1. dotphysics
  2. Pressure demo: suction

Pressure demo: suction

By rallain on March 24, 2010.

How does a suction cup work? It is all about the atmosphere. Here is a demo. Take some type of "suction cup" device. In this case, I used a toy dart. Stick it to something smooth and lift it up. Like this:

i-0d7b27f1b5e150a31e3ea078281ea92b-2010-03-24_vid00863mp_4.jpg

What lifts up the metal block? The atmosphere. Diagram time:

i-e1f6e84c8863330e3b099ffe5b7525e7-2010-03-24_untitled_5.jpg

But this isn't a very realistic diagram. Actually, the suction cup would be pushing down on the block because the force from the atmosphere would be too large to balance with the weight. Let me put some numbers in here. Suppose this is an aluminum block - I just going to pretend it is 4cm on a side (and a cube). In this case, the weight would be (the density of aluminum is 2700 kg/m3):

i-be07cbff8230f539eb705cc46a79afd1-2010-03-24_la_te_xi_t_1_8.jpg

How does this weight compare to the force the atmosphere pushes up on the block? (I am assuming the suction cup on the top covers the whole block). The force will be the pressure times the area of the bottom and the atmospheric pressure is about 105 N/m2. This gives:

i-7c8db74e5d903aa1d42993c29ef0247d-2010-03-24_la_te_xi_t_1_9.jpg

What would happen if I remove the outside air? Can you do this? I can not remove it all, but I can remove enough to make a difference. Here is a video of just that. I put the dart with the block in a jar that I can pump the air out of.

Tags
buoyancy
demo
density
forces
free body diagram
pressure
Video
atmosphere
equilibrium
Physics
suction
vacuum
demo

More like this

Sad Balloon Physics II: How Long Will a Balloon Last?
As a follow up to Wednesday's sad balloon post, the repair that lofted it back to the ceiling was a temporary reprieve, unsurprisingly. After 24 hours, more or less, it had sunk back down to the point where the ribbon was just barely touching the floor. On the one hand, it looks kind of pathetic…
The idealised greenhouse effect model and its enemies
This post comes about as an attempt to write down, slowly and carefully, a simple version of the "idealised GHE model". This apparently simple concept causes lots of confusion, though mostly amongst people who want to believe there are fatal flaws at the heart of climate science. Before I go on:…
A Rock the Size of Jupiter?
For some reason, the topic of really big rocks came up at dinner the other night, and SteelyKid declared that she wanted to find "A rock as big as the solar system." We pointed out that that was pretty much impossible, more or less by definition, rocks being sub-parts of the solar system. "OK, how…
Quantum Viruses?
The Twitter conversation that prompted yesterday's post about composite objects was apparently prompted by a comment somebody made about how a virus left alone would see its quantum wavefunction spread out on a time scale of minutes. This led to wondering about whether a virus could really be…

That's a cool demonstration. Kudos to whoever thought of proving it was the vacuum rather than adhesion.

Let me put some numbers in here. Suppose this is an aluminum block - I just going to pretend it is 4cm on a side (and a cube). In this case, the weight would be (the density of aluminum is 2700 kg/m3):

I have a GPS suction cup that I placed on my kitchen counter top without the GPS. It is not being used. I just place the suction part on the counter for a day intending to pack it away. Every few hours the suction cup falls to the floor by itself. How does it fall from the counter top where it is just resting with the suction part either facing up or to its side. Thanks

By Iram Imran (not verified) on 25 Mar 2012 #permalink