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? (1) x < 0, v < 0, a > 0 (2) x > 0, v < 0, a < 0 (3) x > 0, v < 0, a > 0 (4) x > 0, v > 0, a < 0

Answer: (1) x < 0, v < 0, a > 0

  • Displacement = Downward (x<0)
  • Velocity = Downward (v<0)
  • Acceleration = In direction of velocity (a>0)

Because the lift is coming from the downward direction, the displacement will be negative. We must determine if the motion is retarding or accelerating. We know that the displacement will be negative due to the downward motion. When the lift reaches the fourth floor, it is about to stop; hence, motion is low; thus, x<0 and a>0. Because the displacement is negative, the velocity is also negative, i.e. v<0.

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