showsyour noteraisestry itfor exampledefinessee also

explanation

How does a forest grow?

Trees grow in partnership with what lies beneath them: roots, fungi and the soil they share.

diagram

What feeds a tree

note

Fungi trade sugar for water?

Ask about this next.

definition

Mycorrhiza

A fungus living on a plant's roots, swapping minerals and water for sugar.

example

After the 1988 Yellowstone fires

Lodgepole cones open in the heat. Seedlings carpeted the ground the next spring.

misconception

Trees only compete for light

A forest grows through sharing as much as rivalry.

explanation

Why do bridges vibrate?

Every structure has rhythms it prefers. Push in time with one and small forces add up.

example

London's wobbly bridge

On opening day in 2000, the Millennium Bridge swayed as walkers fell into step with it.

data

Footsteps and sway

Force	Rhythm
Steps, up and down	2 Hz
Steps, side to side	1 Hz
Bridge sway, 2000	1 Hz

note

Is this how a glass shatters?

Singing at just the right pitch.

question

Why do soldiers break step on a bridge?

Marching in time can push a bridge at its own rhythm.

example

Tacoma Narrows, 1940

Wind set the bridge twisting until it tore apart, four months after it opened.

question

Do trees feed seedlings?

Some studies say so. The evidence is debated.

explanation

Why chords sound sweet

Notes whose frequencies form simple ratios blend into one sound.

definition

Natural frequency

The rate a thing swings at when nudged and let go.

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