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Concepts Of Physics MCQ Edition [Volume 1]
Exam
Concepts Of Physics MCQ Edition [Volume 1]
Subject
Physics
Chapter
Circular Motion

Concepts Of Physics MCQ Edition [Volume 1] · Physics · Circular Motion

A simple pendulum of length 1 m has a bob of mass 100 g . If the...

A simple pendulum of length 1 m1 \text{ m} has a bob of mass 100 g100 \text{ g}. If the bob has a speed of 1.4 m/s1.4 \text{ m/s} when the string makes an angle of 0.200.20 radian with the vertical, determine the tension in the string at this instant. You may use the approximations cos⁡θ≈1−θ2/2\cos\theta \approx 1 - \theta^2/2 and sin⁡θ≈θ\sin\theta \approx \theta for small θ\theta.

Options

  1. A0.96 N0.96 \text{ N}
  2. B0.76 N0.76 \text{ N}
  3. C1.16 N1.16 \text{ N}
  4. D1.18 N1.18 \text{ N}

Correct answer

C. 1.16 N1.16 \text{ N}

Step-by-step solution

Given:
Length of the pendulum, l=1 ml = 1 \text{ m}
Mass of the bob, m=100 g=0.1 kgm = 100 \text{ g} = 0.1 \text{ kg}
Speed of the bob, v=1.4 m/sv = 1.4 \text{ m/s}
Angle with the vertical, θ=0.20 rad\theta = 0.20 \text{ rad}
Acceleration due to gravity, g=9.8 m/s2g = 9.8 \text{ m/s}^2

Using the small angle approximation for cos⁡θ\cos\theta:
cos⁡θ≈1−θ22=1−(0.20)22=1−0.02=0.98\cos\theta \approx 1 - \dfrac{\theta^2}{2} = 1 - \dfrac{(0.20)^2}{2} = 1 - 0.02 = 0.98

The forces acting on the bob along the radial direction are the tension TT towards the pivot and the component of weight mgcos⁡θmg \cos\theta away from the pivot. The net force provides the necessary centripetal acceleration:
T−mgcos⁡θ=mv2lT - mg \cos\theta = \dfrac{mv^2}{l}

Rearranging for tension TT:
T=mgcos⁡θ+mv2lT = mg \cos\theta + \dfrac{mv^2}{l}

Substituting the values into the equation:
T=(0.1×9.8×0.98)+0.1×(1.4)21T = (0.1 \times 9.8 \times 0.98) + \dfrac{0.1 \times (1.4)^2}{1}

T=0.9604+0.196T = 0.9604 + 0.196

T=1.1564 NT = 1.1564 \text{ N}

Rounding off to two decimal places, we get:
T≈1.16 NT \approx 1.16 \text{ N}

Answer: 1.16 N1.16 \text{ N}
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