Question

BEGINNER'S BOX-2
1. A person of mass M kg is standing on a lift. If the lift moves stop vertically upwards according to given v-t graph then find out the weight of man at the following instants: g = 10 m/s)
(1)t = 1 second
(2) t = 8 seconds
(3) t = 12 seconds

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Answer 1

Answer:

(1) 20M      (2) 10M  (3) 5m

Explanation:

Weight is a force. A force is mass times acceleration.  In this question, mass is undefined so our problem lies in finding the acceleration of the mass on the lift and to not forget about gravity.

First you need to figure out the acceleration of the lift at the times in question.  At time 1 the lift has changed from a velocity of 0m/s to 10 m/s in one second so the acceleration is 10m/ss.

At time 8 the lift isn't accelerating at all. It is maintaining a constant velocity of 20m/s. There is no acceleration.

At time 12, the lift is decellerating at half the rate that it sped up. It now takes 4 seconds to slow down to a standstill instead of the 2 seconds it previously took to get up to 20m/s from a stand still. The acceleration here is -5m/ss.

At time 1, The lift has an acceleration of 10 m/ss.  At time 8 the lift has an acceleration of 0 m/ss.  At time 12, the lift has an acceleration of -5m/ss.

You add in the acceleration of 10m/ss  due to gravity to each of these accelerations  and you get your answer.