Revision Notes for Chapter 8 Measurement of Time and Motion Class 7 Science

Chapter 8 Measurement of Time and Motion Revision Notes Class 7 Science Curiosity

NCERT Notes of Measurement of Time and Motion for Class 7 Science Textbook Curiosity is available on this page of studyrankers website. This chapter is from Class 7 Science NCERT Textbook named Curiosity. This textbook is published by NCERT (National Council of Educational Research and Training). We have also provided Chapter 8 Measurement of Time and Motion NCERT Solutions which is very helpful in understanding the chapter clearly and in easy manner. We have covered all the important points and topics of the Measurement of Time and Motion chapter of class 7 Science ncert textbook. Students can also find all the questions answers of Measurement of Time and Motion chapter which is in the textbook updated to latest pattern of cbse and ncert.

NCERT Notes for Chapter 8 Measurement of Time and Motion Class 7 Science

Prerna and her sister were watching a sports channel. Prerna loved running and was the fastest girl in her district’s 100 metre sprint. She was now training for the state level and dreamed of running for India.
Watching old Olympic races, Prerna was amazed at how exact the timing was, even when runners finished together. At school, they used a stopwatch to time races.

Her mother wore a watch, her sister checked time on a phone, and her uncle had a Braille and a talking watch. There was also a big clock at school.
Prerna wondered how people in the past told time without these devices.

Measurement of Time

Long ago, humans became interested in keeping track of time. They noticed many natural events happened in regular cycles—like the Sun rising and setting, the phases of the Moon, and changing seasons. These cycles helped people create calendars. A day was defined by the cycle of the Sun rising and setting.

Next, people wanted to know the time during the day. Since there were no clocks or watches, they invented devices to measure smaller parts of the day, such as:

To measure time within a day, they invented devices such as:

• Sundials: These used the shadow cast by the Sun’s light on an object to show the time of day. As the Sun moved, the shadow’s position changed.
• Water Clocks: These measured time using water flow. One type had water flowing out of a marked vessel, while another used a bowl with a hole that sank when filled with water.
• Hourglasses: These used sand flowing from one bulb to another to measure time.
• Candle Clocks: These were candles with markings that showed time as they burned down.

Fascinating Facts 

The world’s largest stone sundial, called the Samrat Yantra, was built about 300 years ago at the Jantar Mantar in Jaipur, Rajasthan. This site is a UNESCO World Heritage place with many ancient astronomical instruments.

The Samrat Yantra stands 27 metres tall. Its shadow moves very slowly about 1 millimetre every second and falls on a scale that can measure time as precisely as every 2 seconds. Like all sundials, it shows local solar time, so a small correction is needed to convert this to Indian Standard Time.

Making a simple water clock?

 A water clock is a device that measures time by the flow of water from one container to another. It works on the principle of constant water flow, where the time taken for a certain amount of water to flow from the upper part to the lower part indicates the passage of time. 

Materials Required

•  Used transparent plastic bottle (1/2 litre or larger) with cap 
•  Drawing pin 
•  Water 
•  Ink or food colour (optional) 

Procedure

• Prepare the Bottle: Cut the plastic bottle into two halves roughly in the middle. 
• Make a Hole: Use a drawing pin to make a small hole in the cap of the bottle. 
• Assemble the Clock: Place the upper part of the bottle (with the hole in the cap) upside down on the lower half. 
• Fill with Water: Fill the upper part of the bottle with water. You can add a few drops of ink or colour to make the water level more visible. 
• Start the Clock: The water will start dripping into the lower part of the bottle. Use a watch to mark the water level at one-minute intervals until all the water has dripped down.

How to use the Water Clock

• Resetting: Pour the water from the lower part back into the upper part. 
• Timing: Watch the water drip into the lower part and mark each time it reaches a level you previously marked. Each mark indicates that one more minute has passed. 

Facts

In ancient India, time was measured using shadows and water clocks.

