Physics in your pocket

Trna J.

A properly methodical experiment in physics education aids understanding of the curriculum through exposition and helps to motivate pupils. Simple physics experiments are a special, deeply motivating type of school experiment.

From an implementation point of view, simplicity means an ease of technical execution of the school experiments. This technical ease brings a valuable opportunity for its realization by students alone, both during lessons and at home. Such simple physics experiments are also thrifty, as they are usually accomplished using simple equipment (e.g. plastic bottles, beverage cans).

A simple pupil’s experiment using simple equipment can become a fundamental educational instrument, especially in teaching physics in primary education. Students’ competence as the most universal goal in education can be developed e.g. in a study group, solving simple or more complex projects based on simple experiments.

The following are examples of easy physics experiments, all of which can be implemented with only the help of the coins as a simple and relatively accessible piece of equipment:

1. Inertia I

We cover a glass with an adequately large strong smooth sheet of paper (or plastic foil), on which we put a heavy coin. Quickly pulling the paper away causes the coin to fall into the glass. The same result we obtain by pushing the paper quickly away. The inertia of the coin and low friction cause the fall of the coin into the glass.

2. Inertia II

On a tall glass (or vase) we put a ring made of thick smooth paper, on top of which a coin is placed. Using a finger we yank the paper ring sharply away to the side - the coin falls into the glass. The inertia of the coin and low friction cause the fall of the coin into the glass.

3. Inertia III

On a smooth table we place a column of a few coins. Using a table knife (or a thin ruler) we strike the bottom coins out alternately from side to side - the coins left are still in the column. The inertia of the coins keeps them in the column.

4. Collisions

On a smooth table we strike the same coins briskly against each other using the thumb and the forefinger. Since the thumb imparts lesser speed on the coin than the index finger, the coins exchange their velocities after the collision. Variations of this experiment include using coins of different masses or striking a stationary coin or a fixed barrier (a metal ruler) or using two forefingers to hit the coins etc. The scientific principle controlling the exchange of the coins’ momenta is the law of conservation of momentum.

5. Newton’s cradle

On a smooth table we place a row of identical coins (from two to four; but it can be more than that) so they are touching each other. Using the first coin in the series, we hit the next coin in the direction of the axis through all the coins. This causes the last coin to jump away. For a more impressive demonstration, the inner coins may be held tightly to the table. The exchange of the coins’ momenta is valid also for these coins (the principle behind Newton’s cradle).

6. Centripetal force

We roll a coin on a level table. Sometimes the trajectory of the coin is a straight line and at other times it is a curve, with the latter occurring almost always at the end of its motion. For the straight-line trajectory, the resultant force and the sum of moments are equal to zero. For the curved trajectory, the resultant force acting on the tilted coin is the centripetal force, which is enhanced by the moment of non-zero forces.

7. Centrifugal force

The coin is lying on the bottom of a glass with smooth surface and conical shape such that it is wider at the top than the base. The task is to get the coin out of the glass without touching the coin or overturning the glass. Moving the glass in a circular accelerating motion causes the coin to spiral upwards on the inner side of the glass until it flies out and away.

8. Flywheel

We spin a heavy and large coin (by flicking) around its vertical axis on a large book (with a smooth cover). Inclining the book does not change the direction of the coin’s axis of rotation, and after straightening the book, the coin spins the same way as at the beginning of motion. According to the law of conservation of angular momentum the axis of rotation of the coin remains constant.

9. Pressure

We place a coin flatwise in a plasticine disc. On the coin we carefully place a weight (e.g. 1 kg). After removing the weight, the coin is slightly submerged. Next we set the coin on its edge in the plasticine disc and weight it down carefully again with the same weight. The coin will submerge deeply. To prevent twisting the coin it is necessary to insert the weight in a vertical tube, e.g. made of paper or better made of plastic foil. The pressure exerted by the constant force upon the area changes with the size of the area.

