Measurement of Time and MotionClass 7 Science Notes

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Section 1 of 4

Measurement of Time

Humans have been interested in tracking time for a long time. They noticed that many natural events repeat themselves regularly, like the rising and setting of the Sun, the phases of the Moon, and the changing seasons. They used these cycles to keep time and developed calendars. A day was defined by the cycle of the Sun's rising and setting. Then, they looked for ways to know the time of day.

To measure shorter periods within a day, they created devices like sundials, water clocks, hourglasses, and candle clocks.

  • Sundial Time is determined by the position of the shadow cast by the Sun's light on an object.
  • Water Clock Time is measured by the flow of water either out of or into a vessel.
    • In one type, water flows out of a vessel with markings.
    • In another type, a bowl with a small hole is floated on water. It fills gradually and sinks after a certain time.
  • Hourglass Time is measured by the flow of sand from one bulb to another.
  • Candle Clock Candles with markings indicate the passage of time as the candle burns.
Example
The world's largest stone sundial, the Samrat Yantra, is located at Jantar Mantar in Jaipur, Rajasthan. It was built around 300 years ago. It is 27 metres tall, and its shadow moves about 1 millimetre per second.

Activity 8.1: Let us construct

A simple water clock can be made using a plastic bottle, a drawing pin, and some water. Cut the bottle in half, make a hole in the cap, invert the top half into the bottom half, and fill it with water. Mark the water level every minute as the water drips.

Example
In ancient India, time was measured using shadows and water clocks. The Arthasastra by Kautilya mentions shadow-based time measurement. Varahamihira gave an accurate expression for time using the shadow of a vertical stick around 530 CE. Water clocks are described in the Arthasastra and other texts. The sinking bowl water clock, or Ghatika-yantra, was mentioned by Aryabhata. Time was measured constantly with Ghatika-yantra at monasteries and royal palaces. When the bowl sank, it was announced by drums or gongs.

As civilization advanced and people travelled long distances, accurate time measurement became essential. This led to better mechanical clocks driven by weights, gears, and springs from the fourteenth century. The invention of the pendulum clock in the seventeenth century was a significant breakthrough.

Know A Scientist

The pendulum clock was invented in 1656 and patented in 1657 by Christiaan Huygens (1629-1695). He was inspired by Galileo Galilei's (1564-1642) investigations of pendulums. Galileo noticed a lamp swinging in a church and found that the time for each swing was the same, using his pulse to measure time. He concluded that the time to complete one oscillation was constant for a pendulum of a specific length.

8.1.1 A simple pendulum

A simple pendulum consists of a small metallic ball, called the bob, suspended from a rigid support by a long thread.

When the bob is moved to one side and released, it begins oscillatory motion. This motion is periodic because it repeats its path after a fixed time.

One oscillation is completed when the bob moves from its mean position (O) to one extreme position (A), then to the other extreme position (B), and back to O. It also completes one oscillation when it moves from A to B and back to A. The time period is the time taken to complete one oscillation.

Activity 8.2: Let us experiment

To measure the time period of a pendulum, collect a string, a bob, a stopwatch, and a ruler. Tie the bob to the string, fix the other end to a support, and let the bob come to rest. Move the bob slightly to one side and release it. Measure the time for 10 oscillations and repeat the activity several times. Divide the time taken for 10 oscillations by 10 to find the time period.

The time period of a pendulum is almost the same every time.

The time period of a simple pendulum depends on its length but not on the bob's mass. All pendulums of the same length have the same time period at a given location.

The time period of a simple pendulum of a given length is constant at a place. This property is used in the measurement of time.

All clocks, old or modern, are based on a continuously repeating process that marks equal time intervals.

Modern clocks use tiny, rapid vibrations from a quartz crystal (quartz clocks) or atoms (atomic clocks). Huygens' early pendulum clocks could lose 10 seconds per day, but today's atomic clocks lose only one second in millions of years. Scientists are always seeking more accurate ways to measure time.

8.1.2 SI unit of time

The SI unit of time is the second, with the symbol s. Larger units are the minute (min) and the hour (h).

60 s=1 min60 min=1 h60 \text{ s} = 1 \text{ min} \quad 60 \text{ min} = 1 \text{ h}

Units of time (second, minute, hour) begin with a lowercase letter, except at the beginning of a sentence. Their symbols ('s', 'min', 'h') are also lowercase and singular. A full stop is not written after the symbol, except at the end of a sentence. "sec" for second and "hrs" for hour is incorrect.

Example
The hole in the bowl of Ghatika-yantra was made to fill and sink in 24 minutes. The time unit measured was called ghatika or ghati, which became the standard unit of time measurement until the end of the nineteenth century. A 24-hour-long day was divided into 60 equal ghatis.

Activity 8.3: Let us identify

The smallest interval of time that can be measured with a typical wall clock is one second.

Science and Society

Measuring tiny fractions of a second is crucial in today's world. In sports, timekeeping devices record events to one-hundredth or one-thousandth of a second (a millisecond). In medicine, heart monitors like Electrocardiogram (ECG) machines measure millisecond variations in heartbeats. In music, digital recordings capture sound thousands of times per second. Smartphones and computers process signals in microseconds (one-millionth of a second). Scientists develop more precise time-measuring tools for space exploration, medicine, and advanced science experiments.