Speed

Speed is a way to measure how fast something is moving. In science, it is called kinematics. The speed of an object tells us how much its position changes over time.

We can talk about average speed, which is the total distance traveled divided by the time it took, or instantaneous speed, which is the speed at one exact moment.

Speed is measured in units like metres per second, kilometres per hour, or miles per hour. Different places use different units. For example, we often see speed limits in kilometres per hour on roads. In air and sea travel, people use a unit called knots.

The fastest speed anything can go is the speed of light. According to special relativity, nothing with mass can reach this speed. The speed of light is the ultimate speed limit in our universe.

Definition

Italian physicist Galileo Galilei is known for first measuring speed by looking at the distance covered and the time it takes. Galileo said speed is the distance covered per unit of time. For example, a cyclist who goes 30 metres in 2 seconds has a speed of 15 metres per second. Objects in motion often change speed (a car might go at 50 km/h, slow to 0 km/h, and then go to 30 km/h).

Speed at one moment, or almost constant for a very short time, is called instantaneous speed. By looking at a speedometer, you can see the instantaneous speed of a car at any moment. A car going at 50 km/h usually does not stay at that speed for a full hour, but if it did, it would travel 50 km.

Different from instantaneous speed, average speed is the total distance covered divided by the time taken. For example, if you drive 80 kilometres in 1 hour, the average speed is 80 kilometres per hour. Also, if you travel 320 kilometres in 4 hours, the average speed is 80 kilometres per hour. When you divide a distance in kilometres (km) by a time in hours (h), the result is in kilometres per hour (km/h).

Speed tells us how fast an object is moving, while velocity tells us both how fast and in which direction the object is moving. If a car is said to travel at 60 km/h, its speed is given. But if the car is said to move at 60 km/h to the north, its velocity is given.

Units

Main article: Conversion of units § Speed or velocity

Different units are used to measure speed. Some common ones include:

• metres per second (symbol m s⁻¹ or m/s), the SI derived unit;
• kilometres per hour (symbol km/h);
• miles per hour (symbol mi/h or mph);
• knots (nautical miles per hour, symbol kn or kt);
• feet per second (symbol fps or ft/s);
• Mach number (dimensionless), speed divided by the speed of sound;
• in natural units (dimensionless), speed divided by the speed of light in vacuum (symbol c = 299792458 m/s).

(* = approximate values)

Examples of different speeds

Main article: Orders of magnitude (speed)

Psychology

According to Jean Piaget, people naturally understand the idea of speed before they fully understand time. He was inspired by a question Albert Einstein asked him in 1928: "In what order do children learn about time and speed?" Children first think about speed when they see one object catching up to and passing another. They decide which object is moving faster by looking at who is ahead and who is behind after some time passes.