Page 121 - Ripples SCIENCE 7 - TEJPUR Edition 2024 Answer Key

7. Discuss the ancient time-measuring devices.

Ans: (a) Sundial: An ancient timekeeping device, a sundial indicates the time of the
day by the position of the shadow of the object exposed to the Sun’s rays. A

mathematician and astronomer, Theodosius of Bithynia, invented a universal
sundial that could be used anywhere on the Earth with accuracy.

(b) Sand clock: Sand clock also known as the hour glass is used to measure time for
shorter intervals. It consists of two glass bulbs connected vertically by a narrow

neck that allows the flow of sand from the upper bulb to the lower bulb. The
sand flows in a fixed time in the sand clock.

8. A car travels 100 km with a speed of 20 km/h and the other 100 km with a speed of
50 km/h. Calculate the total time taken to complete the journey.

Ans: Speed of car in first 100 km = 20 km/h
Speed of car in next 100 km = 50 km/h

Time taken to = Distance
Speed

cover first 100 km 100 5
20 1
= 5 hours.

Similarly,

Time taken to cover = 100 2
50 1
next 100 km = 2 hours.
Total Time = 5 hours + 2 hours

= 7 hours.

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