Study Guides/Physics/Unit of Speed
Study Guide · Physics

What is the SI Unit of Speed?

In Physics (Class 9 Motion), the very first calculation you learn is Speed. Speed is defined as the distance traveled by an object per unit of time. But what is the universally accepted scientific unit to measure it?

Question (Click to Flip)

Is the unit of Velocity different from the unit of Speed?

Answer

No. Because velocity is simply 'speed in a specific direction', they share the exact same SI unit: meters per second (m/s).

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Key Facts

The fastest possible speed in the entire universe is the Speed of Light in a vacuum, which is exactly 299,792,458 meters per second (roughly 3 lakh km per second). Nothing can go faster than this!

The SI Unit: m/s

The official International System (SI) unit of Speed is meters per second (m/s) or m s⁻¹.

Why? Because the SI base unit for distance is the Meter (m), and the SI base unit for time is the Second (s). By dividing distance by time, we get meters/second.

Other Common Units

While scientists use m/s, it is not practical for everyday life (like driving). Therefore, other larger or smaller units are widely used:

  1. Kilometers per hour (km/h or kmph): The standard unit used for the speed of cars, trains, and airplanes worldwide.
  2. Miles per hour (mph): The standard vehicle speed unit used strictly in the United States and the UK.
  3. Centimeters per second (cm/s): Used in chemistry and biology to measure very slow things (like the speed of an ant or a chemical reaction).

The Golden Conversion Trick

Physics exams will frequently ask you to convert a car's speed from km/h into m/s before solving a numerical problem.

The Trick:

  • To convert km/h to m/s: Multiply the number by (5 / 18).
  • To convert m/s to km/h: Multiply the number by (18 / 5).

Example: A car is moving at 72 km/h. What is its speed in m/s? Calculation: 72 × (5/18) = 4 × 5 = 20 m/s.

Questions and Answers

Is the unit of Velocity different from the unit of Speed?+

No. Because velocity is simply 'speed in a specific direction', they share the exact same SI unit: **meters per second (m/s)**.

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