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Rotational Motion question

2021 · 16 Mar · Shift 1 · Q71
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  5. /2021 · 16 Mar · Shift 1 · Q71

Rotational Motion question

2021 · 16 Mar · Shift 1 · Q71

JEE MainPhysicsRotational MotionNumerical+4 / −1
Consider a frame that is made up of two thin massless rods AB and AC as shown in the figure. A vertical force P→\overrightarrow PP of magnitude 100 N is applied at point A of the frame. JEE Main 2021 (Online) 16th March Morning Shift Physics - Rotational Motion Question 124 English Suppose the force is P→\overrightarrow PP resolved parallel to the arms AB and AC of the frame. The magnitude of the resolved component along the arm AC is xN. The value of x, to the nearest integer, is ‾\underline{\hspace{2cm}}​. [Given : sin(35 ∘^\circ∘) = 0.573, cos(35 ∘^\circ∘) = 0.819 sin(110 ∘^\circ∘) = 0.939, cos(110 ∘^\circ∘) = −-− 0.342 J
Numerical answer
View written solutionFree

Correct answer: 82

  1. Interpret the geometry

    The force P⃗\vec PP of magnitude 100 N100\,\text{N}100N acts vertically downward at point AAA.

    It is to be resolved along the two non-perpendicular arms ABABAB and ACACAC.

    From the given trigonometric values, the included geometry is consistent with:

    • arm ABABAB making 35∘35^\circ35∘ with the vertical,
    • arm ACACAC making 35∘35^\circ35∘ with the horizontal on the other side, so the angle between the two arms is ∠BAC=110∘.\angle BAC = 110^\circ.∠BAC=110∘.

    Hence, when resolving a force along two non-orthogonal directions, we use the triangle/law of sines method.

  2. Let the resolved components be

    • along ABABAB: FABF_{AB}FAB​
    • along ACACAC: FAC=xF_{AC}=xFAC​=x

    Their vector sum equals the applied force P=100 NP=100\,\text{N}P=100N.

  3. Use the law of sines for resolution along two inclined directions

    If a force PPP is resolved along two directions with included angle 110∘110^\circ110∘, and the angle between PPP and one arm is 35∘35^\circ35∘, then the angle between PPP and the other arm is also 180∘−110∘−35∘=35∘.180^\circ - 110^\circ - 35^\circ = 35^\circ.180∘−110∘−35∘=35∘.

    For oblique resolution: FACsin⁡35∘=Psin⁡110∘\frac{F_{AC}}{\sin 35^\circ} = \frac{P}{\sin 110^\circ}sin35∘FAC​​=sin110∘P​

    Therefore, FAC=P sin⁡35∘sin⁡110∘.F_{AC} = P\,\frac{\sin 35^\circ}{\sin 110^\circ}.FAC​=Psin110∘sin35∘​.

  4. Substitute the given values

    FAC=100×0.5730.939F_{AC} = 100\times \frac{0.573}{0.939}FAC​=100×0.9390.573​

    FAC≈100×0.6102=61.02 NF_{AC} \approx 100\times 0.6102 = 61.02\,\text{N}FAC​≈100×0.6102=61.02N

    So, x≈61 N.x \approx 61\,\text{N}.x≈61N.

  5. Nearest integer

    x=61\boxed{x=61}x=61​

  6. Compare with stored answer

    Stored correct answer = 828282.

    My derived answer is 616161, which does not match 828282.

    The likely reason is that the actual figure may have a different placement of the 35∘35^\circ35∘ angle than inferred from the text alone. If instead the component along ACACAC is opposite the 35∘35^\circ35∘ angle in the force triangle, one would get x=100sin⁡110∘sin⁡(other angle),x = 100\frac{\sin 110^\circ}{\sin(\text{other angle})},x=100sin(other angle)sin110∘​, which can produce a value near 828282 depending on the exact figure. However, from the standard oblique resolution with the most natural interpretation of the supplied angles, the result is 61 N61\,\text{N}61N.

    Therefore I disagree with the stored answer based on the information visible in the prompt alone.

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