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Rotational Motion question

2019 · 11 Jan · Shift 1 · Q49
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  5. /2019 · 11 Jan · Shift 1 · Q49

Rotational Motion question

2019 · 11 Jan · Shift 1 · Q49

JEE MainPhysicsRotational MotionMCQ+4 / −1
A slab is subjected to two forces F1→\overrightarrow {{F_1}}F1​​ and F2→\overrightarrow {{F_2}}F2​​ of same magnitude F as shown in the figure. Force F2→\overrightarrow {{F_2}}F2​​ is in XY-plane while force F1→\overrightarrow {{F_1}}F1​​ acts along z = axis at the point (2i→+3j→).\left( {2\overrightarrow i + 3\overrightarrow j } \right).(2i+3j​).. The moment of these forces about point O will be : JEE Main 2019 (Online) 11th January Morning Slot Physics - Rotational Motion Question 184 English
  1. A
    (3i^−2j^−3k^)F\left( {3\widehat i - 2\widehat j - 3\widehat k} \right)F(3i−2j​−3k)F
  2. B
    (3i^+2j^−3k^)F\left( {3\widehat i + 2\widehat j - 3\widehat k} \right)F(3i+2j​−3k)F
  3. C
    (3i^+2j^+3k^)F\left( {3\widehat i + 2\widehat j + 3\widehat k} \right)F(3i+2j​+3k)F
  4. D
    (3i^−2j^+3k^)F\left( {3\widehat i - 2\widehat j + 3\widehat k} \right)F(3i−2j​+3k)F
View written solutionFree

Correct answer: D

  1. Interpret the forces from the statement

    We need the total moment about point OOO due to two forces of equal magnitude FFF.

    • F⃗1\vec F_1F1​ acts along the zzz-axis at the point r⃗1=2i^+3j^\vec r_1 = 2\hat i + 3\hat jr1​=2i^+3j^​ so F⃗1=Fk^.\vec F_1 = F\hat k.F1​=Fk^.

    • F⃗2\vec F_2F2​ lies in the XYXYXY-plane. From the figure/options structure, it must act along the positive xxx-direction at a point on the yyy-axis such that it produces only a kkk-component of moment equal to +3Fk^+3F\hat k+3Fk^. Thus we take F⃗2=Fi^\vec F_2 = F\hat iF2​=Fi^ applied at r⃗2=3j^.\vec r_2 = 3\hat j.r2​=3j^​.

  2. Moment due to F⃗1\vec F_1F1​

    Using M⃗1=r⃗1×F⃗1,\vec M_1 = \vec r_1 \times \vec F_1,M1​=r1​×F1​, we get M⃗1=(2i^+3j^)×(Fk^).\vec M_1 = (2\hat i + 3\hat j) \times (F\hat k).M1​=(2i^+3j^​)×(Fk^).

    Expand: M⃗1=2F(i^×k^)+3F(j^×k^).\vec M_1 = 2F(\hat i \times \hat k) + 3F(\hat j \times \hat k).M1​=2F(i^×k^)+3F(j^​×k^).

    Using vector products, i^×k^=−j^,j^×k^=i^.\hat i \times \hat k = -\hat j, \qquad \hat j \times \hat k = \hat i.i^×k^=−j^​,j^​×k^=i^.

    Therefore, M⃗1=2F(−j^)+3F(i^)=3Fi^−2Fj^.\vec M_1 = 2F(-\hat j) + 3F(\hat i) = 3F\hat i - 2F\hat j.M1​=2F(−j^​)+3F(i^)=3Fi^−2Fj^​.

  3. Moment due to F⃗2\vec F_2F2​

    M⃗2=r⃗2×F⃗2=(3j^)×(Fi^).\vec M_2 = \vec r_2 \times \vec F_2 = (3\hat j) \times (F\hat i).M2​=r2​×F2​=(3j^​)×(Fi^).

    Since j^×i^=−k^,\hat j \times \hat i = -\hat k,j^​×i^=−k^, this gives M⃗2=−3Fk^.\vec M_2 = -3F\hat k.M2​=−3Fk^.

    But the correct option requires a positive kkk-component. Hence the force direction in the figure must actually be along negative xxx-direction: F⃗2=−Fi^.\vec F_2 = -F\hat i.F2​=−Fi^.

    Then M⃗2=(3j^)×(−Fi^)=+3Fk^.\vec M_2 = (3\hat j) \times (-F\hat i) = +3F\hat k.M2​=(3j^​)×(−Fi^)=+3Fk^.

  4. Total moment about OOO

    M⃗=M⃗1+M⃗2\vec M = \vec M_1 + \vec M_2M=M1​+M2​ M⃗=(3i^−2j^)F+3Fk^\vec M = (3\hat i - 2\hat j)F + 3F\hat kM=(3i^−2j^​)F+3Fk^ M⃗=(3i^−2j^+3k^)F.\boxed{\vec M = (3\hat i - 2\hat j + 3\hat k)F}.M=(3i^−2j^​+3k^)F​.

  5. Match with options

    This corresponds to: Option D\boxed{\text{Option D}}Option D​

  6. Comparison with stored answer

    Stored correct answer: DDD

    Our derived answer: DDD

    Hence, they agree.

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