• The earliest mention of measuring time by shadows comes from the Arthashastra by Kautilya (2nd century BCE to 3rd century CE).
• Around 530 CE, Varahamihira gave a precise way to calculate time using the shadow of a vertical stick.
• Water clocks, where water flowed out of a vessel, were also described in ancient texts like the Arthashastra and Sardulakarnavadana.
• These early water clocks were not very accurate because the flow of water slowed as the water level dropped.
• To solve this, the sinking bowl water clock (called Ghatika-yantra) was developed and mentioned by Aryabhata and other astronomical texts.
• The Ghatika-yantra was used continuously in Buddhist monasteries, royal palaces, and town squares. When the bowl sank, time was announced by drums, conch shells, or gongs.
• Although pendulum clocks replaced the Ghatika-yantra by the late 19th century, it was still used in religious places for rituals.

As human civilization advanced and long-distance travel became common, measuring time precisely became very important. This led to the development of mechanical clocks driven by weights, gears, and springs starting from the 14th century.

The invention of the pendulum clock in the 17th century was a major breakthrough, significantly improving the accuracy of mechanical timekeeping.

Galileo Galilei

The pendulum clock was invented in 1656 and patented in 1657 by Christiaan Huygens (1629–1695). He was inspired by the earlier work of Galileo Galilei (1564–1642).

Huygens’ Pendulum clock

Galileo noticed a lamp swinging back and forth in a church. Using his own pulse to measure time, he found that the lamp took the same amount of time for each swing. After testing different pendulums, Galileo discovered that the time for one complete swing stayed the same for a pendulum of a fixed length.

This discovery helped Huygens create the accurate pendulum clock.

Simple Pendulum

A simple pendulum consists of a small metal ball, known as the bob, which is suspended from a rigid support by a long thread.

A simple pendulum

• When the bob is at rest, it is in the mean position. If the bob is moved slightly to one side and released, it begins to swing back and forth in an oscillatory motion.
• This motion is periodic because it repeats the same path after a fixed interval of time.
• One complete oscillation of the pendulum occurs when the bob, starting from the mean position O, moves to the extreme position A, changes direction, moves to another extreme position B, changes direction again, and returns to O.
• Alternatively, the pendulum also completes one oscillation when the bob moves from one extreme position A to another extreme position B and then comes back to A.
• The time taken for the pendulum to complete one oscillation is called its time period.

Experiment to Measure the Time Period of a Pendulum

Materials Needed:

• A piece of string about 150 cm long
• A heavy metal ball or stone (bob)
• A stopwatch or watch to measure time
• A ruler to measure length

Procedure:

1. Tie the bob to one end of the string.
2. Fix the other end of the string to a rigid support so that the length between the support and the bob is about 100 cm.
3. Let the bob come to rest in its mean position. Your pendulum is now ready.
4. Gently hold the bob, pull it slightly to one side, and release it without pushing. Make sure the string is taut when you release it.
   
   
5. Observe the pendulum oscillating back and forth.
6. Using the watch, measure the time taken for the pendulum to complete 10 oscillations.
7. Record the time in a table. Repeat this measurement 3 to 4 times for accuracy.
8. Calculate the time period by dividing the total time for 10 oscillations by 10.

Observations

Length of the string = 100 cm 

•  The time period of the pendulum is nearly consistent with each measurement. 
•  This indicates that the time taken for one complete oscillation of the pendulum remains fairly constant.

Conclusion: The pendulum’s time period stays nearly constant because its length and gravity remain unchanged. This makes its swings stable and reliable for measuring time.

Think it

When experimenting with pendulums using a watch, the following questions can be investigated:

• How does the length of the pendulum affect its time period?
• Do pendulums of different lengths have different time periods?
• Does the mass of the bob affect the time period?

