Question 4 Suppose we represent the distance above the ground by a positive integer and that below the ground by a negative integer, then answer the following: (i) An elevator descends into a mine shaft at the rate of 5 metre per minute. What will be its position after one hour?
Elevator goes down at rate of 5 metre per minute
Therefore,
Position of elevator after 1 minute = −5m
Position of elevator after 60 minute = −5 × 60 m
= −300 m
Elevator is 300 m below ground level.
Question 4 Suppose we represent the distance above the ground by a positive integer and that below the ground by a negative integer, then answer the following: (ii) If it begins to descend from 15 m above the ground, what will be its position after 45 minutes?
Initially,
Elevator is 15 m above ground
∴ Initial Position = +15 m
Now,
Elevator moves down for 45 minutes
Distance elevator moves in 1 minute = −5 m
Distance elevator moves in 45 minute = −45 × 5 m
= −225m
∴ Final position of elevator = Initial position
+ Distance it moves in 45 minutes
= 15 + (−225)
= 15 − 225
= − (−15 + 225)
= − (225 − 15)
= −210 m
∴ Position after 45 minutes is −210 m
i.e. Elevator is 210 m below ground level

Davneet Singh has done his B.Tech from Indian Institute of Technology, Kanpur. He has been teaching from the past 13 years. He provides courses for Maths, Science, Social Science, Physics, Chemistry, Computer Science at Teachoo.

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