Class 7 Science Notes Chapter 13 (Motion and Time) – Science Book

Okay, let's prepare detailed notes for Chapter 13: Motion and Time from the NCERT Class 7 Science textbook, focusing on aspects relevant for government exam preparation.

NCERT Class 7 Science: Chapter 13 - Motion and Time
(Detailed Notes for Exam Preparation)

1. Introduction to Motion

• Definition: An object is said to be in motion if it changes its position with respect to time relative to a stationary observer or reference point. If it does not change its position, it is said to be at rest.
• Relativity: Motion is relative. An object can be in motion relative to one observer and at rest relative to another (e.g., a passenger inside a moving bus is at rest relative to other passengers but in motion relative to a person standing outside).

2. Types of Motion (Based on Path)

• Rectilinear Motion: Motion along a straight line.
  • Examples: A car moving on a straight road, a falling stone (ignoring air resistance), march-past of soldiers in a parade.
• Circular Motion: Motion along a circular path.
  • Examples: Blades of a rotating fan, hands of a clock, Earth revolving around the Sun, a stone tied to a string and whirled around.
• Periodic Motion: Motion that repeats itself after regular intervals of time.
  • Examples: Motion of a pendulum, swing, hands of a clock, revolution of the Earth, heartbeat.
  • Note: Circular motion can be periodic (like Earth's revolution), but not all periodic motion is circular (like a pendulum's swing).

3. Slow and Fast Motion: Concept of Speed

• Concept: We often compare motion by how fast or slow objects move. This intuitively relates to the distance covered in a given time.
• Speed: The distance covered by an object in a unit time is called its speed.
  • It indicates how fast an object is moving.
  • Formula: Speed = Total Distance Covered / Total Time Taken
  • Example: If a car covers 100 km in 2 hours, its speed is 100 km / 2 h = 50 km/h.

4. Units of Speed and Time

• Basic Unit of Time: The basic unit of time is the second (s).
• Other Units of Time: Minute (min), Hour (h).
  • 1 min = 60 s
  • 1 h = 60 min = 3600 s
• Basic Unit of Speed: Since speed is Distance/Time, the basic unit (SI unit) is metres per second (m/s).
• Other Common Units of Speed: Kilometres per hour (km/h), metres per minute (m/min).
• Conversion:
  • To convert km/h to m/s: Multiply by (1000 m / 3600 s) = 5/18.
  • To convert m/s to km/h: Multiply by (3600 s / 1000 m) = 18/5.
  • Exam Relevance: Conversions are frequently asked.

5. Uniform and Non-uniform Motion

• Uniform Motion: An object moving along a straight line with a constant speed is said to be in uniform motion. It covers equal distances in equal intervals of time.
• Non-uniform Motion: An object moving along a straight line whose speed keeps changing is said to be in non-uniform motion. It covers unequal distances in equal intervals of time.
  • Examples: A car starting from rest, a car slowing down, a ball rolling down an incline (usually accelerating).

6. Measurement of Time

• Historical Devices: Sundials, water clocks, sand clocks were used in ancient times. These relied on natural periodic events but had limitations (e.g., sundials don't work at night or on cloudy days).
• Modern Devices: Clocks and watches are now used. They rely on the principle of periodic motion.
  • Examples: Quartz clocks use vibrations of a quartz crystal; older clocks used pendulums or balance wheels.

7. Simple Pendulum

• Definition: A simple pendulum consists of a small metallic ball (called a bob) suspended by a light, inextensible string from a rigid support.
• Oscillatory Motion: The to-and-fro motion of a simple pendulum is an example of periodic or oscillatory motion.
• Key Terms:
  • Oscillation: One complete to-and-fro movement of the pendulum bob (e.g., from mean position A to extreme B, then to extreme C, and back to A).
  • Time Period (T): The time taken by the pendulum to complete one oscillation.
  • Frequency (f): The number of oscillations completed in one second. (f = 1/T). (Frequency concept might be less emphasized in Class 7 but good to know).
• Factors Affecting Time Period:
  • The time period of a simple pendulum depends only on its length (Length 'L' from the point of suspension to the centre of the bob). Longer length means a longer time period.
  • The time period is independent of the mass of the bob (for a given length).
  • The time period is independent of the amplitude (the maximum displacement from the mean position), provided the amplitude is small.
  • Exam Relevance: This is a very important point often tested – what affects the time period of a pendulum.

8. Measuring Speed Practically

• To measure the speed of an object, you need to measure:
  1. The distance covered by the object.
  2. The time taken to cover that distance.
• Then use the formula: Speed = Distance / Time.
• Odometer: Device in vehicles that measures the distance travelled.
• Speedometer: Device in vehicles that indicates the speed at that instant, usually in km/h.

9. Distance-Time Graphs

• Purpose: A graphical representation showing how the distance travelled by an object changes with time. It provides a visual understanding of the motion.
• Plotting:
  • Time is usually plotted on the X-axis (horizontal).
  • Distance is usually plotted on the Y-axis (vertical).
  • Choose appropriate scales for both axes.
• Interpretation:
  • Object at Rest: The graph is a straight line parallel to the time axis (X-axis). (Distance does not change with time).
  • Object in Uniform Motion (Constant Speed): The graph is a straight line inclined to the time axis. The steeper the slope, the greater the speed.
  • Object in Non-uniform Motion: The graph is a curved line.
• Calculating Speed from Graph: The slope (or gradient) of the distance-time graph gives the speed.
  • Slope = (Change in Distance) / (Change in Time) = Speed (for uniform motion).

Key Takeaways for Exams:

• Definitions of Motion, Rest, Rectilinear, Circular, Periodic Motion.
• Definition and Formula for Speed (Speed = Distance/Time).
• Units of Time (s, min, h) and Speed (m/s, km/h) and their conversions (especially km/h to m/s and vice-versa).
• Difference between Uniform and Non-uniform motion.
• Principle of time measurement (using periodic motion).
• Simple Pendulum: Definition, Oscillation, Time Period.
• Crucial: Factors affecting the Time Period of a simple pendulum (Only Length, not mass or small amplitude).
• Interpretation of Distance-Time graphs: Shape of graph for rest, uniform motion, non-uniform motion.
• Slope of Distance-Time graph represents speed.
• Functions of Odometer and Speedometer.

Study these points thoroughly. Pay special attention to definitions, formulas, units, conversions, the factors affecting a pendulum's time period, and interpreting distance-time graphs, as these are common areas for questions in competitive exams drawing from basic science concepts.