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11 Aug Three super easy physics experiments to impress friends and foes

A moved picture with different colors of (neon)lightone of the three experiments!?

As a physics student I’m often asked what kind of jobs I can get when I graduate or even what the general purpose of physics is. I can give a long monologue about the beauty of physics, why life is full of physics and why physicists are wanted for their analytical abilities. Or I can show them these three experiments which demonstrate the simplistic beauty of physics perfectly. If you’re not a physics student you can use these experiments to impress housemates and friends at (birthday) parties. You may even win some bets!

Balancing a stick on two fingers

Materials:

  • A stick. Every stick works, as long as all the weight is on one side (like a hammer). Best results with a long and heavy stick (e.g. cane or broomstick).

What you do:
You ask the one to whom you demonstrate this experiment: ‘Can you balance this stick on the tip of two fingers?’ They probably attempt to put their fingers somewhere random on the stick and fail. After a few fails or if they finally succeed, you can say: ‘I can do it in one attempt’. You put your index fingers on opposite sides and slide them together. BOOM! They happen to meet at the centre of mass and stay balanced on your fingers. Every time!

Demonstration gif of the principle with a large pencil.

I used a large pencil, but a bigger and heavier stick will yield more impressive results. You can see it tilt now because of the low mass of the pencil.

How it works:
If one finger gets closer to the centre of mass, relatively more mass will rest on that finger. More mass equals more friction and on the other side the opposite happens: less mass will rest on that finger and thus there will be less friction. Eventually, the finger on the side with less friction will start to move until this finger is closer to the centre of mass compared to the other finger. Then, the other finger will start to move again. This process repeats itself until the fingers meet precisely at the centre of mass. This is the place where you need to put your fingers to balance an object. Win for physics!

Tornado in a bottle

Materials:

  • Two empty plastic bottles
  • water
  • strong tape, e.g. duct tape

What you do:
You fill one bottle almost completely. Now you put the other empty bottle on top of it and tape them together. Test it above the sink to make sure it doesn’t leak. For the best results you could saw off the rigid top part of both bottles and use glue and tape to make the seal sturdier. But it will work with just the tape as well. In any case, you should end up with a sort of hour glass filled with water on one side.

Now you go up to someone and say: ‘I bet I can empty this bottle faster than you can!’ You give them the bottle and they’ll probably just turn it upside down and wait. The upper bottle will empty into the lower bottle in a burping manner. The water flow gets interrupted with bulges of water moving to the upper bottle. It’s your turn: You turn the bottle upside down; give it a clockwise shake and BOOM! You’ve got yourself a tornado in a bottle. And guess what? It empties way faster than just turning it around. Double win for physics and for you!

Demonstration gif of the tornado in a bottle.

Tornado in a bottle. You have to practise a bit with the swirling to get a good vortex.

(Note: You could also do the demonstration with just one bottle and empty it above the sink)

How it works:
Giving a spin to the bottle will result in a small centripetal force. This small force triggers a vortex. This vortex is a more efficient way for the fluid to flow into the other bottle, because air will move through the middle of the vortex into the upper bottle without disturbing the downwards water flow. The liquid wants to be in the lowest energy state (i.e. all the water in the lower bottle) as fast as possible and thus the vortex will sustain itself. Now you have your own spinning tornado in a bottle. Awesome!

Tip: Add some food colouring, oil and or small particles to the water. This will make the tornado more visible and more fun to watch.

Make pepper fear you

Materials:

  • a bowl or a deep plate
  • ground pepper
  • water
  • (dish) soap

What you do:
You start bragging: ‘I can make pepper fear me!’ People will not believe you, so you show them your trick: You grab a small bowl or a deep plate, fill it with water and sprinkle some pepper on the surface of the water. You put a little soap on your index finger (without the audience noticing) and you put that finger in the water, or you could just drop some soap into the bowl. WOW, the pepper on the surface is so impressed by your finger, it flees away from it!

Demonstration gif showing the pepper moving away from the soaped finger when the finger touches the water.

Behold, the mighty index finger makes the ground pepper flee like little babies!

How it works:
Most of the pepper particles can float on the water because of the high surface tension. If the soap is added to the water, the surface tension is reduced. In some cases floating particles will sink, but in our case that’s not what happens. The surface tension isn’t reduced instantly in all the water. When the soap hits the water, a surface tension reducing wave arises and spreads in a circular manner to the rest of the water. This wave carries some water with it. The pepper particles are so light; they can travel on this wave. Ergo, you see pepper moving away from the centre, to the sides, like they’re fleeing.

Did you like these experiments or do you know other simple experiments? Tell me in the comments below!

WTF simon

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WTF Simon

WTF Simon

Runner, Physicist, Dutch 3000m steeplechase champion, day-dreamer, sport minded and science lover
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