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Rotational Motion question

2020 · 5 Sep · Shift 1 · Q43
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  5. /2020 · 5 Sep · Shift 1 · Q43

Rotational Motion question

2020 · 5 Sep · Shift 1 · Q43

JEE MainPhysicsRotational MotionNumerical+4 / −1
A force F→=(i^+2j^+3k^)\overrightarrow F = \left( {\widehat i + 2\widehat j + 3\widehat k} \right)F=(i+2j​+3k) N acts at a point (4i^+3j^−k^)\left( {4\widehat i + 3\widehat j - \widehat k} \right)(4i+3j​−k) m. Then the magnitude of torque about the point (i^+2j^+k^)\left( {\widehat i + 2\widehat j + \widehat k} \right)(i+2j​+k) m will be x\sqrt xx​ N m. The value of x is ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 195

  1. Given data

    Force: F⃗=i^+2j^+3k^\vec F = \hat i + 2\hat j + 3\hat kF=i^+2j^​+3k^

    Point of application of force: r⃗1=4i^+3j^−k^\vec r_1 = 4\hat i + 3\hat j - \hat kr1​=4i^+3j^​−k^

    Point about which torque is to be calculated: r⃗0=i^+2j^+k^\vec r_0 = \hat i + 2\hat j + \hat kr0​=i^+2j^​+k^

  2. Position vector from torque point to point of application

    r⃗=r⃗1−r⃗0\vec r = \vec r_1 - \vec r_0r=r1​−r0​ =(4−1)i^+(3−2)j^+(−1−1)k^= (4-1)\hat i + (3-2)\hat j + (-1-1)\hat k=(4−1)i^+(3−2)j^​+(−1−1)k^ =3i^+j^−2k^= 3\hat i + \hat j - 2\hat k=3i^+j^​−2k^

  3. Torque formula

    Torque about the given point is τ⃗=r⃗×F⃗\vec \tau = \vec r \times \vec Fτ=r×F

    So,

    \begin{vmatrix} \hat i & \hat j & \hat k \\ 3 & 1 & -2 \\ 1 & 2 & 3 \end{vmatrix}$$
  4. Evaluate the cross product

    - \hat j\big(3\cdot 3 - (-2)\cdot 1\big) + \hat k\big(3\cdot 2 - 1\cdot 1\big)$$ $$\vec \tau = \hat i(3+4) - \hat j(9+2) + \hat k(6-1)$$ $$\vec \tau = 7\hat i - 11\hat j + 5\hat k$$
  5. Magnitude of torque

    ∣τ⃗∣=72+(−11)2+52|\vec \tau| = \sqrt{7^2 + (-11)^2 + 5^2}∣τ∣=72+(−11)2+52​ =49+121+25= \sqrt{49 + 121 + 25}=49+121+25​ =195= \sqrt{195}=195​

    Comparing with x\sqrt{x}x​, x=195x = 195x=195

  6. Comparison with stored answer

    Stored correct answer = 195195195

    Our derived answer also is 195195195.

Final Answer: 195\boxed{195}195​

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