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Motion in A Straight Line question

2005 · Shift 0 · Q172
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Motion in A Straight Line question

2005 · Shift 0 · Q172

JEE MainPhysicsMotion in A Straight LineMCQ+4 / −1
A parachutist after bailing out falls 50m50m50m without friction. When parachute opens, it decelerates at 2  m/s2.2\,\,m/{s^2}.2m/s2. He reaches the ground with a speed of 3m/s3m/s3m/s. At what height, did he bail out?
  1. A
    182m182m182m
  2. B
    91m91m91m
  3. C
    111m111m111m
  4. D
    293m293m293m
View written solutionFree

Correct answer: D

  1. First phase: free fall for 50 m50\,\text{m}50m

The parachutist starts from rest and falls freely without air resistance for 50 m50\,\text{m}50m.

Using v2=u2+2gsv^2=u^2+2gsv2=u2+2gs with u=0,g≈10 m/s2,s=50 mu=0,\quad g\approx 10\,\text{m/s}^2,\quad s=50\,\text{m}u=0,g≈10m/s2,s=50m we get v2=0+2(10)(50)=1000v^2=0+2(10)(50)=1000v2=0+2(10)(50)=1000 so the speed just before opening the parachute is v=1000=1010 m/s.v=\sqrt{1000}=10\sqrt{10}\,\text{m/s}.v=1000​=1010​m/s.

  1. Second phase: motion with parachute open

After the parachute opens, the parachutist is decelerated at a=−2 m/s2a=-2\,\text{m/s}^2a=−2m/s2 (taking downward as positive, retardation means acceleration upward, hence negative).

He reaches the ground with speed v=3 m/s.v=3\,\text{m/s}.v=3m/s.

Let the distance covered after the parachute opens be sss.

Again use v2=u2+2asv^2=u^2+2asv2=u2+2as where u2=1000,v2=9,a=−2.u^2=1000,\quad v^2=9,\quad a=-2.u2=1000,v2=9,a=−2.

Thus, 9=1000+2(−2)s9=1000+2(-2)s9=1000+2(−2)s 9=1000−4s9=1000-4s9=1000−4s 4s=9914s=9914s=991 s=247.75 m.s=247.75\,\text{m}. s=247.75m.

  1. Total height from which he bailed out

Total height H=50+247.75=297.75 m.H=50+247.75=297.75\,\text{m}. H=50+247.75=297.75m.

This is closest to 293 m.293\,\text{m}.293m.

Hence the correct option is D.

  1. Note on approximation

If one uses g=9.8 m/s2g=9.8\,\text{m/s}^2g=9.8m/s2, the result is u2=2(9.8)(50)=980,u^2=2(9.8)(50)=980,u2=2(9.8)(50)=980, 9=980−4s,9=980-4s,9=980−4s, s=242.75 m,s=242.75\,\text{m},s=242.75m, H=50+242.75=292.75 m≈293 m.H=50+242.75=292.75\,\text{m}\approx 293\,\text{m}. H=50+242.75=292.75m≈293m.

So with the standard precise value g=9.8 m/s2g=9.8\,\text{m/s}^2g=9.8m/s2, the answer is exactly option D.

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