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Drop jump

By David, 17 October 2019 Activity

Two matcch box drawers side by side with a ten cent coin in the upper drawer.

It appears like the coin is climbing out.

Drop the ruler, watch for the jump… that’s the point of drop jump! Can you work out which matchbox tray the coin will fall into?

You will need

  • 2 trays from inside matchboxes
  • Ruler (a metal ruler works particularly well)
  • Tissues
  • Blu-tack or sticky tape
  • Coin

What to do

  1. Small ball of Blue tac on the end of a a ruler.Put a blob of Blu-tack near the end of the ruler. If you don’t have any Blu-tack, you can roll a piece of sticky tape into a loop with the sticky side out and use that instead.
  2. Matchbox drawer held onto ruler with Blue tac.Stick one of the trays to the Blu-tack, hanging part of the tray over the end of the ruler.
  3. Matchbox drawer on ruler with ball of Blue tac.Put another blob of Blu-tack or sticky tape on the ruler next to the tray.
  4. Two matchbox drawers on a ruler.Use this to stick the other tray next to the first one.
  5. Two matchbox drawers lined with tissue.Take two tissues. Fold each neatly until they just fit inside the trays, then put one tissue in each tray.
  6. Two matchstick drawers, lined with tissue, one has a ten cent coin in it.Put the coin in the end tray.
  7. You’re almost ready!
  8. Put the ruler on a flat surface. A carpeted floor is particularly good because it’s quieter than a hard surface.
  9. Lift up the tray end of the ruler until the ruler is at a 45-degree angle.
  10. Place the coin in the tray at the end of the ruler.
  11. Ten cent coin in the higher of two matchbox drawers, side by side.Adjust the coin so it’s lying flat relative to the flat surface.
  12. Finger holding ruler in place.Put a finger on the flat surface, touching the part of the ruler that is on the surface. This will anchor and stabilise the ruler.
  13. Let go of the tray end of the ruler.
  14. Coin jumping from one matchbox drawer into the other.Which tray does the coin end up in?
  15. If you have a device that can record videos, record it and watch back to see what happens!

What’s happening?

If everything went well, the coin should end up in the other tray from the one it started in. (If it didn’t, a little practice can help you make a clean drop jump.)

This activity is quick and can be a little noisy, so it can be hard to see what’s going on. That’s why it’s handy to watch a video.

You might think that the coin stays in its original tray until the ruler hits the ground. Then it bounces out and into the other tray. However, if you watch closely, that’s not what happens. It looks like the coin starts to ‘climb’ out of its tray as soon as the ruler starts to fall!

Of course, the coin isn’t climbing. Both the coin and trays are falling. But the coin’s tray is falling faster than the coin, making it appear like the coin is climbing out. But that’s strange too – why does this tray fall so quickly?

If you drop a ruler and a coin at the same time, they’ll fall at pretty much the same rate. But in this activity, the ruler isn’t just falling – it’s rotating! And that’s the key to what’s happening.

As the ruler swings down, different parts move at different speeds. The end on the flat surface doesn’t move much at all. The middle moves quite a bit, but not as much as the far end of the ruler, which moves very fast.

The tray, perched right on the end of the ruler, is accelerating faster than gravity, leaving the coin behind. The coin falls almost straight down and ends up in a different tray!

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2 comments

  1. I love your Friday-afternoon posts. I often take them home to my children (late primary, early high school). This particular article is nice – what if the ruler were super, super long! Then the end would go faster than the speed of light. So…. what happens? 🙂 Time and space are stretched and squeezed!

      Reply
    1. Ooh! interesting!

      I’m not sure if a longer ruler goes faster! If you imagine the middle of the ruler falling at normal gravity, and the table end stays still, I think the far end goes at twice gravity!

      I guess if the ruler is really long, it’ll weigh more than the Earth and things start to get weird. Maybe an even longer one would collapse under its own gravity and form a black hole?

        Reply

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