2010-07-18, 07:17 PM
As you said, there's a pseudo-vacuum; it's not a true vacuum. You sealed the needle but you did not seal the plunger, so air can still pass into and out of the syringe from the plunger end. When you draw back the plunger, you increase the volume of the syringe. Ergo, one or both of the following events must take place:
1. The air has to rush in to fill that volume. The only location at which it can do this is through the plunger end.
2. The volume has to restore itself to the surrounding air pressure, since you lowered the air pressure by drawing back the plunger.
The volume of the can't restore itself instantaneously, so before it does, air rushes into the syringe. This produces a force that causes your syringe to fly in the observed direction.
Off topic: My doctor gave me penicillin instead of a syringe.
1. The air has to rush in to fill that volume. The only location at which it can do this is through the plunger end.
2. The volume has to restore itself to the surrounding air pressure, since you lowered the air pressure by drawing back the plunger.
The volume of the can't restore itself instantaneously, so before it does, air rushes into the syringe. This produces a force that causes your syringe to fly in the observed direction.
Off topic: My doctor gave me penicillin instead of a syringe.