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Rotational Motion question

2020 · 9 Jan · Shift 1 · Q45
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Rotational Motion question

2020 · 9 Jan · Shift 1 · Q45

JEE MainPhysicsRotational MotionNumerical+4 / −1
A body of mass m = 10 kg is attached to one end of a wire of length 0.3 m. The maximum angular speed (in rad s–1) with which it can be rotated about its other end in space station is : (Breaking stress of wire = 4.8 × 107 Nm–2 and area of cross-section of the wire = 10–2 cm2) is:
Numerical answer
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Correct answer: 4

  1. Given data
  • Mass of body: m=10 kgm = 10\,\text{kg}m=10kg
  • Length of wire: r=0.3 mr = 0.3\,\text{m}r=0.3m
  • Breaking stress of wire: σ=4.8×107 N m−2\sigma = 4.8 \times 10^7\,\text{N m}^{-2}σ=4.8×107N m−2
  • Area of cross-section: A=10−2 cm2A = 10^{-2}\,\text{cm}^2A=10−2cm2
  1. Convert area into SI units

Since, 1 cm2=10−4 m21\,\text{cm}^2 = 10^{-4}\,\text{m}^21cm2=10−4m2

Therefore, A=10−2×10−4=10−6 m2A = 10^{-2} \times 10^{-4} = 10^{-6}\,\text{m}^2A=10−2×10−4=10−6m2

  1. Maximum tension in the wire

Breaking stress is σ=Tmax⁡A\sigma = \frac{T_{\max}}{A}σ=ATmax​​

So, Tmax⁡=σA=(4.8×107)(10−6)=48 NT_{\max} = \sigma A = (4.8 \times 10^7)(10^{-6}) = 48\,\text{N}Tmax​=σA=(4.8×107)(10−6)=48N

  1. Condition for circular motion in space station

Since the body is rotated in a space station, effectively there is no gravity affecting the rotation. Hence the wire tension alone provides the centripetal force:

T=mω2rT = m\omega^2 rT=mω2r

At maximum angular speed, tension reaches breaking value: Tmax⁡=mωmax⁡2rT_{\max} = m\omega_{\max}^2 rTmax​=mωmax2​r

Thus, 48=10⋅ω2⋅0.348 = 10\cdot \omega^2 \cdot 0.348=10⋅ω2⋅0.3

48=3ω248 = 3\omega^248=3ω2

ω2=16\omega^2 = 16ω2=16

ω=4 rad s−1\omega = 4\,\text{rad s}^{-1}ω=4rad s−1

  1. Final answer

The maximum angular speed is: 4\boxed{4}4​

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