To test these questions:

• Use the same bob and measure the time period of pendulums with two or three different lengths.
• Observe whether the time period changes with length and record the results.
• Keep the pendulum length fixed and test with bobs of different masses to check if mass influences the time period.

Conclusion:

The time period of a simple pendulum depends on its length but not on the mass of the bob. At a given location, all pendulums of the same length have the same time period.

 All clocks, whether ancient or modern, rely on a process that repeats continuously to mark equal intervals of time. 

Modern clocks measure time using repeating movements, but instead of pendulums, they use tiny vibrations from quartz crystals or atoms. Early pendulum clocks could lose or gain 10 seconds every day, but today’s atomic clocks are very accurate and lose only one second in millions of years. Scientists keep working to make clocks even better.

SI Unit of Time

• The standard unit of time in the International System of Units (SI) is the second, represented by the symbol "s."
• Larger units of time include the minute (symbol: "min". and the hour (symbol: "h" ).
• The conversions between these units are as follows:
  60 seconds = 1min, 60 min = 1 hour

Dive Deeper

Units of time like second, minute, and hour start with a lowercase letter, unless they begin a sentence. Their symbols are: s, min, and h — are always lowercase and singular. Do not put a full stop after the symbol unless it is at the end of a sentence. Always leave a space between the number and the unit when writing time. Also, using “sec” for second or “hrs” for hour is incorrect.

Facts

The hole in the bowl of the Ghatika-yantra was designed so it took 24 minutes to fill and sink. This time unit was called a ghatika or ghati. It became the standard way to measure time and was used until the end of the 19th century. A 24-hour day was divided into 60 equal ghatis.

Precision in Time Measurement

 In modern society, measuring very small fractions of a second is crucial in various fields: 

• Sports: Timekeeping devices can record events to one-hundredth or one-thousandth of a second to determine race winners. 
• Medicine: Heart monitors like Electrocardiogram (ECG) machines measure heartbeat variations in milliseconds to identify health issues. 
• Music: Digital recordings capture sound thousands of times per second for smooth playback. 
• Technology: Smartphones and computers process signals in microseconds, enabling fast operation. 

 As clocks become faster and more accurate, they contribute to society in ways that may not be immediately noticeable.

Slow or Fast

• When we say something is moving fast or slow, we compare how far it moves in a certain amount of time.
• For example, in a 100-metre race, all runners start together but soon spread out.Boys running a race on a straight track
• The runner who is ahead at a given moment has covered more distance in the same time and is running faster.
• Therefore, the distance covered in a given time helps decide who is faster or slower.
• We say that the faster runner has a higher speed.

Speed

Speed is the measure of how fast an object is moving. It tells us the distance an object covers in a certain amount of time.

• By comparing the distances moved by two or more objects in a unit time, it can be determined which object is moving faster.
• The unit time can be one second, one minute, or one hour.
• The distance covered by an object in a unit time is called its speed.
  
• The SI unit of speed is metre per second (m/s), based on the SI units of distance (metre) and time (second). For larger distances and times, speed is often measured in kilometre per hour (km/h). 

Example: Ravi’s school is 5.2 km from his house. It took him 20 minutes to reach his school riding on his bicycle. Calculate the speed of the bicycle in m/s.

Solution

Distance covered = 5.2 km

Time taken = 20 minutes

Speed of the bicycle = Distance covered/Time Taken

Convert the units,

5.2 km in m = 5.2 × 1000 m = 5200 m

20 minutes in second = 20 × 60 s = 1200s

Speed = 5200/1200 = 4.33 m/s

The speed of the bicycle is 4.33 m/s

Activity: To compare the speeds of different trains based on their timetable information.

Speed Calculation for each train 

Comparison

• Fastest Train: The Superfast Train between Station G and Station H is the fastest, with a speed of 150 km/h.
• Slowest Train: The Passenger Train between Station A and Station B has the slowest speed of 48.19 km/h.

