Small phenomenon, the truth--analysis of pressure and fluid

(This article was first published on September 22, 2012 in the QQ space into my room and find the title "small phenomenon, the sermon--Analysis of pressure and fluid (my original)")

        http://v.youku.com/v_show/id_xnduyodu1ntm2.html

The simple truth is that for a stationary object, no matter how much force it exerts on the ground, it will not fly, and some may say why the basketball falls on the ground, then I ask you why a lump of mud falls on the ground, which involves the conversion of kinetic energy, elastic potential energy and internal energies. The object can be bounced first need to be elastic, in the collision, elasticity can convert kinetic energy into elastic potential energy, and then the elastic potential energy into kinetic energy, that is, rebound, and for the non-elastic object, the impact of the kinetic energy can only be converted to internal energy (heat).

There is a special case: if a stationary flat ball is placed on the ground, a small amount of liquid nitrogen is injected into it, and then suddenly the swelling volume increases, which can be bounced up, which can be explained in two reasonable ways:

1, the ball and liquid nitrogen can be regarded as a system, liquid nitrogen expansion of the moment produced in the direction of the pressure, the force belongs to the internal force of the system, which points to the surface of this part of the pressure is much larger than the sphere and the internal nitrogen gravity, and at this time the reaction to the ball is greater than the gravity of the sphere system To produce an acceleration that bounces;

2, the more detailed analysis of the more scientific but complex explanation: the expansion of liquid nitrogen also takes time, the ball's bounce also takes time, in this process, we analyze the ball at the bottom of the ground contact with the position of the force situation: the bottom of the ball by its own gravity pressure, liquid nitrogen pressure and the ground pressure, before the ball left the ground , the bottom of the ball remains static, indicating that the ball bottom by the combined force is 0, and when the ball off the ground, the ball bottom is more unaffected by the pressure of the ground, so it can be ruled out is the ground pressure to the ball produced by the acceleration to make its movement, of course, the ground pressure does have a role, is to let the ball The acceleration of the left and right direction can cancel each other out, the upward acceleration makes the upper part of the ball accelerate upward movement, because inertia will drive the whole sphere upward movement away from the ground, that is, bouncing up.
And for the bucket in the video, because the ball and barrel are finally separated, so not as a whole system, so ruled out the first interpretation method. Then in the second interpretation of the analysis: the bottom of the bucket is still under the effect of balance (including the pressure of nitrogen on it and the pressure on the ground), so the bottom of the barrel does not upward acceleration, will not make the barrel upward movement; then do the pressure of nitrogen on the barrel wall? However, according to the slope analysis of the barrel wall, the pressure of nitrogen on the barrel wall is downward in the vertical direction, and the barrel will not move upward; that's what someone said, it's only possible that table tennis has a friction force on the barrel wall, but the friction of ping pong is so big that it makes the barrel bounce so high? The only explanation: fluid and air pressure.
With regard to fluid and pressure, the higher the flow rate, the smaller the pressure. Simply put, when the ball spurt out of the instant, the area above the barrel of air flow is very large, the pressure is very small, and the barrel around and the bottom are subject to a strong atmosphere, the barrel wall pressure = atmospheric pressure x bucket surface area, barrel bottom is the same, to the barrel wall and the bottom of the pressure (due to the wall of the bucket The acceleration causes the barrel to move upward.
Then calculate the approximate pressure: a standard atmospheric pressure: 100000n/m2, the bucket bottom area is probably 0.4m2, the barrel probably receives the upward pressure to be about 40000N, namely 4000kg, what? 4 tons? Yes, 4 tons. Don't underestimate the strength of the atmosphere. In fact, the barrel wall and bottom of the barrel is also under the pressure, the normal state is also 4 tons, but at this time in the barrel and above the air in the rapid flow, pressure is small, so the pressure is relatively small, so the barrel by upward of the external force about 2 tons, fully capable in a moment to play so high. When the air flow rate on the barrel is reduced, the pressure difference decreases, and the barrel naturally falls off.

Small phenomenon, the truth--analysis of pressure and fluid