10. Scales

We create a set of scales by placing a fulcrum under a plastic (or wooden) ruler (or by suspending it) at its centre of gravity. On both sides of the scales we place coins such that balance is maintained. We can vary both the number of coins and their distances from the centre of the scales. The balance of the class 1 lever is a simple case of the Principle of Moments.

11. Elasticity and Strength of Materials

We put a sheet of thin paper on an empty glass. A coin placed on the paper would fall down into the glass. Folding the paper into accordion-like pleat prevents the coin from falling in - on the contrary, we can add further coins. A much greater force is necessary to deform the corrugated paper since it has vertically facing load carrying parts, while the flat paper has none.

12. Drag I

On a coin we put a paper disk of the same diameter as the coin. We drop the coin with the paper from the height onto a pad. The coin falls down faster than the paper. Analogously we can use a coin and circular aluminium foil. The drag and different masses of objects induce a faster fall for the coin.

13. Drag II

We drop two identical coins from the same height into glass cylinders. One of them is empty (air) and the second one is filled with water. The coin in the water takes longer to fall. The experiment can be altered using different liquids etc. The drag force acting on the coin is larger in the water than in the air.

14. Bernoulli’s equation

In front of an open hand (or a bowl) a light coin is lying. By blowing sharply across the top of the coin we generate enough lift to flip the coin into the hand (or the bowl). Perfecting this experiment may take practice. Due to Bernoulli’s equation the air pressure over the coin is reduced and after a subtle lift the coin is moved into the palm by the pressure force of the surrounding air along with the stream of the blowing air.

15. Surface tension I

Into a container filled with water we gently place a light coin afloat on the water surface (e.g. using a wire holder). The surface tension of water keeps the coin afloat.

16. Surface tension II

To a container filled up to the rim with water (e.g. a glass, a transparent plastic cup) we add coins one at a time, until the water overflows. This experiment is suitable as a competition or a guessing game. The experiment can be varied using different liquids (e.g. oil, ethanol). The surface tension keeps the water inside the container. The significant curvature of the water surface provides a nice illustration of the effects of surface tension.

17. Thermal conductivity I

Holding a small coin between thumb and forefinger, we start carefully to heat it over a match flame. Soon the coin gets uncomfortably hot - we are not able to hold it whilst the match is burning. The metal coin has high thermal conductivity and low heat capacity, thus its temperature increases quickly.

18. Thermal conductivity II

When heating over a flame, a sheet of paper will carbonise. If we put a coin on the sheet of paper, the place where the coin is positioned will not char. The metal coin conducts the heat away, thus this spot on the paper cools off.

19. Thermal conductivity III

On a wire gauze attached to a ring stand we place three big coins of the same diameter and thickness, made of different metals (e.g. Al, Cu and Fe). The coins are placed at the same distance from the centre of the wire gauze, and in the centre of each coin we put match heads. Heating the wire gauze precisely in the centre, the match heads flare one by one yet always in the same order. Instead of a wire gauze and a gas burner it is possible to use an electric stove. Different metals have different thermal conductivity.

20. Thermal expansion I

We put a coin between two glass strips (at least 25 cm long) in such a way that the coin is standing at the end of the bottom strip. Heating the top strip will cause the coin to fall out. The lower side of the top strip is heated faster than its upper side, so due to different thermal expansions of the lower and upper side the strip is forced to bend upwards, and the coin is released.

21. Thermal expansion II

We bend a piece of thick wire into the shape of a triangle and attach it to a ring stand horizontally. Into the corner made from the loose ends of wire we insert a coin. After heating the opposite side of the triangle, the coin will fall out. The coin is released due to the thermal expansion of the wire.

22. Thermal expansion III

We hammer two nails into a wooden plank located at such a distance to enable a large coin to barely fall through, if the plank is standing. Using a wooden clothes peg to grip the coin, we heat it over a candle (a lighter or a burner). Then the coin will not fall through. The coin expands under heat, thereby unable to fall through between the nails.

