SUCK

Too many teachers of science talk about milk being sucked up a straw and/or passengers being sucked out of an aeroplane. Unfortunately this generates a false picture in the minds of their students.

The dictionary indicates that "suck" means to draw into the mouth. The milk is in fact forced into the mouth in accordance with Newton's first law of motion. Again, the passengers are forced out of the plane in accordance with the first law.

Pressure is defined as Force divided by Area. P = F/A. If the area is fixed then Pressure will vary directly with Force.

You place your lips over the straw. The pressure of the air inside the straw above the milk is balanced by the pressure of the air above the milk outside of the straw. Hence, the force per unit area is the same. You consciously lower your diaphragm and your lung capacity increases. A greater volume with the same number of air particles must result in a drop in pressure. Hence, the pressure of the air inside the straw drops which means that the force per unit area drops. We now have a greater force per unit area acting on the surface of the milk, outside of the straw, and hence milk is forced into the straw.

You are sitting in an aeroplane, at sixty thousand feet, next to a window. The air pressure acting on the inside of the window is far greater than that on the outside. Hence, the force per unit area is greater inside. The window shatters and in accordance with Newton's first law the pieces of glass move out and away from the plane. In general the net movement of air particles will be towards the hole where the window used to be. The air pressure on the side of your body closest to the window will drop very quickly and hence so will the force per unit area acting on that part of your body. The greater air pressure and force per unit area on the other side of your body will cause you to move in accordance with Newton's first law. You are forced out of the plane. In simple terms the air molecules move into regions of low pressure and if you are in the way then bad luck!