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Physics

The bob of a simple pendulum is imparted a velocity of 5 m s-1 when it is at its mean position. To what maximum vertical height will it rise on reaching at its extreme position if 60% of its energy is lost in overcoming the friction of air?

(Take g = 10 m s-2).

Work, Energy & Power

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Answer

Given,

Energy lost = 0.6 x KE

KE = 12\dfrac{1}{2} x mass x velocity2

Substituting the values we get,

KE=12×mass×52KE = \dfrac{1}{2} \times mass \times 5^2 \\[0.5em]

Amount of energy lost,

Energy lost=12×mass×52×0.6\text{Energy lost} = \dfrac{1}{2} \times mass \times 5^2 \times 0.6 \\[0.5em]

Amount of energy available,

Energy available=12×mass×52×0.4\text{Energy available} = \dfrac{1}{2} \times mass \times 5^2 \times 0.4\\[0.5em]

Applying the rule for the conservation of energy we get,

Kinetic energy available = potential energy

12×mass×52×0.4=mass×gravity×height12×5×5×0.4=10×height12×5×2=10×height5=10×heightheight=0.5m\dfrac{1}{2} \times mass \times 5^2 \times 0.4 = mass \times gravity \times height\\[0.5em] \Rightarrow \dfrac{1}{2} \times 5 \times 5 \times 0.4 = 10 \times height\\[0.5em] \Rightarrow \dfrac{1}{2} \times 5 \times 2 = 10 \times height\\[0.5em] \Rightarrow 5 = 10 \times height\\[0.5em] \Rightarrow height = 0.5m \\[0.5em]

∴ Maximum vertical height reached = 0.5m

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