NCERT Solutions Class 7 Science Chapter 8 Measurement of Time and Motion

NCERT Solutions Class 7 Science Chapter 8: Measurement of Time and Motion chapter is fundamental to understanding the physical world around us, as time and motion are inextricably linked and form the basis of many scientific principles. 

Understanding how to measure time precisely and describe different kinds of motion is crucial not just for academic success but also for developing a deeper appreciation for the mechanics of the universe. In this NCERT Solutions Class 7 Science Chapter we'll explore the historical evolution of timekeeping devices, delve into the concepts of speed and distance, and differentiate between uniform and non-uniform motion. 

1.0Download NCERT Solutions Class 7 Science Chapter 8 : Free PDF

Download the free PDF of NCERT Solutions for Class 7 Science Chapter 8 from below:

2.0Key Concepts in Chapter 8: Measurement of Time and Motion

This chapter introduces several vital concepts that are essential for comprehending the dynamics of objects and the flow of events. Let's explore these in detail.

1. Time and Its Measurement 

Time is a fundamental quantity that helps us sequence events and measure their duration. For centuries, humans have sought accurate ways to measure time.

• Definition of Time: Time is often defined as the indefinite continued progress of existence and events in the past, present, and future regarded as a whole. In physics, it's a dimension in which events can be ordered from the past through the present to the future.
• Need for Measurement of Time: Accurate time measurement is crucial for various activities, including sports, scientific experiments, navigation, and even daily routines like catching a train or baking a cake.
• Units of Time: The standard international (SI) unit of time is the second (s). Larger units include the minute (min), hour (h), day, month, and year.
• • 1 minute = 60 seconds
  • 1 hour = 60 minutes
  • 1 day = 24 hours
  • 1 year = 365 days (approximately)
• Periodic Events: Many phenomena in nature repeat themselves after a fixed interval of time. These are called periodic events. Examples include the rotation of the Earth, the revolution of the Moon around the Earth, and the beating of a heart. Early timekeeping devices often relied on periodic events.
• Time Measuring Devices:
• • Sundial: One of the earliest time-measuring devices, it uses the position of the sun's shadow to indicate time. Its accuracy depends on the sun's position and is limited to daytime.
  • Sand Clock (Hourglass): Measures a specific duration of time by the flow of sand from one bulb to another. Not suitable for measuring varying time intervals.
  • Water Clock (Clepsydra): Measures time by the regulated flow of water. Ancient civilizations used these.
  • Pendulum Clock: Invented by Christiaan Huygens, this clock utilizes the periodic motion of a pendulum to keep time. A simple pendulum consists of a small metallic ball (bob) suspended from a rigid stand by a thread. Its motion is periodic, and for small displacements, the time taken to complete one oscillation (one back-and-forth movement) is nearly constant. This property is known as the period of oscillation.
  • Wristwatches and Stopwatches: Modern timekeeping devices that use oscillating crystals (quartz watches) for high accuracy. Stopwatches are specifically designed to measure short time intervals with precision.

2. Motion and Its Types

Motion refers to the change in the position of an object with respect to its surroundings over a period of time.

• Definition of Motion: An object is said to be in motion if its position changes with respect to a stationary reference point (or observer) over time. If its position does not change, it is said to be at rest.
• Reference Point: To describe motion, a reference point or origin is essential. For example, a car is moving with respect to the road, but a passenger inside the car is at rest with respect to the car itself.
• Types of Motion:
• • Straight Line Motion (Rectilinear Motion): Motion along a straight line. Examples: A car moving on a straight road, a bullet fired from a gun, an apple falling from a tree.
  • Circular Motion: Motion along a circular path. Examples: A point on the blade of a rotating fan, an athlete running on a circular track, the Earth revolving around the Sun (approximately circular).
  • Periodic Motion: Motion that repeats itself after regular intervals of time. All periodic motions are oscillatory, but not all oscillatory motions are periodic. Examples: The oscillation of a simple pendulum, the hands of a clock, the swinging of a child on a swing.
  • Rotational Motion: The motion of an object about a fixed axis. In rotational motion, different parts of the object move in circles of different radii, but the entire object turns around a central axis. Examples: A spinning top, the blades of a ceiling fan, a potter's wheel. (Note: A fan's blades undergo circular motion, while the fan itself undergoes rotational motion about its central axis).

3. Speed 

Speed is a measure of how fast an object is moving. It quantifies the rate at which an object covers distance.

