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Center of Mass question

2020 · 7 Jan · Shift 1 · Q66
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Center of Mass question

2020 · 7 Jan · Shift 1 · Q66

JEE MainPhysicsCenter of MassMCQ+4 / −1
Three point particles of masses 1.0 kg, 1.5 kg and 2.5 kg are placed at three corners of a right angle triangle of sides 4.0 cm, 3.0 cm and 5.0 cm as shown in the figure. The center of mass of the system is at a point: JEE Main 2020 (Online) 7th January Morning Slot Physics - Center of Mass and Collision Question 83 English
  1. A
    2.0 cm right and 0.9 cm above 1 kg mass
  2. B
    0.9 cm right and 2.0 cm above 1 kg mass
  3. C
    0.6 cm right and 2.0 cm above 1 kg mass
  4. D
    1.5 cm right and 1.2 cm above 1 kg mass
View written solutionFree

Correct answer: B

  1. Set up coordinates

Take the 1.0 kg1.0\,\text{kg}1.0kg mass as the origin.

From the usual right triangle arrangement shown:

  • 1.0 kg1.0\,\text{kg}1.0kg at (0,0)(0,0)(0,0)
  • 1.5 kg1.5\,\text{kg}1.5kg at (4,0)(4,0)(4,0) cm
  • 2.5 kg2.5\,\text{kg}2.5kg at (0,3)(0,3)(0,3) cm

This matches the 333-444-555 right triangle.

Total mass: M=1.0+1.5+2.5=5.0 kgM = 1.0+1.5+2.5 = 5.0\,\text{kg}M=1.0+1.5+2.5=5.0kg

  1. Use center of mass formula

The coordinates of center of mass are xcm=∑mixi∑mi,ycm=∑miyi∑mix_{\text{cm}} = \frac{\sum m_i x_i}{\sum m_i}, \qquad y_{\text{cm}} = \frac{\sum m_i y_i}{\sum m_i}xcm​=∑mi​∑mi​xi​​,ycm​=∑mi​∑mi​yi​​

  1. Compute xxx-coordinate

xcm=(1.0)(0)+(1.5)(4)+(2.5)(0)5.0x_{\text{cm}} = \frac{(1.0)(0) + (1.5)(4) + (2.5)(0)}{5.0}xcm​=5.0(1.0)(0)+(1.5)(4)+(2.5)(0)​ xcm=65=1.2 cmx_{\text{cm}} = \frac{6}{5} = 1.2\,\text{cm}xcm​=56​=1.2cm

  1. Compute yyy-coordinate

ycm=(1.0)(0)+(1.5)(0)+(2.5)(3)5.0y_{\text{cm}} = \frac{(1.0)(0) + (1.5)(0) + (2.5)(3)}{5.0}ycm​=5.0(1.0)(0)+(1.5)(0)+(2.5)(3)​ ycm=7.55=1.5 cmy_{\text{cm}} = \frac{7.5}{5} = 1.5\,\text{cm}ycm​=57.5​=1.5cm

  1. Interpretation

So the center of mass is at 1.2 cm right and 1.5 cm above the 1 kg mass\boxed{1.2\,\text{cm right and }1.5\,\text{cm above the }1\,\text{kg mass}}1.2cm right and 1.5cm above the 1kg mass​

  1. Compare with options

Given options:

  • A: (2.0,0.9)(2.0, 0.9)(2.0,0.9)
  • B: (0.9,2.0)(0.9, 2.0)(0.9,2.0)
  • C: (0.6,2.0)(0.6, 2.0)(0.6,2.0)
  • D: (1.5,1.2)(1.5, 1.2)(1.5,1.2)

None matches exactly. The closest is option D if coordinates were interchanged approximately, but it is still not correct.

Hence, based on the standard interpretation of the figure, none of the listed options is correct.

If the masses at the other two vertices were swapped:

  • 2.5 kg2.5\,\text{kg}2.5kg at (4,0)(4,0)(4,0)
  • 1.5 kg1.5\,\text{kg}1.5kg at (0,3)(0,3)(0,3)

then xcm=(2.5)(4)5=2.0 cm,ycm=(1.5)(3)5=0.9 cmx_{\text{cm}} = \frac{(2.5)(4)}{5} = 2.0\,\text{cm}, \qquad y_{\text{cm}} = \frac{(1.5)(3)}{5} = 0.9\,\text{cm}xcm​=5(2.5)(4)​=2.0cm,ycm​=5(1.5)(3)​=0.9cm which gives option A.

So the stored answer BBB does not agree with the physics calculation under either natural placement.

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