CogKnoSys

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Physical

Speed and Velocity

Speed is how fast something moves; velocity is its speed together with the direction it is moving.

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The idea

Speed tells you how fast something is moving. It is the rate at which the distance an object travels changes over time. Velocity adds one more piece of information: the direction of motion. So '5 metres per second' is a speed, while '5 metres per second east' is a velocity.

Scientists call speed a scalar, which has size only, and velocity a vector, which needs both size and direction. In other words, speed is the size, or magnitude, of velocity. If an object's speed, direction or both change, it is accelerating. That means a car going around a curve at a steady speed is still accelerating, because its direction keeps changing.

Average speed is the distance traveled divided by the time it took. Instantaneous speed is the speed at a single moment, like the number on a car's speedometer. The SI unit of speed is the metre per second, but kilometres per hour, miles per hour and knots are also used. Nothing can travel faster than light in a vacuum, which moves at 299,792,458 metres per second.

Check yourself

Question 1 of 2

What is instantaneous speed?

Pick one. The first pick counts.

Composition

what it's made of

  • Distance — How far an object travels; speed compares it with the time taken.
  • Time — How long the motion lasts; average speed is distance divided by this.
  • Direction — The extra piece of information that turns a speed into a velocity.
  • Magnitude — The size of a velocity, which is its speed.
  • Units — Metres per second in science; kilometres per hour, miles per hour or knots in daily life.

Connection

how it relates

Under Motion

Related

  1. Ocean Currentsstrength 60 of 100

    Ocean currents are described by how fast the water moves and in which direction, which is a velocity.

  2. Thermal Energystrength 50 of 100

    The hotter a substance is, the faster its particles move on average.

  3. Climate Patternsstrength 40 of 100

    Weather maps show wind as a speed together with a direction, which is a velocity.

Change

how and why it evolves

Past
People measured speed as distance covered over time long before physicists described velocity as a vector with a direction.
Now
Today speed and velocity are basic ideas in kinematics, the part of physics that describes how objects move.
Next
In relativity physics, a quantity called rapidity replaces the everyday idea of speed. Such ideas may matter more as engineers design faster spacecraft and particle experiments.

Knowledge you can walk. Read the idea, then step up, across or down.

Speed and Velocity

Speed is how fast something moves; velocity is its speed together with the direction it is moving.

Learn, discuss, create, buy or invest

The idea

Speed tells you how fast something is moving. It is the rate at which the distance an object travels changes over time. Velocity adds one more piece of information: the direction of motion. So '5 metres per second' is a speed, while '5 metres per second east' is a velocity.

Scientists call speed a scalar, which has size only, and velocity a vector, which needs both size and direction. In other words, speed is the size, or magnitude, of velocity. If an object's speed, direction or both change, it is accelerating. That means a car going around a curve at a steady speed is still accelerating, because its direction keeps changing.

Average speed is the distance traveled divided by the time it took. Instantaneous speed is the speed at a single moment, like the number on a car's speedometer. The SI unit of speed is the metre per second, but kilometres per hour, miles per hour and knots are also used. Nothing can travel faster than light in a vacuum, which moves at 299,792,458 metres per second.

Check yourself

2 quick questions, instant feedback.