Chapter 9:
Motion and Time

Class 7 Science | CBSE Curriculum

Learn about speed, types of motion, simple pendulum, time period, distance-time graphs, uniform and non-uniform motion explained with more than 100 practice questions.

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Quick Revision: Motion and Time

  • Speed: Distance covered per unit time. Formula: Speed = Distance / Time. SI unit: m/s.
  • Types of Motion: Rectilinear (straight line), Circular (along a circle), Periodic (repeats after fixed interval, e.g., pendulum).
  • Uniform Motion: Constant speed → equal distances in equal time intervals. Distance-time graph is a straight line.
  • Non-uniform Motion: Changing speed → distance-time graph is curved.
  • Simple Pendulum: Bob + string. Time period = time for one oscillation. T = total time / number of oscillations.
  • Time Period of Pendulum: Depends on length (longer length → longer period). Constant for given length (Galileo's discovery).
  • Units of Time: 1 day = 24 hours, 1 hour = 60 minutes, 1 minute = 60 seconds. Microsecond = 10⁻⁶ s, nanosecond = 10⁻⁹ s.
  • Speedometer: Measures instantaneous speed (km/h). Odometer: Measures total distance travelled (km).
  • Distance-Time Graph: Slope = speed. Horizontal line = at rest. Straight line = constant speed. Steeper slope = higher speed.
  • Average Speed: Total distance travelled divided by total time taken.
  • Conversion: km/h to m/s: multiply by 5/18. m/s to km/h: multiply by 18/5.

Chapter Summary:

Motion and Time introduces the fundamental concepts of measuring and comparing motion. Speed is defined as distance covered per unit time (SI unit: m/s). Objects can move in different ways: rectilinear (straight line), circular, or periodic (repeating motion like a pendulum). Uniform motion means constant speed, shown as a straight line on a distance-time graph; non-uniform motion shows changing speed as a curved line.

Time measurement uses periodic events. The simple pendulum is a classic example: its time period (time for one oscillation) is constant for a given length. Galileo discovered this property, leading to pendulum clocks. Modern quartz clocks are more accurate. Time units range from nanoseconds (10⁻⁹ s) to billions of years for astronomical events.

Speed is calculated as distance/time. Instruments like speedometers (instantaneous speed) and odometers (total distance) help measure motion. Distance-time graphs provide visual insights into motion: slope indicates speed, horizontal lines indicate rest, steeper lines indicate faster motion. Understanding these concepts helps analyze everyday motion, from walking to rocket launches.