Experts have designed these Class 7 Science Notes Chapter 8 Measurement of Time and Motion Class 7 Notes for effective learning.
Class 7 Science Chapter 8 Measurement of Time and Motion Notes
Class 7 Measurement of Time and Motion Notes
Class 7 Science Chapter 8 Notes – Measurement of Time and Motion Notes Class 7
Measurement Of Time
- In ancient times, people measured time using some natural events which repeated regularly after fixed time intervals.
- It was observed that full moon occurs after fixed intervals of time and the sunrise also occurs after fixed intervals of time. So the two natural periodic events which were used in ancient times to measure time were the occurrence of full moon and the sunrise.
- The time from one full moon to the next full moon was called a month.
- The time from one sunrise to the next sunrise was called a day.
- Many devices like sundials, water clocks, hourglasses, and candle clocks were also made earlier to measure smaller intervals of time within a day.
- The motion of pendulum was first studied by Galileo.
- The to and fro motion of a simple pendulum is an example of periodic motion.
- The time taken by pendulum bob to make one complete oscillation from A to O, to B and then back from B to O to A is called time period of the pendulum.
Fig. Some ancient time measuring devices
We measure time with the help of clocks and watches.
Fig. Some common clocks and watches
A Simple Pendulum
A simple pendulum consists of a small metal ball (called bob) suspended by a long thread from a rigid support such that the bob is free to swing back and forth.
Fig. Simple Pendulum
- The motion of pendulum was first studied by Galileo.
- The to and fro motion of a simple pendulum is an example of periodic motion.
- The time taken by pendulum bob to make one complete oscillation from A to O, to B and then back from B to O to A is called time period of the pendulum.
SI Unit of Time
- The standard unit of measuring time is second. The symbol of second is ‘s’. The larger units of time are minute and hour. The symbol of minute is min and the symbol of hour is h.
- Units of time, such as second, minute, and hour and also their symbols begin with a lowercase letter.
- A full stop is not written after the symbol, except at the end of a sentence. While writing the time, always leave a space between the number and the unit.
- 1 hour = 60 minutes, 1 minute = 60 seconds
Still bigger units of time for expressing longer time intervals are day, month, and year.
- 24 hours = 1 day
- 30 days = 1 month
- 12 months = 1 year
- 10 years = 1 decades
- 100 years (or 10 decades) = 1 century
- 1000 years (10 centuries) = 1 millenium
Slow Or Fast Motion
- An object which takes a long time to cover a certain distance is known to be in slow motion while the object which takes shorter time to cover the same distance is known to be in fast motion.
- By comparing the distances moved by two or more objects in a unit time, it can be found out which of them is moving faster.
- We call the distance covered by an object in a unit time as the speed of the object.
Speed
- Speed is the distance covered by an object in unit time.
Speed = \(\frac{\text { Total distance covered }}{\text { Total time taken }}\) - SI unit of speed is metre/second and is expressed as m/s.
- Commonly used unit of speed is kilometre/hour, expressed as km/h.
- Total distance covered = \(\frac{\text { Speed }}{\text { Total time taken }}\)
- Total time taken = \(\frac{\text { Total distance covered }}{\text { Speed }}\)
- Speedometer is the device which records speed of a vehicle in Km/h.
- Odometer is the device or meter that measures the distance moved by thee vehicle in km.
Uniform And Non-Uniform Linear Motion
- An object moving along a straight line with a constant speed is said to be in uniform linear motion.
- If the speed of an object moving along a straight line keeps changing, it is said to be in non-uniform linear motion.
→ Non-uniform linear motion: If the speed of an object moving along a straight line keeps changing, it is said to be in non¬uniform linear motion.
→ Oscillatory motion: The to and fro motion of a simple pendulum about its mean position is called oscillatory motion.
→ Speed: It is defined as the distance travelled per unit time. Thus
Speed = \(\frac{\text { Distance covered }}{\text { Time taken }}\)
→ Speedometer: It is a device used to measure speed of an object.
→ Time period: Time taken by the pendulum for one oscillation is called time period.
→ Uniform linear motion: An object moving along a straight line with a constant speed is said to be in uniform linear motion.
→ Unit of time: The S.I. unit of time is second. It is denoted by ‘s’.
→ Unit of speed: The S.I. unit of speed is m/s (metre/second).
