Physics Teachers' Inventions Fair proceedings

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Czech original

Jílek M.

Physics Workshop for Secondary School Students

Jílek M.

Faculty of Mathematics and Physics Charles University Prague

In this contribution I am going to describe several experiments and suggestions for students’ work; these were created over the course of the preparation of a physics workshop of the 2001/2002 school year at the Faculty of Mathematics and Physics, Charles University in Prague. The goal of the workshop was to get a deeper understanding of several physical phenomena (and their practical applications) by examining simple experiments. We also dealt with the solutions of problems and exercises for which there is no time in regular classes. At the moment, we are working on an electronic version of the workshop program, so that they can be used not only as possible suggestions for work with students in various seminars and extracurricular physics classes, but also during regular classroom teaching.

One of the big topics we concerned ourselves with were the collisions of objects and their connection with the laws of conservation of energy and momentum. To shed some light upon this topic we can present an exercise:

What happens to a boat, if we stand on the stern and blow into the sail, or alternatively use a strong fan in place of our breath? Can the boat move? Forwards or backwards? Or does it stand still?

To practically solve this situation we created a simple boat from a yogurt glass lid. A small insulated wire duct taped to the bottom of the jar lid will serve as our mast for a small paper sail (see fig. 1). We use a small plastic tube (e.g. a piece of a ballpoint pen) with a balloon connected to rubber bands to provide a stream of air into the sail. The tube is secured with various sheet metal strips so a wooden board and this was glued to the ship, so that the tube points towards the sail. The ship with a balloon filled with air was released on the water surface of a small bathtub.

The result of this experiment is that the ship starts moving forwards. This can be explained with the use of the law of conservation of momentum. The total momentum of the boat with the balloon filled is zero. If we consider the collision of released air and the sail as elastic, the air bounces backwards off the sail, therefore the vector of momentum for each particle of the air flow points backwards. If the zero momentum of the boat AND the air is to be conserved, the vector of momentum of the boat points forwards.

If this experiment is repeated without the sail, the ship will move in the opposite direction because of the exact same reason. This is a more comprehensible example of the standard reaction engine. However, because the reflection of air from the sail is not ideally elastic and the air flow doesn’t necessarily land on the full surface of it, we can, through a combination of a good sail size and balloon-sail distance, make the boat stand in one place with the air flowing.

Another example of a collision of two objects is the impact of a spinning ball (e.g. volleyball) to the ground. The question is: “How will the ball rotate after bouncing off the ground?”

A simple experiment will confirm that after bouncing off the ground after a long fall, the ball will rotate the other way than before impact. During an impact from a small height, the rotation stays the same as before. This is caused by the fact that when a ball falls from a great height, it deforms more on impact, it touches the ground longer, and this is enough to stop the ball and reaction force then rotates the ball in the opposite direction. A small fall only slows the rotation of the ball down, as it does not have enough time to stop fully. Through a good combination of height, we can achieve a zero rotation after impact.

In technical literature, we can find that this phenomenon is used during airplane landings, where the wheels start turning before contact with tarmac so that their sudden rotation doesn’t change the momentum of the airplane, resulting in a forward tilting of the nose.

Another large topic we concerned ourselves with was the rotation of objects. To give students an idea of the so little talked about topic of Coriolis force, we can use one of the following experiments.

The first experiment shows us the trajectory of a metal ball on a spinning board. A circular board (preferably metal) is placed on a stand with a pivot, an old reel to reel tape recorder or any other swivel that can be freely rotated. Then a simple slingshot (made from weak elastic and a piece of leather or cardboard) is affixed to the board; this slingshot will shoot the ball (at low speed) from the edge towards the center.

We can depict the trajectory of the ball by placing a sheet of carbon paper over a sheet of typing paper, so that the ink from the carbon paper is pressed onto the typing paper, drawing the trajectory of the ball on the printer paper.

If we release the ball when the board is not spinning, the ball will move towards the centre and draw a straight line. If, however, we do the same experiment when the board is turning at the same time, the ball will be affected by Coriolis force in the framework of the board, and this will cause the curve of the trajectory to one side in relation with the board. On the paper, we can observe the curve of the trajectory as well as its relation to the direction and speed of the turning of the board.

