Measurement of Speed: Definition, Types, Speed-Measuring Instruments

In Physics, we come across the topic of the measurement of speed. It is also referred to as measuring rate and frequency. It means the frequency produced by a body. It determines the number of measurements that are taking place in a given period which is generally 1 second. There are various instruments for the measurement of speed such as speedometer, tachometer and accelerometer. Continue reading to learn more about them in detail.

What is Speed?

Speed is defined as the rate of change of an object in any direction. It is the ratio of distance covered by an object with time. 

Speed has a scalar quantity, it has magnitude but no direction. 

The units of speed are as follows: 

CGS units: cm/s 

SI units: m/s

Speed Formula

Speed is the ratio of distance with respect to time. 

The formula of speed is given by,

Speed = distance / time. 

Dimensional Formula of Speed 

The dimensional formula of speed is given by, 

The dimensions of time (seconds) = T 

Distance (in meters or centimeters) = L 

Hence, the dimensional formula of speed is

Types of Speed

The following are the types of speed: 

Uniform Speed

When an object covers an equal distance in an equal interval of time. This is known as uniform speed. 

For example if a car is moving with a speed of 60kms per hour, it will cover a distance of 60 kilometers in one hour. 

Variable Speed

Variable speed is defined when an object covers a different distance at equal intervals of time. 

Instantaneous Speed

Instantaneous speed is defined as the speed of an object at that instant of time

Average Speed

Average speed is defined as the total distance travelled by an object with respect to the total time covered by it.

For example, if a car is travelling at total distance of 50 kms in 2 hours, the average speed of the car shall be 25 kmph.  

Relative Speed

Relative speed of an object is the speed of an object or a person in relation to the other object or the frame of reference. 

Relative speed is the speed of an object observed by an observer with respect to their frame of reference. 

What is the Measurement of Speed?

The speed at which a body moves or performs relative to time is known as the speed of an object. The definition of speed will differ depending on the situation it is used. For example, in the case of melting an ice cube, the measurement of speed does not indicate the motion of the ice cube but rather the rate at which it melts and converts into water. It’s the rate of the transformation of an ice cube into water. Similarly, when we say that the speed of a laptop is not efficient, we do not indicate its motion but rather its efficiency in processing the data which is its measurement of speed.

From the above two examples of the ice cube and the laptop, we can conclude that it is not important that the measurement of speed always implies the measurement of an object having any kind of motion but it may be any sort of change that might result in the working efficiency or change of state or change in frequency of the given object.

Measurement of Speed of Light

The measurement of the speed of light is often referred to as “c,” which is a constant that plays a role in numerous areas of physical science. It is believed that the speed of light is exactly 299,792,458 m/s which is roughly 300,000 kilometres per second, or an average of 186,000 miles, or 671 million miles an hour. Based on the relativity theory, C is the upper limit of the speed at which the conventional energy or matter and therefore any signal that transmits information traverses space.

Ole Romer first demonstrated in 1676 that light travels at a finite speed by studying the apparent motion of Jupiter’s moon Io. More precise and progressive measurements of its speed were made through the decades. In a paper that was published in 1865, James Clerk Maxwell proposed that light was an electromagnetic wave, and consequently, travelled at a speed of c. The year 1905 was the time Albert Einstein postulated that the speed of light relative to any reference frame inertial is a constant, and independent of the movement of the source of light. He analyzed the implications of this postulate by constructing his theory of relativity. He, while doing so, proved that the parameter of c has significance beyond the realm of electromagnetism and light.

Measurement of Speed of Sound

The speed of sound is the distance travelled per minute by the soundwave when it travels across the flexible medium. At 20℃ it is estimated that the velocity of sound in the air is approximately 343 metres each second which is equivalent to one kilometre in 2.91 seconds or one mile in 4.69 seconds. It is strongly influenced by temperature, as well as the medium in which a sound wave travels. When the temperature is 0℃ the sound wave’s speed in air is approximately 331 milliseconds.

Sound’s speed in the perfect gas is dependent only upon its composition and temperature. The speed is a slight relationship between frequency and pressure in normal air, varying little from its ideal behaviour. In informal speech, speed of sound refers to the speed at which sound waves travel through the air. However how fast sound travels differs depending on the substance through which it travels, and it is the slowest in gas, a little fast when it is in liquid, and at a very fast rate when it is in solids.

Measurement of Speed of Wind

In the meteorology field, the measurement of the speed of wind is an important atmospheric variable that is caused by the movement of air from low to high pressure, typically due to temperature changes. The wind speed is currently measured using the aid of an anemometer.

