One of the difficulties some students have with any engineering course is to have a feel for the scale of particular forces or the scale of different parameters. This can be hard to communicate. Here is an easy demonstration to illustrate the scale of atmospheric pressure.
- Two suction cup plungers
- A chap stick (or some petroleum jelly)
- Two student volunteers
- Rub the chap stick or petroleum jelly along the rim of the two plungers
- Align the two rims and push them together pushing out as much of the air between them as possible
- Have the two student volunteers try and pull the plungers apart. This will be quite difficult if the seal is good enough and the bulk of the air is removed from between the plungers.
It is not possible to easily calculate or measure the force required to pull the plungers apart. The answer mainly depends on the air pressure and volume in the gap between the plungers before you start to pull. The easiest way to get a rough (over) estimate of the force is to assume that there is a vacuum between the plungers and that the chap stick and the plungers do not provide any significant mechanical resistance. In that case one can draw a free body diagram of one plunger and sum forces along the direction of the axis of symmetry.
∑Fx=0=Patm πD2/4 – Fstudent
Substituting in values for atmospheric pressure (14.7 psi) and the plunger diameter (5½” in my case) then each student will need to provide approximately 350 lb of force to pull the plungers apart. In reality the plungers are unlikely to be perfectly aligned and the seal will also be less than perfect so this will be an upper estimate of the required force. Even if the force actually required is a bit less than this the students will still get a sense that atmospheric pressure is quite large.
An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (firstname.lastname@example.org). I also welcome comments (through the comments section or via email) on improving the demonstrations.