Question

Two light balls are suspended with light strings very close to each other. What happens when we blow air between the balls?

Answer 1

Concept used:

1. Pressure and velocity both are the macroscopic parameters governing plenty of natural phenomena. Pressure is the measure of force per unit area. Velocity is the measure of the rate of change of displacement.
2. In thermodynamics, for any in-compressible, non-viscous fluid, the relation between pressure and velocity is given by Bernoulli’s equation.

$P+\frac{1}{2}\rho v^2+\rho gh=\mathrm{Constant}$

Where, $P$ is the pressure of the in-compressible, non-viscous fluid, $\rho$ is the density of the in-compressible, non-viscous fluid, $v$ is the velocity of the in-compressible, non-viscous fluid, $g$ is the acceleration due to gravity, and, $h$ is the vertical height of the pipe from the reference level.

Explanation:

1. Pressure and velocity are inversely proportional to each other. If pressure increases, the velocity decreases to keep the algebraic sum of potential energy, kinetic energy, and pressure constant.
2. Similarly, if velocity increases, the pressure decreases to keep the sum of potential energy, kinetic energy, and pressure constant.
3. When we blow air between two balls, suspended with light strings, and the velocity($v$) of flow increases, the pressure($P$) decreases hence the lower pressure created attracts the balls towards each other as the pressure($P$) created is much lower than the standard atmospheric pressure, which is in accordance with the Bernoulli’s theorem.