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Notes Chapter 8 Class 7 Science Curiosity NCERT

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Measurement of Time and Motion


1. Measurement of Time

Ancient Timekeeping Devices:

  • Sundial: Uses shadows from sunlight to tell time.
  • Water Clock: Measures time by water flow (outflow or floating bowl types, e.g., Ghatika-yantra).
  • Hourglass: Time based on sand flow.
  • Candle Clock: Time marked by burning candle.

Evolution of Clocks:

  • Mechanical clocks (14th century): Used weights, gears, springs.
  • Pendulum clocks (17th century): Invented by Christiaan Huygens, inspired by Galileo.
  • Modern clocks: Quartz (crystal vibrations) and atomic clocks (atom vibrations, highly accurate).

Fascinating Fact: Ghatika-yantra measured time in ghati (24 minutes); a day had 60 ghatis.


2. Simple Pendulum

  • Definition: A bob (small metal ball) hung by a thread from a fixed support.
  • Oscillation: One complete swing (e.g., mean position → one side → other side → back).
  • Time Period: Time for one oscillation; depends on pendulum length, not bob mass.
  • Key Property: Time period is constant for a given length at a specific location.
  • Activity: Measure time for 10 oscillations, calculate time period (time ÷ 10).

3. SI Unit of Time

  • Second (s): SI unit of time.
  • Larger units: 60 s = 1 min, 60 min = 1 h.
  • Rules: Use lowercase (s, min, h), no full stop, space between number and unit (e.g., 5 s).

4. Speed

Definition: Distance covered in unit time (Speed = Distance ÷ Time).

SI Unit: Metre/second (m/s); also kilometre/hour (km/h).

Formulas:

  • Speed = Distance ÷ Time
  • Distance = Speed × Time
  • Time = Distance ÷ Speed

Average Speed: Total distance ÷ total time (used when speed varies).

Activity: Calculate train speeds using railway timetable data.


5. Uniform and Non-Uniform Linear Motion

  • Linear Motion: Motion along a straight line.
  • Uniform Linear Motion: Constant speed, equal distances in equal time intervals.
  • Non-Uniform Linear Motion: Changing speed, unequal distances in equal time intervals.
  • Example: A train moving at constant speed (uniform) vs. speeding up/slowing down (non-uniform).
  • Real Life: Non-uniform motion is more common (e.g., cars in traffic).

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