Practical puzzles and what happens next!

This blog post comes with huge thanks to David Featonby with whom I was able to converse at some length about fun ways to get students thinking at a fairly early stage of my teaching career. Some of the below feature in his What happens next series” others have come from conversations with teachers when I have been doing my Woah! — The physics of toys and viral videos” CPD sessions. A few are original. I will add more as I remember them and as others contribute!

Each scenario should be described and shown, and pupils in teams asked to

  1. Indicate what they expect to see happen next

  2. Explain why what happens does

The difficulty of these various scenarios obviously varies considerably, and you should grade them according to the class’s ability and the amount of fun you want to have. Some of the simpler ones are described first.

  1. Two conical beakers are both filled to approximately ¾  depth of water (that is 3/4 depth) (or any other liquid). What will happen when one is emptied into the other?1

  2. Float an orange in water. Ask what will happen once it is peeled 2

  3. Float an egg in a container of water. What will happen when a considerable amount of salt is dissolved  in the water ?3

  4. A new unopened can of coke and a can of diet coke are floated in a large tank of cold water. What will happen?4

  5. Take a single serving ketchup sachet and balance it by adding paperclips to its bottom so that it just floats in water. Add to a 2 litre bottle and fill with water. Ask what will happen if the top is screwed into the bottle and the bottle is squeezed?5

  6. Peanuts in Lemonade. Get a bottle of cheap lemonade, the cheaper the better as long as it is still fizzy. Put a few peanuts in the top…6

  7. The Tea bag trick! Buy single-serve teabag, Unfold bag , remove staple and  empty contents into bin. Stand the bag on its end as a square based cylinder. Set fire to the top of the cylinder.7

  8. Fork Handles. Set up 4 candles of different lengths so that they can fit inside an inverted glass jar. Light the candles and place the inverted jar over them.8

  9. A hardboiled egg and a raw egg are spun horizontally on a table. Try stopping each one…9

  10. Place some (ideally heavy) crockery on a table cloth. When the cloth is pulled away and off the table, will the crockery stay on the table?10

  11. A large book or mass (about 1-2 kg if possible) connected to some thick string which in turn is wrapped around a cylinder — I use a rolling pin, a wine bottle will do. When the rod is lifted what will happen?11

  12. The mug trick! About 1m of string is tied to the handle of a mug. The other end of the string is tied to a washer. The string is draped over a pencil with the washer held out horizontally. Let go of the washer so that the mug falls toward the floor… 12

  13. Drop a small piece of paper and a coin.

    1. Separately13

    2. With the paper on top of the coin14

    3. With the paper below the coin but close to it15

  14. Throw into the air a plastic bottle with water issuing from a hole in the bottle. What happens to the jet?16

  15. A groan tube is dropped vertically and caught. What happens?17

  16. A slinky spring is held vertically and then dropped, Looking carefully at the bottom of the slinky — explain what you see.18

  17. Place a tennis ball on top of a basketball, and drop both together. What happens to the tennis ball?19

  18. Rest a meter rule horizontally on your fingers, one about ¼ way along and the other at the opposite end. Slide your fingers are towards the centre. What happens?20

  19. Throw a raw egg as hard as you can at a vertical sheet which is held by two people…21

  20. Spin a Cadbury’s cream egg as quickly as possible horizontally on a slightly rough surface.22

  21. Place a single sheet of newspaper on a table over a half metre length of thin wood, one end of which protrudes. Hit the protruding end sharply.23

  22. Wine glass with (half a) cork inside.  What happens when I blow sharply over the top of the glass?24

  23. Shape half a lolly stick into the shape of a boat. Place in bowl of clean still water and place a drop of washing up liquid at the stern.25

  24. Burning the candle at both ends! Put a needle through the centre of a long candle and shave both ends. Balance the candle at its centre and light both ends, what happens?26

  25. Using an empty milk bottle and hard boiled egg, drop a lighted match into the bottle and then seal the top with the egg. What happens?27

  26. Suspend a broom handle horizontally by two thin threads from its ends. Take another broom handle and whack the centre of the suspended one. What breaks, one thread, two threads, or the broom handle?28

  27. One ice cube is placed on a plastic box, which feels warm, and a second on an upturned, thick bottomed frying pan which feels cold. What happens?29

  28. Place a medium size (2cm diameter by 1 cm depth) neodymium magnet on top of a Japanese yen. Lift the magnet quickly.30

  29. Suspend a yen on a thread. Move the neodymium magnet towards the yen . What will happen?31

  30. Float” a yen on some still water utilizing the surface tension, Hold a charged rod beside the yen. What happens? 32

  31. Obtain two test tubes such that the second just fits inside the first. Fill the first with water and float the second inside the tube. Turn the two tubes upside down. What will happen ?33

  32. What happens when a small but strong cylindrical magnet is rolled down a gentle slope at In a N-S direction ?34

  33. Plastic fantastic! - Fill a polythene bag with water. What happens when you push barbeque skewers through the bag?35

  34. Hysteresis — Touch an elastic band to your lip to feel the temperature. Stretch it and it should feel warm. Hold it stretched and repeat…36

  35. Two balloons - Inflate two identical balloons , one so that it is fairly large, the other just started. Connect the balloons in such a way that the air can flow from one to the other. (I use a bit of silicone hose with a bulldog clip to seal it) What will happen when the air is allowed to flow. Will the big balloon blow the little one up or vice versa?37