Wind speed can affect the forecasting of weather, maritime and aviation operations, construction, as well as the growth and metabolism rate of a variety of plant species. It has numerous other implications as well. The direction of the wind is generally nearly identical to that of isobars because of the rotation of the Earth.

Measurement of Speed Instruments

Many instruments help in measuring the speed, but for general calculation of speed or motion taking place in an object we use the following instruments as mentioned below:

• Speedometer
• Odometer
• Tachometer
• Accelerometer

Speedometer

A speedometer or a speed meter is a gauge that displays and measures the current speed of the vehicle. Today they are fitted to automobiles. Speedometers are also used for other vehicles and have distinct names, and it is also used in different ways for detecting speed. For boats, it is known as the pit log. In case of an aircraft, it is used as an airspeed indicator.

Charles Babbage created the early speedometer, which was generally fitted to locomotives. The electric speedometer was invented by Croatian Josip Belušić in 1888. The image given below depicts the speedometer assembled on an automobile.

Tachometer

A tachometer is a tool that measures the rotational speed of a disk or shaft, like a motor present in a machine. This instrument typically shows the number of revolutions per minute (RPM) on an analog dial calibrated over a vehicle.

Its word is derived from the Greek “takhos” meaning speed and “metron” meaning measure. According to the arbitrary convention, in the world of automobiles, a tachometer is utilized for engine rotations. The image given below depicts the tachometer assembled on an automobile.

Accelerometer

An accelerometer is a device that measures the correct acceleration of a vehicle. Proper acceleration is the rate of acceleration of a body within its instantaneous rest frame. It differs from the coordinate acceleration, that is the acceleration within a fixed coordinate system. For instance, an accelerometer at rest at the top of the Earth will be determined as the acceleration caused by gravity on Earth. In contrast, accelerometers that are in a free fall will show zero.

Accelerometers can be used for a variety of purposes in science and industry. They are extremely sensitive and are utilized for inertial navigation systems for missiles and aircraft. Vibrations in rotating machines are also tracked by accelerometers. They are employed in tablets and digital cameras as well so that the images displayed on the screens always appear in a straight line. In the case of unmanned aerial vehicles accelerometers aid in stabilising the flight. The image given below depicts the accelerometer assembled on an automobile.

What is Velocity?

Velocity of an object is defined as the rate at which an object changes its position with respect to time. Velocity has a direction and is a vector quantity. 

The formula for velocity is given by, 

Velocity = change in distance/change in time 

Difference Between Speed and Velocity

The following are the differences between speed and velocity 

Speed	Velocity
The speed of a moving body is the distance traveled by a body with respect to time.	The velocity of a body is the change in distance travelled in a particular direction with respect to time.
Speed is a scalar quantity.	Velocity is a vector quantity.
The average speed of a moving body is zero.	The average velocity of a moving body can never be zero.
A moving body’s speed can never be negative.	The velocity of a moving body can be negative depending on the direction.

Measurement of Speed Solved Examples

Example 1. A bus travels from one point to another at a speed of 40 kmph and comes back at a speed of 30 kmph. What is the average speed of the bus. 

Solution 1. 

Given data, 

Speed of the object while travelling to the destination (S1) = 40 kmph 

Hence, time taken

T1 = S / 40 

Speed of the object while coming back (S2) = 30 kmph 

Time taken by the bus, 

T2 = S / 30

The total time taken by an object (T) = T1 + T2 = (S / 40) + (S / 30)

= 7S / 120 

The formula of average speed is, 

Average speed =

The average speed of the object is 34.3 kmph.

Example 2. A train covered a distance of 120 km in one hour. Determine the speed of train.

Solution 2. 

Given data, 

Distance (D) = 120 x 1000 m = 120000 m. 

Time taken (t) by train in seconds (t) = 60 x 60 = 3600 s

Using the formula for speed, 

Speed = distance / time = 33.3 m/s 

The speed of train is determined by 33.3 m/s.

Example 3. Find the average speed of a vehicle which covers 200 kms in 4 hours and next 160 kms in another 4 hours.

Solution 3. 

Given data, 

Total distance covered by the vehicle = 200 km + 160 km = 360 km

Total time taken by bus = 4 hours + 4 hours = 8 hours 

Average speed = total distance / total time covered 

Average speed = 360 km / 8 hours = 45 kmph 

The average speed of the vehicle is 45 kmph.

Measurement of Speed: Things to Remember

Few important things to remember about measurement of speed. 

• Speed is defined as the ratio of distance of time travelled to move from one point to another point in the given time. 
• The SI unit of speed is meter per second. 
• Speed does not have a direction. It is a scalar quantity. 
• Speedometers are used to measure the speed of an object.

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