How is Speed Measured? Formula, Instruments, and Exam Solved Questions
The measurement of speed is an essential concept in physics and is fundamental to understanding motion, whether in daily life or advanced scientific studies. Speed indicates how quickly an object moves from one place to another and is used to describe everything from the motion of vehicles, aircraft, and trains to waves, sound, and light.
What is Speed?
Speed is defined as the rate at which an object covers distance, regardless of its direction of travel. It is a scalar quantity, meaning it only has magnitude and not direction. The standard units used for speed are meters per second (m/s) in the SI system and centimeters per second (cm/s) in the CGS system. When describing motion, speed helps us compare how fast different objects are moving in various contexts.
Speed Formula
The basic formula for speed is:
Dimensional Formula of Speed
Speed is derived from the ratio of distance to time. The dimensional formula is as follows:
Types of Speed
Speed can be categorized based on how an object moves during a given time interval. The key types include:
- Uniform Speed: Equal distances are covered in equal intervals of time. For example, a car moving steadily at 60 km/h will cover 60 km every hour.
- Variable Speed: The object covers different distances in equal time intervals.
- Instantaneous Speed: The speed of an object at a specific instant.
- Average Speed: The total distance traveled divided by the total time taken.
- Relative Speed: The speed of an object as observed from another moving object or frame of reference.
| Type of Speed | Definition | Example |
|---|---|---|
| Uniform Speed | Equal distance in equal time intervals | Train on a straight track at constant speed |
| Variable Speed | Unequal distance in equal time intervals | Car in city traffic |
| Instantaneous Speed | Speed at a particular moment | Speedometer reading |
| Average Speed | Total distance / Total time | Overall journey by car |
| Relative Speed | Speed relative to another object | Two trains moving towards each other |
What is the Measurement of Speed?
The measurement of speed means determining how quickly something changes or how fast a process occurs. In mechanics, it is usually the rate at which an object changes position. In other contexts, such as computing or chemistry, speed can represent the efficiency or rate of change (like the speed of a chemical reaction or data processing).
Measurement of Speed of Light
The speed of light, denoted as c, is a fundamental constant of nature. Its exact value is 299,792,458 meters per second. Light is the fastest phenomenon in the universe, and no signal or object can exceed this speed. The concept and measurement of the speed of light play a crucial role in modern physics, especially in the theory of relativity.
Measurement of Speed of Sound
The speed of sound depends on the medium and its temperature. In air at 20°C, sound travels at about 343 m/s. In solids, sound travels faster, while in gases, it is slower. Understanding this speed is vital for studying acoustics and wave phenomena.
Measurement of Speed of Wind
Wind speed is an essential parameter in meteorology. It is typically measured using an instrument called an anemometer, which helps in weather forecasting and many technical applications. Wind speed impacts aviation, shipping, and plant growth.
Instruments for Measuring Speed
Several instruments are widely used for measuring speed in different domains:
- Speedometer: Measures the current speed of vehicles (cars, bikes, etc.).
- Tachometer: Measures rotational speed (usually in revolutions per minute - RPM).
- Odometer: Measures the total distance covered by a vehicle.
- Accelerometer: Measures changes in velocity (acceleration) of vehicles and machines.
- Anemometer: Used to measure wind speed.
| Instrument | Where Used | Measure Provided |
|---|---|---|
| Speedometer | Vehicles (cars, motorcycles) | Instantaneous speed |
| Tachometer | Engines, motors | Rotational speed (RPM) |
| Odometer | Vehicles | Distance traveled |
| Accelerometer | Electronics, vehicles, UAVs | Acceleration |
| Anemometer | Weather stations | Wind speed |
Solved Examples
Example 1: A bus travels from one point to another at a speed of 40 km/h and returns at 30 km/h. What is its average speed?
Solution:
- Time on return: T2 = S / 30
- Total time = S/40 + S/30 = 7S/120
- Average speed = Total distance / Total time = (2S) / (7S/120) = 240/7 = 34.3 km/h
Example 2: A train covers 120 km in one hour. Find its speed in m/s.
