##### Fact Sheet:

See The Bigger Picture

# When things interact in nature what happens?

### Flocks and Shoals

Birds of a feather flock together and a flock swoops, twists and dives in the air as if it were a single animal. But there is no leader: the flock behaviour emerges from every individual birdâ€™s response to its neighbours. Each bird delicately balances its position and speed to keep close but avoid collisions. The patterns are the result.

Mathematical models containing only these ‘local’ ingredients help us predict the dynamics of flocks and shoals and explain why, for very different species, the patterns often look remarkably similar.

### Rocks and Cans

Soft sedimentary rock layers are slowly crushed when continents collide. Crushing produces a seemingly endless variety of zigzag and crease patterns as the layers buckle and fold up in different ways.

We can understand why the patterns form by analyzing how the layers interact when they are crushed.

Sometimes folding occurs only in part of the material even though all of it is being crushed.

This can has only buckled in the middle and gives us an insight into how folding occurs in rocks.

### Bouncing and Colliding

Bouncing is more complex than you think.

One ball, even a ‘super-bouncy’ one, dropped onto the ground, does not bounce back up to its original height.

But if you drop two balls together they interact. Extra momentum is transferred to the upper ball which can then bounce much higher! The complex interactions between bouncing balls are exploited in lottery machines.

### Muesli and Sand

Shaking is more complex than you think.

Have you ever noticed that in your box of breakfast cereal the biggest bits are at the top? When you shake the box the smaller bits fall below the bigger bits until the biggest ones reach the surface. This is called the Brazil Nut Effect.

The Brazil Nut Effect occurs through the interactions between cereal bits, and shows that ‘granular materials’ such as muesli and sand behave in complex ways: sometimes they can flow like liquids, but they certainly don't mix like liquids...

When a flat tray with a mixture of big and small granules is shaken horizontally, the small granules line up in stripes.

The shaking produces different collisions between grains of the same or different sizes. Small granules fly off big ones until they meet other small granules. So after a while the small granules have separated out.