Question

Lift A is going up with velocity 20m/s and acceleration as $8m/s^2$ in the downward direction. While lift B is going down with 10m/s and acceleration of $+2m/s^2$ in upward direction. At this moment a bolt in the ceiling of lift A gets loose and falls. Height of the lift is 16 m.

What is the velocity of bolt relative to lift B when it hits the floor of A.?

• A. 18 m/s
• B. 20 m/s
• C. 22 m/s
• D. None of these

Answer 1

The correct option is A

18 m/s

(From previous question you know that the bolt took 4 sec to hit the floor.)

Velocity of bolt at 4s:
vb = 20 - g $\times$ 4 = -20 m/s
= 20 m/s downward
Velocity of lift B at 4 s:
vB = 10 - 2 $\times$ 4 = 2 m/s downward.
So relative velocity
$=\frac{v_b}{B}=20-2=18m/s$