A person in a lift holding lower end of a vertical uniform rope, which is hanging from roof of the lift. Person produces a pulse on the lower end of the string. Pulse reaches at the roof (other end) i

Question

A person in a lift holding lower end of a vertical uniform rope, which is hanging from roof of the lift. Person produces a pulse on the lower end of the string. Pulse reaches at the roof (other end) in 1.2 sec when lift is at rest. In the following, state of lift's motion is given in List-I and the time when the pulse reaches at the roof in List-II. Match the statements from List-I with those in List-II and select the correct answer using the code given below the lists. (Assume man applies no force on the string)

List - I	List - II
(P) Lift is accelerating vertically up	(1) $t = 1.2 s$
(Q) Lift is accelerating vertically down with an acceleration less than the gravitational acceleration	(2) finite time $t > 1.2 s$
(R) Lift is moving vertically up with constant speed	(3) finite time $t < 1.2 s$
(S) Lift is falling freely	(4) pulse will not reach at roof
	(5) None of these

TestHub · Accepted Answer

Pulse speed at a point: v = T / μ ​ , tension set by effective gravity g eff ​ . (P) Lift accelerating up: g eff ​ = g + a so tension and wave speed increase, t < 1.2 s . (Q) Accelerating down with a < g : g eff ​ = g − a , speed drops, finite t > 1.2 s . (R) Constant speed: g eff ​ = g , unchanged, t = 1.2 s . (S) Free fall: g eff ​ = 0 , tension zero, pulse never reaches. So P → 3, Q → 2, R → 1, S → 4. Ans (D).

Physics - Wave on String Question with Solution | TestHub

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