Verified Solution • 2025–26 Session moderate

QJEEM19Ph8b05: JEE Main 2019 Physics - Class 11 JEEM19Ph8b05 Solution: A block of mass m is kept on a platform which starts from rest…

A block of mass m is kept on a platform which starts from rest with constant acceleration g/2 upward, as shown in figure. Work done by normal reaction on block in time is -

JEE Main 2019 (Online) 10th January Morning Slot Physics - Work Power & Energy Question 100 English

Multiple Choice Options

  • A mg2t28{{m{g^2}{t^2}} \over 8}
  • B 3mg2t28{{3m{g^2}{t^2}} \over 8}
  • C −- mg2t28{{m{g^2}{t^2}} \over 8}
  • D 0

Step-by-Step Verified Solution

N −- mg = mg2{{mg} \over 2} ⇒\Rightarrow N = 3mg2{{3mg} \over 2}

The distance travelled by the system in time t is

S = ut + 12at2=0+12(g2)t2=12g2t2{1 \over 2}a{t^2} = 0 + {1 \over 2}\left( {{g \over 2}} \right){t^2} = {1 \over 2}{g \over 2}{t^2}

Now, work done

W = N.S = (32mg)(12g2t2)\left( {{3 \over 2}mg} \right)\left( {{1 \over 2}{g \over 2}{t^2}} \right)

⇒\Rightarrow   W = 3mg2t28{{3m{g^2}{t^2}} \over 8}

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