Motion and Time – Notes for Class 7 Science

Introduction to Motion and Time

Motion and time are fundamental concepts in science that we observe in our daily lives. From a moving car to the ticking of a clock, these phenomena help us understand how objects change their position and how we measure the duration of events. This chapter explores the types of motion, how to measure it, and the relationship between distance, speed, and time.

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1. What is Motion?

• Definition: Motion is the change in the position of an object with respect to its surroundings over time.
• Examples: A car moving on a road, a bird flying in the sky, or a ball rolling on the ground.
• Reference Point: To determine if an object is in motion, we need a fixed point called a reference point. For instance, a tree or a building can serve as a reference point to observe a moving bicycle.
• Rest vs. Motion: An object is at rest if it does not change its position with respect to its surroundings (e.g., a book on a table). It is in motion if its position changes (e.g., a falling leaf).

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2. Types of Motion

Objects can move in different ways. The chapter classifies motion into three main types:

1. Rectilinear Motion:
   • Motion in a straight line.
   • Examples: A car moving on a straight road, a stone falling from a height.
2. Circular Motion:
   • Motion along a circular path.
   • Examples: The blades of a fan, the Earth revolving around the Sun.
3. Periodic Motion:
   • Motion that repeats itself after a fixed interval of time.
   • Examples: The swinging of a pendulum, the heartbeat of a person.

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3. Distance and Displacement

• Distance: The total path length covered by an object during its motion. It is a scalar quantity (has magnitude only).
  • Example: If a person walks 5 meters forward and 3 meters back, the total distance is 8 meters.
• Displacement: The shortest distance between the initial and final positions of an object. It is a vector quantity (has both magnitude and direction).
  • Example: In the same case, if a person walks 5 meters forward and 3 meters back, the displacement is 2 meters forward.
• Key Difference: Distance depends on the path taken, while displacement depends only on the starting and ending points.

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4. Speed

• Definition: Speed is the rate at which an object covers distance. It tells us how fast or slow an object is moving.
• Formula: Speed=DistanceTime\text{Speed} = \frac{\text{Distance}}{\text{Time}}Speed=TimeDistance​
• Units:
  • Meters per second (m/s) – SI unit.
  • Kilometers per hour (km/h) – Commonly used for vehicles.
• Example: If a car travels 100 meters in 10 seconds, its speed is: Speed=10010=10 m/s\text{Speed} = \frac{100}{10} = 10 \, \text{m/s}Speed=10100​=10m/s
• Types of Speed:
  1. Uniform Speed: When an object covers equal distances in equal intervals of time (e.g., a car moving at a constant 60 km/h).
  2. Non-Uniform Speed: When an object covers unequal distances in equal intervals of time (e.g., a car slowing down in traffic).

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5. Measurement of Time

• Definition: Time is the duration between two events.
• Units: Seconds (s), minutes (min), hours (h) – SI unit is seconds.
• Tools for Measuring Time:
  1. Clocks and Watches: Use hands or digital displays to show time.
  2. Stopwatch: Measures small intervals of time accurately (e.g., in sports).
  3. Sundial: An ancient device that uses the shadow of an object to indicate time based on the Sun’s position.
  4. Pendulum Clock: Works based on the periodic motion of a pendulum.
• Simple Pendulum:
  • A simple pendulum consists of a small weight (bob) tied to a string, suspended from a fixed point.
  • It exhibits periodic motion, swinging back and forth.
  • Time Period: The time taken for one complete oscillation (one to-and-fro swing).
  • Factors Affecting Time Period:
    • Length of the string (longer string = longer time period).
    • Gravity (does not change much on Earth).
    • It does not depend on the mass of the bob.

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6. Speed and Distance-Time Graph

• Distance-Time Graph: A graph that shows the relationship between distance and time for a moving object.
  • X-axis: Time.
  • Y-axis: Distance.
• Observations:
  1. Straight Line: Indicates uniform speed (constant slope = constant speed).
     • Slope of the line = Speed.
  2. Curved Line: Indicates non-uniform speed (speed changes over time).
  3. Horizontal Line: Indicates the object is at rest (distance does not change with time).
• Example: If a car travels 20 meters in 4 seconds at a constant speed, plotting the graph gives a straight line with a slope of 5 m/s (speed).

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7. Units of Speed and Conversion

• Common Units:
  • Meters per second (m/s).
  • Kilometers per hour (km/h).
• Conversion:
  • To convert km/h to m/s: Multiply by 518\frac{5}{18}185​.
    • Example: 36 km/h = 36×518=10 m/s36 \times \frac{5}{18} = 10 \, \text{m/s}36×185​=10m/s.
  • To convert m/s to km/h: Multiply by 185\frac{18}{5}518​.
    • Example: 10 m/s = 10×185=36 km/h10 \times \frac{18}{5} = 36 \, \text{km/h}10×518​=36km/h.

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8. Historical Methods of Measuring Time

• Sundial: Used shadows cast by the Sun to estimate time.
• Water Clock: Measured time by the regulated flow of water.
• Sand Clock (Hourglass): Measured time by the flow of sand from one chamber to another.
• These ancient methods were less accurate than modern clocks but laid the foundation for timekeeping.

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9. Real-Life Applications

• Motion: Understanding motion helps in designing vehicles, predicting the movement of planets, and creating sports equipment.
• Time: Accurate time measurement is crucial for scheduling, navigation (e.g., GPS), and scientific experiments.
• Speed: Speed calculations are used in traffic management, athletics, and machinery design.

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10. Key Points to Remember

• Motion is classified into rectilinear, circular, and periodic types.
• Speed = Distance ÷ Time; it can be uniform or non-uniform.
• Time is measured using clocks, stopwatches, or pendulums.
• Distance-time graphs help visualize motion.
• A pendulum’s time period depends on its length, not the bob’s mass.
• Units like m/s and km/h are used for speed, with conversions between them.

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Sample Questions

1. Define motion and give two examples.
2. What is the difference between distance and displacement?
3. Calculate the speed of a bicycle that travels 50 meters in 10 seconds.
4. Draw a distance-time graph for an object moving at uniform speed.
5. How does the length of a pendulum affect its time period?

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Conclusion

Chapter 9 – Motion and Time introduces students to the basics of how objects move and how we track the passage of time. By understanding these concepts, we can analyze everyday phenomena, from the speed of a running athlete to the ticking of a clock. The chapter also builds a foundation for more advanced topics in physics, such as velocity and acceleration, in higher classes.

Ch 09 Motion And Time