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Nails trick

 
 

Overview

Materials needed, per group

  • 1 flat piece of wood or ball of modelling clay
  • 15 nails (with ends rounded off for safety)

Practicalities

If you want to assemble the equipment quickly, fix the nail upright in a ball of modelling clay or use any smallish flat piece of wood and hammer the nail into the centre. If you want to repeat this activity, cut a piece of wood or MDF approximately 10 cm square and drill a partial hole in the centre that is just wide and deep enough to accept the first nail. This apparatus can then be taken apart and stored flat when not in use.

The larger the nails, the easier and more impressive the activity is, so get the largest ones you can find (make sure you sand down the sharp points).

Discussion

  • How many of these nails can you balance on the head of one nail?
  • What’s the best way to balance one nail?
  • How can you get the other nails to balance on this one? (You can take it off and lay it flat on the table at this point.)
  • What can you use to hold down the nails lying across the first nail so that they don’t fall off when you lift them back on the nail head?
  • How does this trick work?

Look at a pciture of a crane. How is it able to pick up very heavy weights without falling over?

Extensions

  • What is the maximum number of nails you can balance in this way? Try making a second row and balance it on top of the first. How many layers can you balance in total and how many nails does this use?
  • Can anyone find a different way of balancing the nails?
  • Can you scale up this activity, for example using hockey sticks instead of nails?
  • Can you scale down this activity using pins? (This is much harder; again, watch out for the sharp points.)

Links to everyday life

Cranes
Cranes use a counterbalance - a heavy weight at one end of the arm that stops the crane toppling over when it picks up a weight at the other end of the arm. This makes cranes very heavy and hard to transport, so some have been designed to use a refillable water tank as the counterbalance to make them lighter and easier to transport.

Cantilever bridge
The railway bridge across the Firth of Forth in Scotland contains almost 54,000 tons of steel. When it was completed in 1890 the 2.2-km-long bridge was the biggest in the world. It is the world’s oldest cantilever railway bridge and remains in use to this day.