Speed, distance, and time

What is speed?

Speed is defined as the change of position of an object over time. In other words, it is a measure of the rate at which an object travels over a given distance. In the International System of Units (SI), speed is measured in units of meters per second (m/s). Other units of speed include kilometers per hour (km/h), miles per hour (mph), feet per second (ft/s), and many more. The chosen unit of speed is dependent on the measurement system used in a given country and also on what is being measured. For example, it would not make sense to measure the speed at which a snail moves in terms of meters per second, since a snail moves relatively slowly. Similarly, while we could measure the speed of a race car in terms of millimeters per second, this would result in a large number that would be unnecessarily difficult to deal with in calculations.

The relationship between speed, distance, and time

Speed, distance, and time are related by the following formula:

speed = distance / time

This formula shows that:

• Speed increases if you cover more distance in the same amount of time, or if you cover a distance in a shorter period of time.
• Distance can be calculated if you know the speed and time using the formula: distance = speed × time
• Time required to cover a distance can be found if you know the speed and distance, using the formula: time = distance / speed

Understanding the relationship between speed, distance, and time is not just about solving physics problems; it helps us in everyday situations. Whether you are trying to calculate how long it will take to get to school at a certain speed, how fast you need to run to win a race, or even how speed limits on roads are determined to ensure safety, these concepts are incredibly useful.

Common units of speed

	m/s	km/h	mph	kn	ft/s
1 meter/second [m/s] =	1	3.6	2.236928	1.943844	3.280840
1 kilometer/hour [km/h] =	0.277778	1	0.621369	0.539957	0.911344
1 mile/hour [mph] =	0.44704	1.60935	1	0.868979	1.466672
1 knot [kn] =	0.514444	1.852	1.150775	1	1.687810
1 foot/second [ft/s] =	0.3048	1.09728	0.681816	0.592484	1

Examples of different speeds

	m/s	km/h	mph
Average walking speed	1.4	5	3.1
Peak human running speed	12.42	44.7	27.8
Peak cheetah running speed	33.53	120.7	75
Average orbital speed of the Earth	29,783	107,218	66,623
Average orbital speed of the Sun	251,000	904,000	561,000
Speed of sound in air (sea level, 20°C)	343	1,235	768
Speed of light in vacuum	299,792,458	1,079,252,848	670,616,629