Kristine L Ming

How to: test Galileo’s scaling laws

Mark Miodownik, presenter from Dara O Briain’s Science Club on BBC Two, reveals how you can perform simple science experiments at home. By making ballistic jelly you can see for yourself the difference size makes to surviving a fall.

Ballistic jelly is made from gelatine and water and used by the police to test out the impact of firearms as it mimics human tissue. Make your own following Mark’s step-by-step instructions.

Mark holding ballistic jellyYou’ll need to make one large and one small ballistic jelly for this experiment. Leave a total of 15 hours for it to set in the fridge.

Mark pours powderPour 900ml of hot water into a measuring jug. Then pour in 100g of gelatine powder. Repeat the process with the second jug.

Mark stirringCarefully stir the mixture in both jugs with a spoon trying not to add in any extra air.

Mark with jelly liquidAfter fully mixing them, place the jugs in the fridge for three hours.

Mark stirs jugTake the jugs out of the fridge and place them one at a time in a bowl of just boiled water for 10 minutes. Stir carefully until the gelatine has completely liquefied.

Mark rubs oil over his bowl.Rub the containers you will be using as moulds with oil. This will stop the gelatine from sticking to it as it cools.

Mark pouring from jugCarefully pour around 450ml liquid gelatine into your small container and 1,350ml into your large container. Place them in the fridge for 12 hours.

Mark presses jellyWhen you take out the jellies they should feel elastic but firm

Mark with gelatineThe jelly should tip out quite easily from both containers. You can use a blunt knife to ease them out.

Mark drops jellyAfter making sure the area is clear beneath you, drop the jelly from a height onto a hard surface. You can test the effect of surface area to volume ratio by dropping different sized jellies.

jelly by Mark's shoesYou should find that while the small jelly remains intact, the larger jelly will sustain some damage. The bigger the jelly, the worse it will fare!


Things you’ll need

  • Two 1 litre plastic measuring jugs
  • One large bowl or bucket
  • One small container (the size of a cereal bowl)
  • One large container (the size of a mixing bowl)
  • 200g (around 16 regular sachets) of gelatine
  • Cooking oil
  • Spoon
  • Fridge
  • Kettle to boil water
  • Outdoor space to drop ballistic jelly from height

How to do it:

Be very careful when using the just-boiled water and make sure it’s safe to drop the jellies without danger to you or anyone else.

1.

Pour 900ml of warm water into a measuring jug and add around 100g of gelatine mixture. Repeat the process with the second jug.

2.

Carefully stir the mixture in each jug with a spoon, trying not to add in any extra air.

3.

When fully mixed, place the jugs in the fridge for three hours.

Continue reading the main story

What are Galileo’s scaling laws?

mouse on corn

Fairly large animals like ourselves are likely to sustain serious injury if we were to fall from a great height. However, a smaller animal would be unharmed.

This is due to a fundamental ratio in nature, discovered by Galileo Galilei, which states as the width of an object is doubled, the surface area is squared and the volume is cubed.

So something that is 10 times wider, has 100 times the surface area but 1,000 times the volume. And as the volume increases, so does the mass proportionally.

It means the larger the object, the smaller the surface area to volume ratio will be.

We as humans are therefore ruled by gravitational forces since our surface area is relatively small compared to our volume.

However, gravity is negligible to very small animals who have a large surface area to volume ratio.

They live in a world dominated by surface forces such as friction and surface adhesion, which overcome the relatively weak hold of gravity.

So this explains how spiders and flies can walk up walls.

4.

Pour some just-boiled water into a bowl or bucket. Take the jugs out of the fridge and place them in the bowl making sure the hot water doesn’t spill over into the jugs. After 10 minutes the gelatine should have melted into a liquid. Stir the mixture carefully in the jugs to remove any remaining lumps.

5.

Rub the small and large containers with oil.

6.

Pour 450ml of the gelatine mixture into the small container and the rest (1,350ml) into the large container.

7.

Place the containers in the fridge and leave for 12 hours. Once fully set, tip them out onto a table.

8.

Now it’s time to test out the laws of scale. Find an outdoor space where you can safely drop the jellies from a height of at least 10ft. Firstly drop the small jelly and note the effect. Then drop the large jelly.

9.

You should find that while the small jelly will bounce the larger jelly will split. This is because the surface area per volume ratio decreases as objects get bigger, meaning gravity has more of an impact.



Mark Miodownik and Dara O Briain

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BBC Two’s Science Club Mark Miodownik and Dara O Briain explain why bigger objects sustain more damage than smaller ones when dropped from a height

Article source: http://www.bbc.co.uk/science/0/22615354

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