Another way to observe the effect of Coriolis force from the point of view inside and outside of the rotating framework is the use of a swivel chair. We can create a simple slingshot from an approximately 60 cm long wooden board, clothing elastic and a piece of cardboard or leather, so that the board serves as a guide rod for our projectile, a simple ping-pong ball (see fig. 3).

We sit in the chair, the elastic pulled back (there is not a lot of force behind the shot) and aim perpendicularly to our forehead, spin the chair and release the slingshot. In the turning framework of the chair, the ball is affected by Coriolis force which is changing its trajectoryand misses us.If we don’t spin in the chair, the ball will inevitably hit us, but from the materials used we can deduce that this is not dangerous in any way.

The chair can serve one more purpose, in case we want to feel the Coriolis force first-hand. We sit in the chair and train a quick movement with our hand going from the knees towards our forehead. If we don’t concentrate on the movement and do it while simultaneously spinning, we will feel Coriolis force working upon our hand on its way up, causing us to miss our head.

During the lessons on object rotationflywheel experiments are often mentioned. To explain the behaviour of a spinning flywheel acted upon by an outside force, we need a relatively thorough examination of the situation and we cannot describe this in-depth here.

We can, however, notice that this behaviour occurs in everyday situations. If we e.g. release a coin on an inclined plane so that it rolls down, we can observe its peculiar movement. Even if we don’t release it perpendicularly to the surface, and therefore we might think it will fall on its side, it will turn in the direction of the movement so that it tips to the other side partially. This phenomenon repeats, the coin tips less with each repetition and in the end it balances out. The same effect facilitates e.g. bicycle/motorcycle riding.

The movement very typical for the flywheel can be observed (together with many other phenomena) in the flight of a boomerang flight. If the boomerang is thrown correctly and returns to us, this is solid proof of it doing one precession revolution.

Simple boomerang models (with two or three wings) suitable for classroom flight observation can be cut out of paper using fig. 4 as a template (the boomerang diameter should be approximately 12 cm) from cardboard, e.g. the back cover of a notepad. The wings of the boomerang need to be twisted to one side or the other, depending on the direction of the spin we will throw it with. For stability purposes, the boomerang needs to be released by placing it on a flat surface (e.g. a book) which we hold in one hand in front of us and strike one of the overhanging wings with a pencil. The thus spun boomerang leaves the surface and, if the strength of the strike and the speed of rotation are correct, follows the characteristic trajectory and returns back.

The last experiment I would like to mention is the simple, effective combustion engine model; using this model, we can observe different energy conversions. We create the model from a small wooden board, a photographic film container and a battery-powered electric lighter we can buy, quite cheaply, in any store.

We poke two holes in the lid of the container and pull two wires through, with the insulation removed from the wires around 0.5 cm from the ends. The ends of the wires are bent towards each other so that there is a 3 – 5 mm gap between them. Then, we nail the lid down with a nail, lid edge facing upwards so that the container can be closed and the wires stay on the inside. Unscrew the cover of the lighter and find two wires which form the spark gap. Connect these to the wires leading to the film container and voilà, our model is complete (see fig. 5)

We put a few drops of alcohol inside the container, snap it into the lid and let the fluid evaporate for a little while. We trigger the lighter, a spark inside the container ignites the alcohol fumes and the container flies approximately 5 m into the air with a loud bang. If the fumes don’t ignite, it often means the fumes are supersaturated, in which case the rest of the alcohol needs to be poured out and the container oxidized by blowing inside of it. Otherwise, the container may be too cold, in which case it must be heated in our hands. Apart from using an electric lighter, the fumes can also be ignited using a Wimshurst machine or another high-voltage energy source.

If we want to measure the amount of energy transformed into kinetic energy of the container, we can weigh the container down with e.g. a piece of modelling clay, measure its weight and then calculate the energy from the height to which it travels. In our case, the energy result was approximately 0.6 J. If we take into account the fact that combustion heat of 1 mm³ of alcohol is around 23 J, we can see how ineffective such a simple machine is.

I would like to conclude my contribution by pointing out that during my presentation it was of course impossible to demonstrate all the suggestions collected over the course of the physics workshop. Detailed information about the workshop program, accompanied by explained experiments and solved tasks will, after compilation, be available e.g. on our website: http://fyzweb.cuni.cz.