What Speed Actually Means in Physics

Light streaks going around a corner to represent speed.
Dove Lee/Getty Images

Speed is the distance traveled per unit of time. It is how fast an object is moving. Speed is the scalar quantity that is the magnitude of the velocity vector. It doesn't have a direction. Higher speed means an object is moving faster. Lower speed means it is moving slower. If it isn't moving at all, it has zero speed.

The most common way to calculate the constant velocity of an object moving in a straight line is the formula:

r = d / t


  • r is the rate, or speed (sometimes denoted as v, for velocity)
  • d is the distance moved
  • t is the time it takes to complete the movement

This equation gives the average speed of an object over an interval of time. The object may have been going faster or slower at different points during the time interval, but we see here its average speed.

The instantaneous speed is the limit of the average speed as the time interval approaches zero. When you look at a speedometer in a car, you are seeing the instantaneous speed. While you may have been going 60 miles per hour for a moment, your average rate of speed for ten minutes might be far more or far less.

Units for Speed

The SI units for speed are m/s (meters per second). In everyday usage, kilometers per hour or miles per hour are the common units of speed. At sea, knots (or nautical miles) per hour is a common speed. 

Conversions for Unit of Speed

km/h mph knot ft/s
1 m/s = 3.6 2.236936 1.943844 3.280840

Speed vs Velocity

Speed is a scalar quantity, it does not account for direction, while velocity is a vector quantity which is aware of direction. If ran across the room and then returned to your original position, you would have a speed — the distance divided by the time. But your velocity would be zero since your position didn't change between the beginning and the end of the interval. There was no displacement seen at the end of the time period. You would have an instantaneous velocity if it were taken at a point where you had moved from your original position. If you go two steps forward and one step back, your speed isn't affected, but your velocity would be.

Rotational Speed and Tangential Speed

Rotational speed, or angular speed, is the number of revolutions over a unit of time for an object traveling in a circular path. Revolutions per minute (rpm) is a common unit. But how far from the axis an object is its radial distance as it revolves determines its tangential speed, which is the linear speed of an object on a circular path?

At one rpm, a point that is at the edge of a record disk is covering more distance in a second than a point closer to the center. At the center, the tangential speed is zero. Your tangential speed is proportional to the radial distance times the rate of rotation.

Tangential speed = radial distance x rotational speed.

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Jones, Andrew Zimmerman. "What Speed Actually Means in Physics." ThoughtCo, Apr. 5, 2023, thoughtco.com/speed-2699009. Jones, Andrew Zimmerman. (2023, April 5). What Speed Actually Means in Physics. Retrieved from https://www.thoughtco.com/speed-2699009 Jones, Andrew Zimmerman. "What Speed Actually Means in Physics." ThoughtCo. https://www.thoughtco.com/speed-2699009 (accessed June 6, 2023).