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Circular Motion question

2022 · 25 Jun · Shift 2 · Q48
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  5. /2022 · 25 Jun · Shift 2 · Q48

Circular Motion question

2022 · 25 Jun · Shift 2 · Q48

JEE MainPhysicsCircular MotionMCQ+4 / −1
A disc with a flat small bottom beaker placed on it at a distance R from its center is revolving about an axis passing through the center and perpendicular to its plane with an angular velocity ω\omegaω. The coefficient of static friction between the bottom of the beaker and the surface of the disc is μ\muμ. The beaker will revolve with the disc if :
  1. A
    R≤μg2ω2R \le {{\mu g} \over {2{\omega ^2}}}R≤2ω2μg​
  2. B
    R≤μgω2R \le {{\mu g} \over {{\omega ^2}}}R≤ω2μg​
  3. C
    R≥μg2ω2R \ge {{\mu g} \over {2{\omega ^2}}}R≥2ω2μg​
  4. D
    R≥μgω2R \ge {{\mu g} \over {{\omega ^2}}}R≥ω2μg​
View written solutionFree

Correct answer: B

  1. Identify the force needed for circular motion

For the beaker to rotate with the disc without slipping, it must have centripetal acceleration toward the center:

ac=ω2Ra_c = \omega^2 Rac​=ω2R

So the required centripetal force is

Fc=mω2RF_c = m\omega^2 RFc​=mω2R

  1. What provides this centripetal force?

The only horizontal force available is static friction between the beaker and the disc.

Maximum static friction is

fmax⁡=μNf_{\max} = \mu Nfmax​=μN

Since the disc is horizontal,

N=mgN = mgN=mg

Hence,

fmax⁡=μmgf_{\max} = \mu mgfmax​=μmg

  1. Condition for no slipping

For the beaker to revolve with the disc, the required centripetal force must not exceed maximum static friction:

mω2R≤μmgm\omega^2 R \le \mu mgmω2R≤μmg

Cancel mmm:

ω2R≤μg\omega^2 R \le \mu gω2R≤μg

Therefore,

R≤μgω2R \le \frac{\mu g}{\omega^2}R≤ω2μg​

  1. Match with the options

This corresponds to:

Option B:

R≤μgω2R \le \frac{\mu g}{\omega^2}R≤ω2μg​

  1. Comparison with stored correct answer

Stored correct answer is B, which matches the derived result.

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