# Pressure demo: suction

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:

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

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):

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:

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

### More like this

##### Vacuum Spider-Man
Check out this thing. That is where the guy (Jem Stansfield of BBC's Bang Goes the Theory) shows how he built this thing. Here is part 2 where he uses it to climb a building. Here are some questions: Why does it not matter how powerful the vacuum is? How does a vacuum cleaner work anyway? How does…
##### An example of two collisions
Suppose you want to move an empty paper clip box by shooting it with a toy dart gun. Why would you want to do this? Don't worry about that - this is my example and I am sticking with it. Should you shoot a dart that sticks to the box or should you shoot one that bounces off? I made a video of…
##### More Dangerous Kids
My car had a flat tire. When you get a flat tire, you might as well make something useful of it - right? As I was jacking the car up, I had a great idea. Use this for one of my "Spoof Science" videos. The only problem is that this takes a ton of work to put together a short video. So, I am…
##### What is Arnold made of?
Of course I am talking about Arnold Schwarzenegger. After looking at how many bullets he carries in Commando, I remembered this scene (also from Commando) (warning: maybe some not great language and some killing. You have been warned) If you don't want to watch that clip, here is a shot (sorry…

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