JAMB Physics 2012 Paper

Concept:Upthrust is the buoyant force exerted by air on the balloon and equals the weight of the air displaced by the balloon.Explanation:The upthrust is given by the formula $F = \rho_{\text{air}} V g$, where $\rho_{\text{air}}$ is the density of air, $V$ is the volume of the balloon, and $g$ is the acceleration due to gravity.From the question, $\rho_{\text{air}} = 1.3 \, \text{kg m}^{-3}$, $V = 300 \, \text{m}^{3}$, and $g = 10 \, \text{m s}^{-2}$.The density of hydrogen is not needed because upthrust depends only on the displaced air, not on the gas inside the balloon.Substitute the values into the formula:$F = 1.3 \times 300 \times 10 = 3900 \, \text{N}$.Thus, the upthrust on the balloon is $3900 \, \text{N}$.Answer:C. $3900 \, \text{N}$