I don't quite understand the physic involved in operating a plunger.

I thought that using a plunger on a clogged sink, for example, would create a difference in pressure inside the tube so that, after releasing the plunger, the "force of the air coming from this difference in pressure" would push on the things blocking the sink making them go away, eventually. I told that to a friend of mine who's more knowledgeable in physics than I am but he told me it doesn't work this way and to think more about it.

So, how does a plunger really work?

  • 2
    $\begingroup$ If you press it down, or pull it up later, you make the pressure "above the dirt" greater, or smaller, than the pressure "below the dirt", which means that there is an overall force from above, or from below (later), which has a chance to move the dirt up or down. It doesn't really matter in which way you move the dirt, the goal is to make it move. $\endgroup$ Oct 29, 2014 at 7:46
  • $\begingroup$ @LubošMotl so I was basically right, isn't it? $\endgroup$
    – Geeo
    Oct 29, 2014 at 9:23
  • $\begingroup$ I think so. But we haven't really heard what his complaint was so it's hard to say that he was wrong. $\endgroup$ Oct 29, 2014 at 10:52

3 Answers 3


The movement of the "dookey" is caused by pressure created from a snug, well fitted toilet plunger. When you press and and lift the plunger, it pushes and pulls the dookey. Rocking it back and forth until its free. So there is pressure when you push, and suction when you pull.


When you press the plunger down, it forces air into the drain and increases the atmospheric pressure on it. If the item is dislodged, the pressurized air is free to travel throughout the rest of the piping. When you then pull back up on the plunger, the vacuum created will force anything inside the tube to be forced upwards.

Boyle's law is: $$ p_1 V_1=p_2 V_2\\ 1 = \text{plunger not depressed}\\ 2 = \text{plunger depressed} $$

If the item does NOT get dislodged:
The $p_2$ (pressure after the plunger is pressed down) must increase since the volume that the air can take up decreases by forcing down the plunger. When it is pulled back up, the volume can then increase and the pressure decreases.

If the item DOES get dislodged:
The $V_2$ will increase greatly because the air is now free to move throughout the entire pipe. The $p_2$ is subsequently lowered. We now have: $$ p_3 V_3=p_4 V_4\\ 3 = \text{plunger depressed}\\ 4 = \text{plunger not depressed} $$ Now the rest state is the plunger depressed. When we decompress the plunger, we gain volume and the pressure then falls.

Does this make sense to you?


Plungers are more effective when pushing water instead of air because water doesn't compress. If you let the bell of the plunger fill up with water before thrusting, you have a "solid" column of water from the plunger to the clog. Thrusting the plunger moves column of water like it was a solid piston and applies the entire force of the thrust against the clog.

If there us only air in the plunger bell, you get less force because the change in pressure is dependent on the change in volume of the bell, which isn't all that much once the water moves in response.


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