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 because it would need an endless amount of energy. The speed of light is about 299,792,458 metres per second. This is the ultimate speed limit in our universe.

Definition

Italian physicist Galileo Galilei is usually credited with being the first to measure speed by considering the distance covered and the time it takes. Galileo defined speed as the distance covered per unit of time. A cyclist who covers 30 metres in a time of 2 seconds, for example, has a speed of 15 metres per second. Objects in motion often have variations in speed (a car might travel along a street at 50 km/h, slow to 0 km/h, and then reach 30 km/h).

Speed at some instant, or assumed constant during a very short period of time, is called instantaneous speed. By looking at a speedometer, one can read the instantaneous speed of a car at any instant. A car travelling at 50 km/h generally goes for less than one hour at a constant speed, but if it did go at that speed for a full hour, it would travel 50 km.

Different from instantaneous speed, average speed is defined as the total distance covered divided by the time interval. For example, if a distance of 80 kilometres is driven in 1 hour, the average speed is 80 kilometres per hour. Likewise, if 320 kilometres are travelled in 4 hours, the average speed is also 80 kilometres per hour. When a distance in kilometres (km) is divided by a time in hours (h), the result is in kilometres per hour (km/h).

Speed denotes only how fast an object is moving, whereas velocity describes both how fast and in which direction the object is moving. If a car is said to travel at 60 km/h, its speed has been specified. However, if the car is said to move at 60 km/h to the north, its velocity has now been specified.

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 fully understanding 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.