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

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

Vishal kumarUpdated on 02 Jul 2025, 04:32 PM IST

Imagine a car traveling on a road. The distance it covers in a certain amount of time tells us how fast it is moving—this is its speed. Speed is the rate at which an object moves or changes its position over time. Speed is an important phenomenon in Physics, Let's explore the concept of speed along with its calculation in detail.

This Story also Contains

  1. What is Speed?
  2. How to Calculate Speed?
  3. Types of Speed
Measurement of Speed - Definition, Formula, Unit, Types, FAQs
Measurement of Speed

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?

Mathematically 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.

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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)

Uniform 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.

 non-uniform 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 $60+40=100 \mathrm{~km} / \mathrm{h}$.

Frequently Asked Questions (FAQs)

Q: How does the speed of a shock wave compare to the speed of sound?
A:
A shock wave travels faster than the speed of sound in the medium. The ratio of the shock wave speed to the speed of sound is called the Mach number. When this number is greater than 1, it indicates supersonic flow and the formation of a shock wave.
Q: How do we measure the speed of nerve impulses in the human body?
A:
The speed of nerve impulses is typically measured by stimulating a nerve at one point and recording the time it takes for the signal to reach another point. This can be done using electrodes placed on the skin or inserted near the nerve.
Q: What is the concept of superluminal motion in astronomy?
A:
Superluminal motion is an apparent faster-than-light movement seen in some astronomical radio sources. It's an optical illusion caused by the source moving at a high velocity at a small angle to the line of sight, not actual faster-than-light travel.
Q: How does the concept of speed apply to chemical reactions?
A:
In chemistry, the speed of a reaction refers to the rate at which reactants are converted into products. It's often expressed as the change in concentration of a reactant or product per unit time.
Q: What is the concept of phase velocity?
A:
Phase velocity is the speed at which the phase of a wave propagates in space. It's given by the equation: v_p = ω / k, where ω is the angular frequency and k is the wave number. In some cases, phase velocity can exceed the speed of light, but this doesn't violate relativity as it doesn't carry information.
Q: How does temperature affect the speed of molecules in a gas?
A:
As temperature increases, the average kinetic energy of gas molecules increases, causing them to move faster. This relationship is described by the kinetic theory of gases, where the average speed of gas molecules is proportional to the square root of the temperature.
Q: How do we measure the speed of tectonic plate movement?
A:
The speed of tectonic plate movement is typically measured using GPS technology, which can detect minute changes in position over time. Other methods include studying magnetic patterns in rocks and measuring the spread of seafloor at mid-ocean ridges.
Q: What is the relationship between speed and time dilation in special relativity?
A:
According to special relativity, as an object's speed approaches the speed of light, time passes more slowly for that object relative to a stationary observer. This effect, known as time dilation, becomes significant at very high speeds.
Q: How does the speed of neutrinos compare to the speed of light?
A:
Neutrinos are subatomic particles that travel at speeds very close to, but not exceeding, the speed of light. Recent experiments have measured neutrino speeds to be consistent with the speed of light within the margin of error of the experiments.
Q: How do we measure the orbital speed of planets?
A:
The orbital speed of planets can be calculated using Kepler's laws of planetary motion. We can also measure it directly using the Doppler effect, observing how the light from the planet (or its star) is shifted as it moves towards or away from Earth.