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Episode 6: Milk Carton Sprinkler

This week we bring you an amazing invention that will make watering your plants a breeze and leave you dizzy with excitement. You’ll never look at a milk carton quite the same way again…

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View the full transcript of Episode 6: Milk Carton Sprinkler

Before you start

These experiments have not been specifically safety tested for home use but we believe them to be safe if the instructions are followed. Adult supervision or direction is recommended as appropriate. All experiments are carried out at your own risk.

What you’ll need

  • A piece of string (how long is up to you!)
  • An empty juice carton / plastic milk bottle
  •  A pair of scissors
  • Water
  • A washing up bowl / or just a regular bowl

What to do

Make sure you supervise this experiment very carefully. Not only will you be using sharp scissors as a stabbing tool, but there’s also the risk of getting water absolutely everywhere. You might need to do some of these steps either with (or for) your family, depending on their ages and how sensible they are.

  1. Take your scissors and carefully poke a hole in the bottom left corner of each side of your juice carton – or you could do bottom right if you feel strongly about it, as long as you’re consistent.
  2. Poke another hole in middle of the top flap. If you have a hole punch, it might work a bit better than scissors for this one.
  3. Thread your string through the hole at the top of your carton and tie a knot so you can hang it up later. Now head to your designated sprinkle zone.
  4. Put some water in your bowl, stand the carton in it and then fill the carton up with water.
  5. Lift the carton up, watch (and maybe get out of the way) as it starts to spin.
  6. Wonder why you ever spent money on a watering can.

What to talk about

  • Why do you think the water squirts out more slowly as the carton empties?
  • Have you seen anything else that spins around like this? What was pushing it around?

What’s going on?

The water at the bottom of the carton is under pressure because the water above it is pressing down on it due to gravity. Without any holes, the water at the bottom of the carton pushes outwards equally on all sides. When you make a hole, the water squirts out, so it isn’t pushing on that part of the carton anymore – but on the opposite side of the carton there’s still a pushing force.

As water squirts out of the hole in one corner, it creates a pushing force in the opposite direction.

It’s important that the holes are in the corner, not the middle of each side. Making the holes off-centre makes the pushing forces off-centre as well. The turning effect of a force is called a moment and the bigger the distance of the force from the turning point (string), the larger the moment. You’ll have seen moments in action before – they’re what make windmills turn and Catherine wheels spin.

The experiment also illustrates an important difference between solids and liquids. If we filled the carton with water and froze it, no water would come out of the holes and the carton wouldn’t spin at all. That’s because ice is a solid and that means all of the very tiny particles that make it up (water molecules) are stuck together tightly. When water is a liquid, the molecules can slide over each other and so rush out of the holes.

What next?

If you have enough cartons you can have a go at modifying and perfecting the design of your sprinkler. Think together about what would happen if:

  • You added more holes
  • You changed the pattern of holes on each side
  • You use a different shaped carton
  • You made your holes bigger or smaller

If you want to know more about why the pushing force is opposite to the hole, tune in for the next episode of Do Try this at Home: Rocket Balloon

Did you know?

There was a physicist called Richard Feynman who built a sprinkler that sucked in water instead of pushing it out, because he wanted to find out what would happen. The sprinkler exploded because the pressure inside became too high.


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