Solution:
- Time = 1 hour = 3600 s
- Speed = 120,000 m / 3600 s = 33.3 m/s
Example 3: A vehicle covers 200 km in 4 hours, then 160 km in another 4 hours. Find its average speed.
Solution:
- Total time = 4 + 4 = 8 hours
- Average speed = 360 / 8 = 45 km/h
Difference Between Speed and Velocity
| Speed | Velocity |
|---|---|
| Distance traveled per unit time | Displacement per unit time (in a specific direction) |
| Scalar quantity (no direction) | Vector quantity (has direction) |
| Value cannot be negative | Can be negative, positive, or zero |
| Used for describing general motion | Describes motion regarding direction |
Things to Remember
- Speed is always the distance traveled divided by the time taken.
- The SI unit of speed is meter per second (m/s).
- Speed is a scalar quantity and does not indicate direction.
- Speedometers measure the instantaneous speed of vehicles.
For deeper insights, practice problems, and related explanations, explore these resources:
- Difference Between Speed and Velocity
- Speed of Moving Objects
- Velocity
- Average Velocity
- Instantaneous Speed and Velocity
- Acceleration
Continue to Graphical Representation of Motion for more on motion and mechanics.
FAQs on Measurement of Speed in Physics: Definitions, Formulas & Real-Life Examples
1. What is speed and how is it measured?
Speed is the rate at which an object covers distance; it is calculated as the distance travelled divided by the time taken. It is measured using the following formula:
Speed = Distance / Time.
SI unit: metres per second (m/s).
2. What are the different types of speed in Physics?
Types of speed include:
- Uniform speed: When an object covers equal distances in equal intervals of time.
- Non-uniform speed: When an object covers unequal distances in equal intervals of time.
- Instantaneous speed: The speed of an object at a particular moment.
- Average speed: Total distance travelled divided by total time taken.
- Relative speed: Speed of one object as observed from another moving object or observer.
3. Which instruments are used to measure speed?
Common instruments to measure speed include:
- Speedometer: For vehicles, measures speed in km/h or m/s.
- Anemometer: Measures wind speed in m/s.
- Pitot tube: Used in aircraft and watercraft.
- Log: Measures speed of ships in knots.
- Tachometer: Measures rotational speed (revolutions per minute).
4. What is the SI unit of speed?
The SI unit of speed is metre per second (m/s). In CGS units, it is centimetre per second (cm/s).
5. What is the formula for calculating speed?
Speed = Distance travelled / Time taken
Expressed as v = d / t where ‘v’ is speed, ‘d’ is distance, and ‘t’ is time.
6. What is the difference between speed and velocity?
Speed is a scalar quantity showing how fast an object is moving, regardless of direction. Velocity is a vector quantity showing both speed and the specific direction of movement.
- Speed: No direction, scalar, always positive.
- Velocity: Has direction, vector, can be negative.
7. How is average speed different from instantaneous speed?
Average speed is the total distance divided by the total time taken for a journey, while instantaneous speed is the speed at a specific instant or moment during motion.
8. How is the speed of light measured and what is its value?
The speed of light is measured using precise experimental setups involving time-of-flight or reflection methods. Its exact value in a vacuum is 299,792,458 m/s (about 3 × 108 m/s).
9. What factors affect the speed of sound, and how is it measured?
The speed of sound depends on the medium (air, water, solids) and temperature. In air at 20°C, it is about 343 m/s. It is measured using tuning forks, resonance tubes, or electronic sensors.
10. How can you calculate the speed of a vehicle using a speedometer?
A speedometer gives the instantaneous speed of a vehicle by displaying how many kilometres (or miles) are covered in one hour (km/h or mph). Read the dial or digital display to find the current speed.
11. Why is speed considered a scalar quantity?
Speed is considered a scalar quantity because it has only magnitude (how fast an object moves) and does not include direction.
12. What is the dimensional formula of speed?
The dimensional formula of speed is [M0 L1 T−1], meaning it depends on length divided by time.
