Figure 1. The vacuum “lift”
This is a photo of a science fair project that sought to test how much weight a canister vacuum cleaner could ”lift.” It started with a demonstration that can be done with any vacuum cleaner: we tried picking up balls of various sizes. No ordinary shop vac with a normal-sized hose can lift a bowling ball, but with a simple modification, any vacuum cleaner can easily lift a bowling ball. The key to the whole experiment is surface area.
Before getting into the details, I should point out there really is no such thing as suction as most people (myself included) think of it. The common thought is that vacuums exert a force that pulls objects into them. Hence the terms “lift” “suck” and “pick up” which are often used to describe vacuuming. That is not the case. In actuality, objects entering the hose of a vacuum cleaner are being pushed into it.
Most people recognize that vacuum cleaners work by blowing air out of the canister, which creates a vacuum inside which in turn, “draws” air in on the suction side (figure 2). However, many people don’t realize that the low-pressure vacuum inside the canister isn’t actually pulling air into the vac. Instead air near the inlet hose is getting pushed in by the surrounding atmosphere because there is greater pressure outside the canister than in.
Figure 2. How a canister vacuum works.
Another way to think about it is to consider what happens to a dust bunny when it is vacuumed.
The figure below shows how my mind thinks a vacuum cleaner works: that is, when the hose is placed above a dust bunny, the air rushing into the hose pulls the bunny into the hose. My mind wants to think of suction as a pulling force.
Figure 3. Wrong concept of suction (it is not a pulling force)
However, what is actually happening when a dust bunny gets “sucked” into or “picked up by” a hose. The object isn’t pulled in at all. Instead, atmospheric air is pushing the object into the hose from underneath.
Figure 4. Correct concept of suction (a pushing force):
For practical purposes, it may not matter to most people whether the dust bunny is pushed or pulled, only that it comes out from under the end table. However knowing that it is the air pushing objects toward the hose takes on a new level of impressiveness when one observes air holding up a 409 pound box!
This is how the original experiment went: there was a collection of balls of different mass, each having a diameter larger than the mouth of the hose, that I would ask the students which ones they think a vacuum cleaner could “lift” They all make their votes, then we try them one at a time. With the vacuum running, the hose is lowered down onto a ball until it forms a seal, then I attempt to rase the hose with the ball and hose in tact.
I use a 5 gallon shop style vacuum cleaner, so it easily lifts a racket ball, baseball and softball. It even lifts a 5 pound sand-filled workout ball. But when we get to the bowling ball, the hose forms a good tight seal, but as soon as I try to lift, the seal is broken, end the hose separates from the ball. I then introduce the concept of pressure, and see if we can devise a way to lift the ball.
P=F/A
This formula states that pressure is equal to force divided by area. On earth, we have a very dense atmosphere; it exerts a force of 15 pounds per square inch. If you don’t think that is very much, try to remove a glass-carrying suction cup that has been sealed to a smooth floor. It has a surface area of about 7 inches. So when it forms a seal to a smooth surface with the air underneath removed, it takes 100 pounds of force to overcome the air pressure pushing down on it!
The vacuum cleaner experiments illustrates the force of the air pushing on an object as well. When the seal forms between the object and the hose, air pressure pushes on the object to keep the object pressed against the mouth of the hose. And it takes several pounds of force to counter act the pushing force of the atmosphere to break the seal. So if the object is heavy enough, its weight will exert enough force to break the seal.
Notice from the equation above, however that pressure has to do with both force and area. So, for a given pressure if the area is increased, so is force. So in our experiment, we secure a funnel to the end of the hose to increase the area (figure 6). In so doing, the same air pressure can “lift” more weight.
Figure 5. A funnel increases area to create a larger area of low pressure.
This is an experiment you can try at home. Just make sure the funnel is securely taped to the end of the vacuum cleaner hose, and try lifting up something heavy. Pretty impressive!
The homeschooler who performed the experiment shown in figure 1 wanted to try an even larger surface area. So he built a disk having a diameter of 18 inches. This created an area over 80 times greater than the hose, and therefore a force over 80 times greater. The picture (figure 1) shows the device being held PUSHED against the disk on top by air pressure. We loaded over 409 pounds in the box before the seal finally broke!
