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Q & A: Trees in a windy day

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Most recent answer: 02/23/2017
Q:
Why do trees sway when winds blow?
- Mohor Banerjee (age 12)
Navi Mumbai,Maharshtra,India
A:

When wind is blowing, a force is pushing the trees in the direction of the wind. The wind flow is not constant, it changes with time, and hence the force is also not always there. That is why trees bend in one direction and then they return when the wind flow is reduced momentarily. They will sway until the process continues and will be static again when the wind stops. The amount of sway also depends on how strong the tree is.

Think of a pole firmly attached to ground. If you try to push the pole sideways near the edge which close to ground, it is not very easy to move it. However, if you push it at a point which is near the free edge, it is much easier. In technical terms, it is the "torque" that is being applied on tree tops due to wind is the responsible quantity for the sway. The torque is dependent on 1) the amount of force (here, how fast the wond is blowing), and 2) how far from the pivot the force is being applied (here, tree edge close to ground). By making either of these larger, you can make the sway larger. In our specific case, there are a series of torques working on different parts of the tree, and the net torque determines which way and how much the tree will sway.

Mithun


(published on 02/23/2017)

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