• Definition of Speed: Speed is defined as the distance covered by an object in a unit time.
• Formula for Speed:
• Units of Speed: The SI unit of speed is metres per second (m/s). Other common units include kilometres per hour (km/h) and centimetres per second (cm/s).
• Please convert into these image to latex
• Average Speed: When an object travels different distances in different time intervals, or its speed changes during its journey, we often calculate its average speed.
  Average Speed=Total Time TakenTotal Distance Covered​
• Uniform Motion: An object is said to be in uniform motion if it covers equal distances in equal intervals of time along a straight line. In uniform motion, the speed remains constant.
• Non-Uniform Motion: An object is said to be in non-uniform motion if it covers unequal distances in equal intervals of time, or equal distances in unequal intervals of time. In non-uniform motion, the speed varies. Most real-life motions are non-uniform.

4. Distance-Time Graphs

Distance-time graphs are powerful visual tools used to represent and analyze the motion of objects.

• Purpose: These graphs show the relationship between the distance traveled by an object and the time taken.
• Plotting the Graph:
• • Time is typically plotted along the x-axis (horizontal axis).
  • Distance is typically plotted along the y-axis (vertical axis).
• Interpreting the Graph:
• • Object at Rest: A horizontal line parallel to the time axis indicates that the object is at rest (distance is constant over time).
  • Uniform Speed: A straight line sloping upwards indicates uniform speed. The steeper the slope, the greater the speed.
  • Non-Uniform Speed: A curved line indicates non-uniform speed. If the curve is bending upwards, the speed is increasing (acceleration); if bending downwards, the speed is decreasing (deceleration).
• Calculating Speed from Graph: The slope of the distance-time graph gives the speed of the object.

3.0NCERT Class 7 Science Chapter 8 Measurement of Time and Motion: Detailed Solutions

• Calculate the speed of a car that travels 150 metres in 10 seconds. Express your answer in km/h. Solution: A car travels 150 m in 10 s . Speed $=\frac{\text{ Distance }}{\text{ Time }}=\frac{150}{10}=15\mathrm{m}/\mathrm{s}$. Convert to $\mathrm{km}/\mathrm{h}$ : multiply by 3.6 $15\times 3.6=54\mathrm{km}/\mathrm{h}$.
• A runner completes 400 metres in 50 seconds. Another runner completes the same distance in 45 seconds. Who has a greater speed and by how much? Solution: Distance $=400\mathrm{m}$ for both. Runner A: time $=50\mathrm{s}\to {\mathrm{V}}_{\mathrm{A}}=400/50$ $=8.0\mathrm{m}/\mathrm{s}$ $=8.0\times 3.6=28.8\mathrm{km}/\mathrm{h}$. Runner B: time $=45\mathrm{s}\to {\mathrm{V}}_{\mathrm{B}}=\frac{400}{45}\approx$ $8.888\mathrm{m}/\mathrm{s}=8.888\times 3.6=32.0\mathrm{km}/\mathrm{h}$. Who is faster? Runner B is faster. By how much? ${\mathrm{V}}_{\mathrm{B}}-{\mathrm{V}}_{\mathrm{A}}\approx 0.888\mathrm{m}/\mathrm{s}$ or $32.0-28.8=3.2\mathrm{km}/\mathrm{h}$
• A train travels at a speed of $25\mathrm{m}/\mathrm{s}$ and covers a distance of 360 km . How much time does it take? Solution: Speed $=25\mathrm{m}/\mathrm{s}$. Distance $=360\mathrm{km}=360000\mathrm{m}$. Time $=\frac{360000}{25}=14400\mathrm{s}$ $=\frac{14400}{3600}=4\mathrm{h}$.
• A train travels 180 km in 3 h . Find its speed in: (i) $\mathrm{km}/\mathrm{h}$ (ii) $\mathrm{m}/\mathrm{s}$ (iii) What distance will it travel in 4 h if it maintains the same speed throughout the journey? Solution: Train covers 180 km in 3 h . (i) Speed in km/h: $180/3=60\mathrm{km}/\mathrm{h}$. (ii) Convert to $\mathrm{m}/\mathrm{s}:60\times \frac{1000}{3600}$ $=60\times \frac{5}{18}=\frac{50}{3}\approx 16.67\mathrm{m}/\mathrm{s}.$ (iii) Distance in 4 h at same speed: $60\mathrm{km}/\mathrm{h}\times 4\mathrm{h}=240\mathrm{km}$.
• The fastest galloping horse can reach the speed of approximately $18\mathrm{m}/\mathrm{s}$. How does this compare to the speed of a train moving at $72\mathrm{km}/\mathrm{h}$ ? Solution: Speed of horse $=18\mathrm{m}/\mathrm{s}$ Speed of train $=72\mathrm{km}/\mathrm{h}=72\times \frac{5}{18}$ $=20\mathrm{m}/\mathrm{s}$ The train is faster by $2\mathrm{m}/\mathrm{s}$ than the fastest galloping horse.
• Distinguish between uniform and nonuniform motion using the example of a car moving on a straight highway with no traffic and a car moving in city traffic. Solution: Uniform motion: object covers equal distances in equal intervals of time (speed constant). Example: A car cruising on a straight highway at fixed cruise speed with no traffic. Non-uniform motion: object covers unequal distances in equal intervals of time (speed changes). Example: A car in city traffic (stopping, starting, slowing), or a bicycle going up and down a hill.
• Data for an object covering distances in different intervals of time are given in the following table. If the object is in uniform motion, fill in the gaps in the table.

