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Rotational Motion question

2025 · 4 Apr · Shift 1 · Q71
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Rotational Motion question

2025 · 4 Apr · Shift 1 · Q71

JEE MainPhysicsRotational MotionNumerical+4 / −1
A circular ring and a solid sphere having same radius roll down on an inclined plane from rest without slipping. The ratio of their velocities when reached at the bottom of the plane is x5\sqrt{\frac{x}{5}}5x​​ where x=‾x=\underline{\hspace{2cm}}x=​.
Numerical answer
View written solutionFree

Correct answer: 4OR3.5

  1. Use energy conservation for rolling without slipping

For a body rolling down an incline from height hhh without slipping:

mgh=12mv2+12Iω2mgh = \frac{1}{2}mv^2 + \frac{1}{2}I\omega^2mgh=21​mv2+21​Iω2

Since rolling without slipping,

ω=vR\omega = \frac{v}{R}ω=Rv​

So,

mgh=12mv2+12Iv2R2mgh = \frac{1}{2}mv^2 + \frac{1}{2}I\frac{v^2}{R^2}mgh=21​mv2+21​IR2v2​

mgh=12v2(m+IR2)mgh = \frac{1}{2}v^2\left(m + \frac{I}{R^2}\right)mgh=21​v2(m+R2I​)

Hence,

v2=2gh1+ImR2v^2 = \frac{2gh}{1+\frac{I}{mR^2}}v2=1+mR2I​2gh​


  1. Velocity of the circular ring

For a ring,

Iring=mR2I_{\text{ring}} = mR^2Iring​=mR2

Thus,

vring2=2gh1+1=ghv_{\text{ring}}^2 = \frac{2gh}{1+1} = ghvring2​=1+12gh​=gh

So,

vring=ghv_{\text{ring}} = \sqrt{gh}vring​=gh​


  1. Velocity of the solid sphere

For a solid sphere,

Isphere=25mR2I_{\text{sphere}} = \frac{2}{5}mR^2Isphere​=52​mR2

Thus,

vsphere2=2gh1+25=2gh75=10gh7v_{\text{sphere}}^2 = \frac{2gh}{1+\frac{2}{5}} = \frac{2gh}{\frac{7}{5}} = \frac{10gh}{7}vsphere2​=1+52​2gh​=57​2gh​=710gh​

So,

vsphere=10gh7v_{\text{sphere}} = \sqrt{\frac{10gh}{7}}vsphere​=710gh​​


  1. Find the ratio

Taking the ratio of ring to sphere velocities:

vringvsphere=gh10gh/7=710\frac{v_{\text{ring}}}{v_{\text{sphere}}} = \sqrt{\frac{gh}{10gh/7}} = \sqrt{\frac{7}{10}}vsphere​vring​​=10gh/7gh​​=107​​

Given,

x5=710\sqrt{\frac{x}{5}} = \sqrt{\frac{7}{10}}5x​​=107​​

Squaring both sides:

x5=710\frac{x}{5} = \frac{7}{10}5x​=107​

x=72=3.5x = \frac{7}{2} = 3.5x=27​=3.5


  1. Final answer

x=72\boxed{x = \frac{7}{2}}x=27​​

If integer format is enforced, the problem statement is inconsistent, because the correct value is 3.53.53.5.


  1. Comparison with stored answer

Stored correct answer: 4 OR 3.54 \text{ OR } 3.54 OR 3.5

Our derived answer is 3.53.53.5, which matches one of the accepted stored answers.

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