  36. Float a beaker containing some weighty object in a 2l plastic lemonade bottle containing water. What happens to the water level in the bottle when the weight is placed separately in the bottle?38

  37. Float and empty sealed glass jar, on a boat” in a bowl of water. What happens to the water level when you take the glass jar out of the boat and float it in the bowl separately alongside the boat” ? 39

  38. A paper clip on a piece of thread is lowered  towards a strong magnet. What happens? 40

  39. 59.Take two measuring cylinders, with different diameters ( say no.1 with twice diameter of mumber 2) Pour some liquid from the larger meauring cylinder into the smaller, ..and ask how high will the liquid be in the second, (given the different diameters) (in the case cited 4X)

  40. Place a beaker of water on a sensitive balance - What happens when you lower your finger into it ?

  41. Balance a beaker of water on some scales with an orange (or similar object that floats) beside the beaker. Note the reading.  What will happen to the reading when the orange is moved into the water in the beaker so that it floats ?41

  42. Place a candle in a beaker and fill with water so that the water is just up to the top of the candle, (but doesn’t wet the wick). What happens when the candle is lit ?42

  1. Answer (The beaker fills exactly)

  2. the peeled orange sinks

  3. the egg floats in the salty water, compare with the dead sea or blue lagoon, Iceland

  4. The diet coke floats higher as the dissolved sugar in the non-diet is denser than the replacement sweetner!

  5. The packet will sink — this is a really neat example of a cartesian diver. Also easy to make with pipettes!

  6. They rise and fall as the bubbles form on their surface)

  7. The tea bag burns establishing a convection current around it — when enough has burnt the friction on the outside of the bag/ashes is enough to lift it into the air.

  8. The candles go out in turn as the Co2 extinguishes them. The really cool thing is that if the container is small they will go out tallest-shortest as the thermal buoyancy of the Co2 causes it to rise. If the jar is tall enough to allow the gas to cool sufficiently then they will extinguish smallest to tallest!

  9. The hard boiled egg will stop. The raw one will begin spinning again because the liquid inside is still spinning!

  10. Yes! Tips: remove the hem and pull very suddenly and slightly downwards off the edge of the table.

  11. A few turns provides sufficient friction to easily lift the object into the air!

  12. This is a variant of the one above. The mug stops short of the floor because the string wraps round the pencil many times. There is a lot of physics in this demo from friction to conservation of angular momentum.

  13. Coin lands first

  14. They land together

  15. Coin lands first as paper slides out of the way.

  16. The jets stops whilst the bottle is in the air as they are both effectively falling together, weightless.

  17. The groan stops whilst in freefall — see above

  18. The bottom stays stationary for a while as the spring falls. This is because the two ends are being pulled in toward the centre of mass of the spring. The center of mass is falling, the two cancel out. The top of the spring falls twice as fast as the center of mass.

  19. The mechanics of the collision with the floor mean that most of the kinetic energy of them both is transferred to the tennis ball and it is blasted” into the air — this is a viral craze with beer cans in 2020.

  20. Your fingers will meet at the middle of the rule — this is because the friction between them and your hand is dependent on the proportion of the weight resting on each finger.

  21. As long as you hit the sheet, the egg will not break. The rate of change of it’s velocity is always low enough that the force is lower than that needed to break the shell.

  22. The egg rises onto an end — just like a tippie top!

  23. The wood snaps cleanly — there is such a large impulse due to the air resistance on the sheet of paper.

  24. The cork jumps out of the glass. The fast air over the top creates a low pressure and the air rushes out of the glass, carrying the cork along with it.

  25. The washing up liquid lowers the surface tension behind the stick. The resulting imbalance causes the stick to move forward. Try this in a long bit of guttering!

  26. The rate of wax melting is dependent on angle so the candle rocks back and forth!

  27. The match heats the air in the bottle which expands. The bottle is then sealed by the egg. As the air cools inside, the pressure reduces and the egg is driven into the bottle by the atmospheric pressure.

  28. The broom handle! This is a trick similar to smashing a brick on a volounteer’s chest. The inertia of the broom handle and a very sharp, sudden strike is critical.

  29. The one on the frying pan melts very much faster. They are at the same temperature but the pan feels cold because of the increased thermal conductivity. This conductivity allows the transfer of more energy to the ice cube over time.

  30. The yen are carried upward into the air by the action of lens’s law

  31. Lens’s law again, the yen moves away!

  32. The water is attracted to the charged rod and imperceptibly lifts. The yen appears to move away from the rod as it floats along the surface tension!

  33. The second floats inside the first

  34. The magnet needs to be thin, it will move in a circular arc!

  35. The plastic deforms around the skewer forming a tight seal and no water is spilt. Good for a discussion on plastic materials

  36. It will now feel cold — there is a lot of transfer to and from thermal energy stores as the internal bonds are broken and re-formed

  37. Counter-intuitively the small balloon collapses and inflates the larger. This is a neat result of pressure being force per unit area.

  38. The level falls

  39. No change in level

  40. The clip spins one way on lowering, the other on raising. This is a consequence of FLHR.

  41. It stays the same

  42. The candle continues to burn down into the candle wax, leaving a wall to protect the wick from the water.

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