Speed is the rate at which an object moves. It is distance divided by time. A car traveling 100 kilometers in one hour has a speed of 100 kilometers per hour. Speed is a scalar quantity, meaning it has magnitude but no direction. Velocity is speed with direction. A car going 60 mph north has a velocity. A car going 60 mph has a speed. The distinction matters when direction changes. A car going around a track at constant speed has changing velocity, because its direction is changing. That change in velocity is acceleration.

Speed is measured in many units. Meters per second is the SI unit. Kilometers per hour and miles per hour are common in everyday use. Knots are used in aviation and maritime navigation. The speed of light is the ultimate speed limit. Nothing can travel faster. The speed of sound depends on the medium. It is about 343 meters per second in air at room temperature. Speed is not the same as velocity. Velocity includes direction. Speed does not. Average speed is total distance divided by total time. Instantaneous speed is the speed at a specific moment. A speedometer shows instantaneous speed.

Speed facts

• Definition. Distance divided by time.
• Scalar. No direction.
• Units. m/s, km/h, mph, knots.
• Light speed. 299,792,458 m/s.
• Sound speed. 343 m/s in air.

Speed is not the same as velocity. Velocity is speed with direction.