Question

Balloons are often filled with helium gas because it weighs only about one-seventh of what air weighs under identical conditions. The buoyancy force, which can be expressed as Fb=ρairg Vballoon F_b=\rho_{\text {airg }} V_{\text {balloon }}, will push the balloon upward. If the balloon has a diameter of 12 m12 \mathrm{~m} and carries two people, 70 kg70 \mathrm{~kg} each, find the acceleration of the balloon when it is first released. Assume the density of air is ρ=1.16 kg/m3\rho=1.16 \mathrm{~kg} / \mathrm{m}^3, and neglect the weight of the ropes and the cage.

Solution

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Answered 2 years ago
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First we need to find the volume of a balloon that in shape of a ball.

V=4πr33V=4π633V=904.32 m3\begin{align*} &V = \frac{4 \cdot \pi \cdot r^3}{3}\\ &V = \frac{4 \cdot \pi \cdot 6^3}{3}\\ &V = 904.32 \text{ m}^3\\ \end{align*}

The mass of the balloon is defined by the density of helium ρh\rho_h and the volume of the balloon (helium is 1/7 lighter than air).

ρh=mhVmh=ρhVmh=171.16904.32mh=149.85 kg\begin{align*} &\rho_h = \frac{m_h}{V}\\ &m_h = \rho_h \cdot V\\ &m_h = \frac{1}{7} \cdot 1.16 \cdot 904.32\\ &m_h = 149.85 \text{ kg}\\ \end{align*}

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