problem. Therefore, it is very important to practice “using the same units.”
Rules for Changing Units
- To change kilometers per hour to kilometers per minute: divide by 60 (because 1 hour＝60 minutes)
- To change kilometers per minute to kilometers per second: divide by 60 (because 1 minute＝60 seconds)
- To change kilometers per hour to kilometers per second: divide by 3600 (1 hour＝3600 seconds)
- To change in the opposite direction, multiply by 60 or 3600
Example 1: Express 72 km per hour in meters per minute and meters per second.
Solution:
① Change to meters per minute → 72 ÷ 60 = 1.2 km/min. Changing this to meters gives 1200 m/min
② Change to meters per second → 72 ÷ 3600 = 0.02 km/sec. Changing this to meters gives 20 m/sec
Solution:
① Change to meters per minute → 72 ÷ 60 = 1.2 km/min. Changing this to meters gives 1200 m/min
② Change to meters per second → 72 ÷ 3600 = 0.02 km/sec. Changing this to meters gives 20 m/sec
To change kilometers per hour to meters per second, simply do “÷ 3600” and “km → m (×1000)” together.
Putting this together gives the following useful formula.
a km per hour = a ÷ 3.6 m/sec
Example 2: Express 15 m/sec in kilometers per hour.
Solution: 15 m × 3600 = 54000 m/hour = 54 km per hour
Solution: 15 m × 3600 = 54000 m/hour = 54 km per hour
Once you can quickly use “× 3.6” and “÷ 3.6,” calculations become much faster. Remember “a km per hour ⇔ a ÷ 3.6 m/sec.”