The elementary particles accelerated in the CERN's Large Hadron Collider have many interesting behavior. It is amazing how interesting can a pencil behave which have a velocity of approximately 200 m/s. Although its velocity seems to be negligible compared to the velocity of the hadrons the collision with it can be as inconvinient as  it is in the case of the elementary particles.

Elementary “Pencil Shoot” - potato & staw

A much lower energy version of the pencil shoot (see below) can be reproduced with soda straws and potatoes. I will present the experiment in the following way: At first I show, that  it is impossible to push the straw trough the potato with a slow speed. The successful solution is, that the straw must be driven at a very high speed so that it will become able to penetrate the potato. The properties of materials are governed by state-variables. Such a state-variable is the speed of deformation. If the speed of deformation exceeds a well-defined threshold then the straw cuts through the potato unhamperedly. The idea of this experiment came from an observation of the Nature. Anybody can observe that in extremely strong wind (tornado, hurricane) small and even soft objects can cause serius damages. One potato can serve several students in a group, making this a low-cost experience.

Fastest Surface Wind Speed ever measured:
The fastest surface wind speed at a low altitude was registered on March 8, 1972 at the USAF base at Thule, Greenland, when a peak speed of 333 km/h (207 mph) was recorded. (

Real Pencil Shoot
This demonstration falls into the category of those whose details are difficult to explain but the overall effect is quite obvious. Due to the pencil’s shape and a speed of about 150 m/s, it is capable of transferring its energy due to motion (kinetic energy) in a rather amazing way. When the pencil strikes a 2.5cm piece of plywood, the plywood is easily penetrated by the pencil, with little or no visible damage to the point of the pencil. Since the plywood is much thinner than the length of the pencil, and the energy transfer is focused on a very small region of the plywood, the individual layers or strata of the plywood shatter rather than breaking the pencil. Wood is very strong under compression, and since the entire pencil travels as a unit, the forces of collision act only to compress it. The nature of the collision creates a very large and sudden shearing force on the layers of plywood, and wood grains are much easier to break under shearing forces than under compression.

  1. 2 kilogram cylinder filled with carbon dioxide at a pressure of 6.2 MPascal. At this pressure, most of the carbon dioxide has condensed to liquid.
  2. Ordinary pencil (wooden, hardness HB, sharpened just before the experiment)
  3. Wooden board (60 cm by 10 cm by 3 cm)
  4. 1.5 meter long stainless steel tube (inner diameter 8 mm, wall thickness 2mm) mounted on a table.
The equipment is displayed to the audience and then discussed. The pencil is dropped into the stainless tube held vertically and then the tube is fixed to the table. First the safety stick is removed for the valve to be possible to open. Then I squeeze the handle of the gas cylinder quickly. The pencil goes shooting down the stainless tube and busts through the wooden board!

Basic Ideas:
A gas exerts pressure on all sides of the container which contains the gas. The amount of pressure is related to the energy of the gas and the amount of gas. The higher the energy, the more pressure is exerted, and the more gas contained, the more pressure is exerted. The process is as follows: a short burst of CO2 gas from a fire extinguisher provides enough force to accelerate a pencil along a length of stainless steel tubing to a velocity sufficient to allow the pencil to penetrate a piece of wooden board of 3 cm thick.

During the setup, nothing exciting is happening. The important aspects of this step are to be sure that the small wooden board is securely fastened inside the joiner’s clip, that all the tubes are properly connected, and that the stainless steel tube is aimed at the wooden board. The gas inside the cylinder is being held under very high pressure. When I squeeze the handle of the cylinder, some of the gas inside is released. This gas travels through the flexible tube until it reaches the pencil. The pressure inside the container becomes so great that there is enough force to shoot the pencil down the stainless steel tube. The pencil is accelerated so much that it goes shooting through the small wooden board!

Warning: This is a potentially lethal demonstration!
All high pressure fittings must be properly connected!
Use only objects and hose that hold high pressure!

Let me take a conclusion of all the above:
This Trinity, Observation (inducing appetite for making) Experiments (which are the basis for) Application in real life, is the very Heart of the Science of Physics.

Electromagnetic metal forming

Electromagnetic metal forming is a high energy rate cold forming technique which brings to industry an assembly of components. The tool operates by electromagnetic pulse and not by mechanical or hydraulic means. The forming equipment can be made compact, and easy to operate.

The principle of the electromagnetic metal forming

The electrical energy stored in the capacitor (C) is discharged by high current switch device (S) and will establish an impulse current in the coil (T). In our case the current switch is a low-cost ,,home-tinkered’’ electromagnetic relay.

By this impulse current a magnetic field is created around the coil. This field includes an eddy current in the work piece (W) which has the same frequency as the impulse current in the coil. If the frequency is big enough – (from technical reasons it must be in the 10 kHz frequency range) – the eddy current is condensed in the surface layer of the work piece (skin effect).

From electrotechnical point of view the coil and the work piece have a transformer coupling. The coil is the primary winding of the transformer and the work piece is considered a short-circuited secondary single turn coil.  According to the law of induction antiparallel currents repell each other.

If this force exceeds the forming resistance of the metal of the work piece, the plastic deformation of the work piece will be resulted. When the work piece (W) is inside the coil (T), then shrinking deformation if outside then enlargement deformation will happen.

Of course in practice there are more complicated coil-work piece systems than the simplest case in the figure but the functional principles are the same.

Advantages of the electromagnetic forming