Speed
Speed is the rate of distance change over time.
Speed is a fundamental concept in kinematics, defined as the rate at which distance changes with time. It is the magnitude of velocity, a vector that also indicates direction. The SI unit of speed is the metre per second (m/s), though other units such as kilometres per hour (km/h) and miles per hour (mph) are also common. The Italian physicist Galileo Galilei is generally recognized as the first to measure speed by relating distance covered to the time taken, defining it as distance per unit time. For example, a cyclist covering 30 metres in 2 seconds moves at 15 metres per second. Objects in motion often experience variations in speed, such as a car that slows or accelerates over time. Instantaneous speed refers to the speed at a specific instant, as read from a speedometer; mathematically, it is the magnitude of instantaneous velocity, the derivative of position with respect to time. If distance travelled over time is considered, speed equals the time derivative of that distance. Average speed, by contrast, is the total distance covered divided by the total time interval, and does not capture variations during the trip. For instance, driving 80 kilometres in one hour yields an average speed of 80 km/h, regardless of any changes in pace. On a distance-time graph, the slope of a tangent line gives instantaneous speed, while the slope of a chord line gives average speed over the interval. Speed differs from velocity in that velocity includes direction; moving around a circle and returning to the start results in an average velocity of zero, but the average speed is the circumference divided by the time. Common units include metres per second, kilometres per hour, miles per hour, knots for air and marine travel, feet per second, and the dimensionless Mach number and natural units based on the speed of light. According to special relativity, the fastest possible speed for energy or information is the speed of light in vacuum; matter cannot reach this speed as it would require infinite energy, and the concept of rapidity replaces classical speed in relativistic physics. Psychologically, Jean Piaget found that children’s intuition for speed precedes that of duration, based on the idea of overtaking—judging an object as faster if it starts behind and then moves ahead of another.
- field
- Kinematics, Physics
- SI unit
- Metre per second (m/s)
- Other units
- km/h, mph, knot, Mach number
- Key concept
- Instantaneous speed is the limit of average speed as time interval approaches zero
Quick Facts
- Unit
- m/s, m s / −1
- Symbols
- v
- Dimension
- L T / −1
Facts from the source article.
Lore & Background
Italian physicist Galileo Galilei is usually credited with being the first to measure speed by considering the distance covered and the time it takes. He defined speed as the distance covered per unit of time, expressed as v = d/t. For example, a cyclist covering 30 metres in 2 seconds has a speed of 15 metres per second. Objects in motion often have variations in speed, such as a car travelling at 50 km/h, slowing to 0 km/h, then reaching 30 km/h. Instantaneous speed is the speed at some instant, often read from a speedometer. Mathematically, it is the magnitude of instantaneous velocity, the derivative of position with respect to time. Average speed, by contrast, is the total distance divided by the total time interval. For constant velocity in a straight line, speed simplifies to s/t. Speed differs from velocity in that velocity includes direction. For circular motion, average velocity can be zero while average speed is positive. According to special relativity, the fastest possible speed for energy or information is the speed of light in vacuum; matter cannot reach this speed as it would require infinite energy.
Reader's Guide
Speed is a core concept in physics, essential for describing motion. Its definition by Galileo laid the groundwork for classical mechanics. The distinction between average and instantaneous speed is crucial for analyzing varying motion, as seen in everyday examples like driving. Speed's role as the magnitude of velocity clarifies the difference between scalar and vector quantities. The speed of light as an ultimate limit, from special relativity, has profound implications for physics, setting a universal speed cap. Units like knots for marine travel and Mach numbers for aerodynamics show its practical applications. Piaget's psychological research indicates that children's concept of speed develops from the notion of overtaking, preceding their understanding of duration. Overall, speed remains a fundamental measure in both theoretical and applied contexts.
Did You Know?
- Galileo Galilei is usually credited with being the first to measure speed by considering distance and time.
- The SI unit of speed is the metre per second (m/s).
- According to Jean Piaget, children's intuition for speed precedes that of duration and is based on outdistancing.
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