Measurement of Speed - Definition, Formula, Unit, Types, FAQs

What is Speed?

"The rate at which anybody moves or operates with respect to time is called the speed of the body."

In physics, the measurement of speed refers to the rate of change of position of a body with respect to time. The measurement of speed may vary according to the condition it is applied to.

The unit of speed is the meter per second ( $\mathrm{m} / \mathrm{s}$ ).

How to Calculate Speed?

speed is represented as:

Speed $=\frac{\text { Distance }}{\text { Time }}$
Where:

• Distance is measured in meters (m) in the SI system.
• Time is measured in seconds (s) in the SI system.

Dimensional Formula for Speed

The dimensional formula for speed is:

$[$ Speed $]=[L][T]^{-1}$
Where:

• [L]: Length (meters, m)
• $[T]^{-\mathbf{1}}:$ Time inverse (seconds, $s^{-\mathbf{1}}$ )

This confirms that speed is measured in $\mathbf{m} / \mathbf{s}$ in the SI system.

Also read -

• NCERT Notes For All Subjects
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Types of Speed

1. Average Speed
2. Instantaneous Speed
3. Uniform Speed
4. Non-Uniform Speed
5. Relative Speed

Average Speed

Average speed can be defined as the total distance covered divided by the total time taken.

Average Speed $=\frac{\text { Total Distance }}{\text { Total Time }}$

Example:

A car covers 100 km in 2 hours, resulting in an average speed of $\frac{\mathbf{1 0 0}}{2}=50 \mathrm{~km} / \mathrm{h}$.

Instantaneous Speed

It is the measure of speed of a body at any instant of time, or measuring the rate of change of position with respect to an infinitesimally small time period.

Instantaneous Speed $=\left|\frac{d s}{d t}\right|$
Where:
$d s$ : Infinitesimal change in distance
$d t$ : Infinitesimal change in time

Uniform Speed

Uniform speed indicates a case of motion of an object where the rate of change of position to time is constant throughout the motion, in which the object covers an equal distance in equal intervals of time.

When considering the graph it will be a straight line with a constant slope, that is if we take the slope of the graph between any two points on the graph, it will remain the same always.(we have seen what the slope in a distance vs time graph indicates, it indicates the speed)

​

Non-Uniform Speed

An object is said to be in non-uniform motion when there is an acceleration on the body that is the speed does not always remain the same throughout the motion, it may increase or decrease accordingly.

The rate of increase or decrease of velocity with respect to time is called acceleration, we have talked about uniform and non-uniform speed similarly there can be uniform or non-uniform acceleration also. Acceleration is a vector quantity so it cannot be mentioned in terms of speed but the magnitude of acceleration can be the rate of change of speed.

​

Relative Speed

The speed of one object with respect to another object is called as relative speed.

Relative Speed $=\left|v_1-v_2\right|$

Example:

If two cars are moving in opposite directions at $60 \mathrm{~km} / \mathrm{h}$ and $40 \mathrm{~km} / \mathrm{h}$, their relative speed is $60+40=100 \mathrm{~km} / \mathrm{h}$.

Measurement of Speed

Speed is measured by calculating the distance covered by an object in a given time. Devices like speedometers are used to measure the speed of vehicles.

Example
If a car travel $\mathbf{1 0 0 ~ m}$ in $\mathbf{5 ~ s}$, its speed is:

$
\text { Speed }=\frac{100}{5}=20 \mathrm{~m} / \mathrm{s}
$

What is used to measure speed?

Speed is measured using an instrument called a speedometer. A speedometer is commonly found in vehicles such as cars, bikes, and buses, where it shows the speed of the vehicle at any moment. The speed is usually displayed in kilometres per hour (km/h) or metres per second (m/s).

What Units Are Used to Measure Speed?

The SI unit of speed is metre per second (m/s). It is commonly used in science and physics calculations.

Other commonly used units of speed include:

• Kilometre per hour (km/h) – widely used for vehicles and road transport
• Centimetre per second (cm/s) – used for small-scale motion
• Miles per hour (mph) – used in some countries like the USA and UK

Practical Applications of Speed in Daily Life

• Vehicles: Speedometers show how fast a car or bike is moving.
• Sports: Used to measure the speed of runners, cyclists, and swimmers.
• Public transport: Helps buses and trains follow time schedules.
• Weather forecasting: Wind speed helps predict storms and weather changes.
• Machines: Machines work at fixed speeds for safety and efficiency.
• Aeroplanes: Speed is important for safe take-off, landing, and flight.