Solution:

This corresponds to a constant speed of 8 m every $10\mathrm{s}\to 0.8\mathrm{m}/\mathrm{s}$.

• A car covers 60 km in the first hour, 70 km in the second hour, and 50 km in the third hour. Is the motion uniform? Solution: Car covers 60 km (first hour), 70 km (second hour), 50 km (third hour). No - because equal time intervals (each $1\mathrm{h})$ do not have equal distances ( 60,70 , 50).
• Which type of motion is more common in daily life-uniform or non-uniform? Provide three examples from your experience to support your answer. Solution: Non-uniform motion: Here the object covers unequal distances in equal intervals of time (speed changes). Three examples: walking in a crowd (speed changes), cars in city traffic (stop-and-go), a cyclist pedaling up and down slopes (speed varies).
• Data for the motion of an object are given in the following table. State whether the speed of the object is uniform or nonuniform. Find the average speed.

Solution The speeds vary from interval to interval $\to$ non-uniform motion. Average speed: total distance / total time $=60\mathrm{m}/100\mathrm{s}=0.6\mathrm{m}/\mathrm{s}$.

• A vehicle moves along a straight line and covers a distance of 2 km. In the first 500m, it moves with a speed of 10 m/s and in the next 500 m, it moves with a speed of 5m/s. With what speed should it move the remaining distance so that the journey is complete in 200 s? What is the average speed of the vehicle for the entire journey? Solution: Total distance = 2 km = 2000 m. First 500 m at 10 m/s → time t1 = 500/10 = 50 s. Next 500 m at 5 m/s → time t2 = 500/5 = 100 s. So, time used so far = 50+100 = 150 s. Total allowed time = 200 s → Remaining time for the last 2000−1000 = 1000 m is 200 – 150 = 50 s. Required speed for remaining distance = 1000/50 = 20 m/s

4.0Key Features of NCERT Solutions Class 7 Science Chapter 8: Measurement of Time and Motion

When utilizing NCERT Solutions for Class 7 Science Chapter 8, students can expect several key features that facilitate effective learning:

• Comprehensive Coverage: The solutions provide detailed explanations for all exercises and questions presented in the NCERT textbook, ensuring a thorough understanding of all topics, from the basics of time measurement to complex distance-time graphs.
• Step-by-Step Solutions: Complex problems, especially those involving calculations of speed, distance, or time, are broken down into easy-to-follow steps. This approach helps students grasp the problem-solving methodology and build confidence.
• Conceptual Clarity: Each solution is designed to reinforce the underlying scientific concepts. Definitions are clear, and explanations are provided with examples to ensure conceptual clarity. For instance, the distinction between uniform and non-uniform motion is made explicit.
• Accurate and Reliable Content: The solutions are aligned with the updated NCERT syllabus and are meticulously checked for accuracy, providing reliable information for exam preparation.
• Visual Aids and Diagrams (where applicable): While not directly in text, good solutions often refer to or implicitly describe diagrams and graphs (like distance-time graphs) that are crucial for understanding motion concepts.
• Preparation for Examinations: By working through the solutions, students can practice answering various question types, including short-answer, long-answer, and numerical problems, thereby preparing them effectively for school examinations and competitive tests.