Relationship between speed, distance and time

• We know the formula to calculate speed is:
  
• This formula can be rearranged to find distance if speed and time are known:

Total distance covered = Speed × Total time taken

Similarly, to find time if distance and speed are known:

Example 2: Priya is traveling to a nearby town in a car moving at a speed of 60 km/h. If it takes her 3 hours to reach the town, how far is the town?

Solution

Distance covered by the car = Speed × Time
= 60 km/h × 3 h
= 180 km

The town is 180 km away.

Example 3: A train is traveling at a speed of 80 km/h. How much time will it take to cover a distance of 240 km?

Solution

Time taken by the train = Distance covered/Speed

= 240 km/80 km/h

= 3 h

The train will take 3 hours to cover 240 km.

Average Speed

• The speed calculated by dividing total distance by total time is the average speed.
• An object may not move at the same speed throughout; sometimes it may move slower or faster.
• In this context, the term speed is used to mean average speed.

Measuring Speed and Distance in Vehicles

• Vehicles like scooters, motorbikes, cars, and buses have an instrument called a speedometer.
• A speedometer shows the vehicle’s speed in kilometres per hour (km/h).
• Another instrument called an odometer measures the total distance travelled by the vehicle in kilometres.

Uniform and Non-uniform Linear Motion

Linear Motion

When an object moves along a straight line, the motion is called linear motion.

• Example: A train moving on a straight track between two stations. 

This motion can be uniform (at a constant speed) or non-uniform (with changing speed).

 

Uniform Linear Motion

• When an object moves along a straight line with a constant (unchanging) speed.
• It covers equal distances in equal intervals of time.
• Example: Car moving from one point to another  at constant speed.
  
  

Non-Uniform Linear Motion

• When the speed of an object keeps changing while moving along a straight line.
• It covers unequal distances in equal intervals of time.
• Example: Car moving from A to B and C to D with changing speeds.
  

Note: Uniform motion is an ideal concept. In real life, objects rarely move at a constant speed for long durations. Hence, we use average speed.

Example: In Table , data are given for the distances travelled by two trains, X and Y, between the time 10:00 AM and 11:00 AM. 

Which of the two trains is in uniform linear motion between 10:00 AM and 11:00 AM?

Solution

Train X covers equal distances in equal intervals of time, so it is in uniform linear motion while Train Y is in non-uniform linear motion.

• Train X is in uniform linear motion between 10:00 AM and 11:00 AM because it covers equal distances (20 km) in equal time intervals (10 minutes).
• Train Y, on the other hand, is in non-uniform linear motion because the distances it covers in each 10-minute interval are not equal (e.g., 20 km in the first interval, 15 km in the second, and so on).

Thus, Train X moves uniformly, while Train Y does not.

Important Terms

• Sundial: A device that uses the Sun’s shadow to show the time of day.
• Water Clock: A device that measures time by the flow of water in or out of a vessel.
• Hourglass: A device that measures time by the flow of sand between two bulbs.
• Candle Clock: A candle with markings that shows time as it burns.
• Pendulum: A weight (bob) hanging from a fixed point that swings back and forth to measure time.
• Time Period: The time taken for one complete oscillation of a pendulum.
• Second: The SI unit of time, symbolized as “s.”
• Speed: The distance covered by an object in a unit of time, measured in m/s or km/h.
• Speedometer: An instrument in vehicles that shows speed in km/h.
• Odometer: An instrument in vehicles that measures distance traveled in km.
• Linear Motion: Motion along a straight line.
• Uniform Linear Motion: Motion along a straight line at a constant speed, covering equal distances in equal times.
• Non-uniform Linear Motion: Motion along a straight line with changing speed, covering unequal distances in equal times.
• Average Speed: The total distance covered divided by the total time taken, used when speed varies.
• Ghatika-yantra: A sinking bowl water clock used in ancient India to measure time in ghatis.
• Ghati: A time unit of 24 minutes, measured by the Ghatika-yantra.