Weight and lift: Chicken style

For reasons best known to their small chicken-brains, Harriet and Henrietta have decided to abandon the coup and roost in a tree. Maybe this is because it is rather hot in the coup at night, or possibly because a neighbour’s cat enjoys sitting outside the coup at six in the morning. (One day that cat is going to push his luck too far and find out what sixteen chicken-claws and two beaks feel like.)  Whatever the reason, they feel that a tree is the better place.

They manage to hop and waddle up their separate trees without too much problem and sit out on branches as thin as they dare go about three metres off the ground, presumably happy that they are safe up there for the night. Getting down again in the morning is more interesting. Henrietta usually chooses the same route down as up, through the low branches. But Harriet, suffering delusions of  aeronautical ability, takes a more direct approach. And believe me you don’t want to be in her flight path when she launches.

Her flight-time is beyond the ability to time with a stop-watch, but I’d say this morning it was about one second. Given that she has dropped, I reckon, about three metres, she can’t be generating much lift. A quick estimate can be made at this point.

First, recall that motion can be separated out into the vertical component and horizontal component. Under constant force (in this case gravity plus lift), the components don’t affect each other. That means we can use simple kinematics to work out her acceleration. For an object accelerating from rest, the distance it travels in a time ‘t’ is given by half times ‘a’ times ‘t’ squared, where ‘a’ is the acceleration.  If distance is about three metres, time about one second, then we obtain an acceleration of about 6 metres per second squared. 

That’s made up of gravity (10 metres per second squared) minus the contribution due to lift. This means the lift she’s generating with her barely coordinated flapping equates to about 4 metres per second squared. In other words, she’s managing to lift only about half her weight. Little wonder she has to climb the tree to get up it.




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