Question

Take a syringe and draw out its piston to the limit. Close the nozzle of the syringe with your finger and try to push the piston
A. Are you able to push the piston in a forward direction?
B. How do you feel at the finger?
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Answer 1

Answer:

1..    It is all about pressure. When you plug upon the nozzle, there is no way for the pressure of the fluid to rise above a certain value, so pushing in is opposed by the fluid pushing back on your finger, it gets hard! However, if you open the valve and allow fluid to squeeze out, then the pressure can be relieved a bit and that allows you to keep the piston moving down. That is applying an unbalanced force that allows the fluid to accelerate from zero velocity inside the barrel to some higher velocity through the nozzle.

2..       When you put your finger over the nozzle, you prevent any air from entering or leaving the syringe. Because the air cannot escape from inside the syringe, when you then try to push in the plunger, the air inside the plunger is compressed into a smaller volume. This creates a higher pressure inside the syringe. This higher pressure pushes outwards against the plunger, which is why it becomes much harder to push the plunger further into the syringe.

hope i help you

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