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JEE Advanced 2014 · Mechanical Properties of Fluids

A person in lift is holding a water jar, which has a small hole at the lower end of its side. When the lift is at rest, the water jet coming out of the hole hits the floor of the lift at a distance dd of 1.2 m1.2\ \text{m} from the person. In the following, state of the lift's motion is given in List-I and the distance where the water jet hits the floor of the lift is given 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. (P) Lift is accelerating vertically up (1) d=1.2 md = 1.2\ \text{m} (Q) Lift is accelerating vertically down with an acceleration less than the gravitational acceleration (2) d>1.2 md > 1.2\ \text{m} (R) Lift is moving vertically up with constant speed (3) d<1.2 md < 1.2\ \text{m} (S) Lift is falling freely (4) No water leaks out of the jar

Answer: C. P−1,Q−1,R−1,S−4P-1, Q-1, R-1, S-4

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Written solution

Step 1: Define the effective acceleration due to gravity.

The effective acceleration due to gravity geffg_{\text{eff}} inside the lift depends on its state of motion.

For a lift accelerating vertically upward with acceleration aa:

geff=g+ag_{\text{eff}} = g + a

For a lift accelerating vertically downward with acceleration a<ga < g:

geff=g−ag_{\text{eff}} = g - a

For a lift moving with constant speed:

geff=gg_{\text{eff}} = g

For a lift in free fall:

geff=0g_{\text{eff}} = 0

Step 2: Calculate the horizontal distance of the water jet.

Let hh be the height of the water column above the hole, and HH be the height of the hole above the floor of the lift.

The horizontal velocity of efflux vv is given by Torricelli's law.

v=2geffhv = \sqrt{2 g_{\text{eff}} h}

The time of flight tt for the water to reach the floor is determined by the vertical motion.

t=2Hgefft = \sqrt{\frac{2H}{g_{\text{eff}}}}

The horizontal distance dd traveled by the jet is the product of velocity and time.

d=v⋅td = v \cdot t d=2geffh⋅2Hgeffd = \sqrt{2 g_{\text{eff}} h} \cdot \sqrt{\frac{2H}{g_{\text{eff}}}} d=2hHd = 2\sqrt{hH}

The distance dd is independent of geffg_{\text{eff}}, meaning it remains constant at 1.2 m1.2\,\text{m} as long as geff>0g_{\text{eff}} > 0.

Step 3: Match the states of motion.

For case (P), the lift is accelerating vertically up, so geff=g+a>0g_{\text{eff}} = g + a > 0.

d=1.2 md = 1.2\,\text{m}

For case (Q), the lift is accelerating vertically down with a<ga < g, so geff=g−a>0g_{\text{eff}} = g - a > 0.

d=1.2 md = 1.2\,\text{m}

For case (R), the lift is moving vertically up with constant speed, so geff=g>0g_{\text{eff}} = g > 0.

d=1.2 md = 1.2\,\text{m}

For case (S), the lift is falling freely, so geff=0g_{\text{eff}} = 0.

v=0v = 0

Answer: (P) →\to (1), (Q) →\to (1), (R) →\to (1), (S) →\to (4)

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