Sunday, 30 January 2011
The floating ball
What keeps the ball in the air?
Hold the ball in the air current and let go of it.
The ball floats in the air current while swaying only slightly back and forth.
In accordance with Bernoulli’s principle, the sum of the dynamic and static pressures of a flowing gas is constant. The greater the speed of the flow, the more dynamic pressure there is at work in the direction of the current and the less static pressure there is sideways. The dynamic pressure of the air current is always larger than that of the surrounding stationary air, and, correspondingly, the static pressure is smaller than that of the surrounding stationary air. In an air current that blows directly upwards, a ball will rise to the height at which the buoyancy created by the dynamic pressure and the weight of the ball are equal. The ball will remain in the air current, because the greater static pressure of the stationary air outside of the current pushes the ball back as it tries to escape the current. A situation in which the air current blows at an angle is more complicated. The fact that the ball does not fall is due to the combined effect of the dynamic and static pressures. The counter force exerted by the ball on the air also makes the air current turn slightly downwards.
Large birds, such as cranes, are especially talented in utilising the dynamic and static pressures of rising air currents to gain height in flight. They do not actually need to do anything other than hold out their wings. The dynamic pressure of the current lifts the birds up and the static pressure holds them automatically circling inside the current.