23. Thermal expansion of air I

In a low bowl (a plate) there is a coin flooded with strongly coloured water (milk) that its nominal value is not visible. The task is to find out the nominal value of the coin without touching the bowl (the plate). We heat a glass using hot water (or over a flame) and settle the inverted glass next to the coin into the coloured water (milk). After a while the liquid rises into the glass and the coin becomes visible. The air in the heated glass cools down, hence its pressure is reduced, and due to the higher pressure outside the glass the liquid is pushed into the glass.

24. Thermal expansion of air II

On the moistened mouth of an empty glass bottle (or a hard plastic bottle) we place a coin. Then we place our hands around the body of bottle - this way we heat the air in the bottle (we recommend to cool the bottle in a stream of water before). After a while the coin begins to jump up and down almost periodically. By warming the air, the pressure in the bottle increases. The pressure force of the warmed air lifts the coin.

25. Magnetic properties of materials

We position a permanent magnet (fridge magnet) near to different coins. Some of them are attracted, some not. The coins are made of different metals with different magnetic properties.

26. Magnetic polarisation

We place a few ferromagnetic (soft steel) coins in a row with small gaps. On the first coin we put a permanent magnet (fridge magnet), and bring it closer to its neighbour – which jumps to it. In this way we make a line of attracted coins and slowly drag it over the table. By putting the magnet aside, we observe the line of coins separate. The ferromagnetic materials can be magnetised. After removing the magnetic field, magnetically soft materials tend to return to a non-magnetic state.

27. Eddy current damping

A bifilarly suspended permanent magnet (fridge magnet) oscillates freely, almost without damping. Close underneath the magnet we place a nonmagnetic coin. At the same initial amplitude of displacement the resulting oscillations are now damped. The magnet movements induce Eddy currents within the coin, and thus induce magnetic fields which act against the magnet motion to damp it.

28. Plane mirror I

We stand a coin on its edge in front of and parallel to a plane mirror. We can observe a laterally inverted image of the coin in the plane mirror. The image of the coin is formed by the plane mirror in accordance with the law of reflection.

29. Plane mirror II

We place a coin in front of an upright plane mirror. We can observe a laterally and vertically inverted image of the coin in the mirror. We can change the distance between the coin and the mirror and the inclination of the mirror. The image of the coin is formed by the plane mirror in accordance with the law of reflection.

30. Plane mirrors I

Two plane mirrors are connected along an edge and placed perpendicular to each other. In front of mirrors we place a coin on its edge. We observe an uninverted image of the coin in the mirrors. The image of the coin is formed by the plane mirror in accordance with the law of reflection due to the reflections in two mirrors.

31. Plane mirrors II

We place a coin in front of two upright plane mirrors connected along one edge. We change the angle between the mirrors and thus change the number of reflected images of the coin. The images of the coin are produced in accordance with the law of reflection due to the reflections in two mirrors.

32. Plane mirrors III

Between two upright parallel plane mirrors we place a coin. We observe images of the coin in both mirrors. The images of the coin are produced in accordance with the law of reflection due to the reflections in two mirrors.

33. Reflection

On a table we place a coin in front of an upright glass slab. Behind the glass slab we position a glass container to be in a place of a reflected image of the coin. The glass partially reflects and partially transmits light. Hence we are able to observe the real container behind the glass and at the same moment the image of the coin in the glass.

34. Refraction

On the bottom of an opaque (plastic) cup we position a coin so that it is not visible from the side. After filling the cup with water the coin can be seen. The surface of water inside the cup becomes the interface between media of different refractive indices, at which the light rays coming from the coin are bent (away from the normal) and thus these rays can reach the eye and the coin is seen.

35. Blind spot

We place three smaller coins next to each other with distances of 8-10 cm between them. Move your head toward the coins. At the distance of from 25 to 30 cm the middle coin will disappear. When you pull your head away, the coins situated on the very right will disappear. Light reflected from the disappearing coins falls on the blind spot in the retina where the eye can not see.

References:

[1] Öveges J.: Fyzikální kratochvíle. Praha, SNDK 1965.

[2] UNESCO: Základy přírodních věd v pokusech. Praha