Notes Chapter 8 Class 7 Science Curiosity NCERT

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).