Now to the question. How can the ducklings get away with falling from a tall tree without getting hurt? Even with a cushion of leaves, if we as people attempted something like this, we would get badly hurt if not killed. So how do the ducklings get away with it?
The short answer is that 1) they don't hit the ground as hard as we would and 2) they are sturdy little animals.
1) is in the realm of physics and deserves some more explanation. Why is it that the ducklings hit the ground less hard than we would? It's a question of terminal velocity and also a matter of how large a force the ground exerts on the ducklings to stop their fall.
When something falls toward the ground, it speeds up. At the same time, air slows down the fall. The drag due to air gets stronger and stronger as the speed of the object increases. Eventually, an equilibrium situation is reached, where the air drag exactly balances out the downward pull of gravity. The object quits accelerating and falls at a steady, maximum speed, called terminal velocity.
Terminal velocity depends on a number of things and can be higher or lower depending upon the situation. For ducklings, terminal velocity is not as high as it is for humans, because ducklings are a lot lighter than humans. If a duckling and a human jump off a tall tree, they will both reach their own terminal velocity. For the human, the higher terminal velocity means that they will hit the ground first, and also harder.
Because of their smaller speed and smaller mass, the ducklings have less momentum than a person would. Having less momentum when they land, they require less force from the ground to come to rest below the tree. Less force means a softer landing.
Now your 3-year old may ask "How do the ducklings know they will be fine?". I can't help with that.
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