First slide

Instantaneous acceleration

Question

A lift is coming from 8th floor and is just about to reach 4th floor. Taking ground floor as origin and positive direction upwards for all quantities, which one of the following is correct?

Moderate

A

x < 0, v < 0, a > 0
B

x > 0, v < 0, a < 0
C

x > 0, v < 0, a > 0
D

x > 0, v > 0, a < 0

Solution

As the lift is coming in downward directions, displacement will be negative. We have to see whether the motion is accelerating or retarding.
We know that due to downward motion, displacement will be negative. When the lift reaches 4th floor and is about to stop, hence motion is retarding in nature, hence x < 0; a > O.
As displacement is in negative direction, velocity will also be negative, i.e. v < 0.

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