volume_mute
Net Force Action on Atmospheric Objects
publish date: 2026/05/20 22:17:30.505784 UTC
volume_mute
A rigid fiberglass sphere filled with standard air weighs exactly \(12\text{ lb}\) in a vacuum. When it is placed into the outside atmosphere, it displaces a volume of air that weighs \(2\text{ lb}\). What is the net vertical force acting on the sphere, and in which direction?
Correct Answer
10 lb downward
Explanation
The downward gravitational force (true weight) is \(12\text{ lb}\), and the upward buoyant force is equal to the weight of the displaced air, which is \(2\text{ lb}\). The net force is the difference between them:
\[ F_{\text{net}} = 12\text{ lb (down)} - 2\text{ lb (up)} = 10\text{ lb downward} \]
Reference
Basic Physics: A Self-Teaching Guide