Newton’s laws in action

For reasons best known to their little chicken-brains, Hyacinth and Brigitta (our chickens) have decided that their coop is no-longer the des-res that it once was and a far better location for a night in the wind and rain is on top of the garden shed. The problem is that neither (especially Brigitta) is particularly proficient at flying yet, so getting on top of the shed is right at the limit of their abilities.

Watching them in the evening is hilarious. They strut around the base of the shed, looking very agitated (probably because they’ve got an audience of me, Karen and the cat), trying to locate the best launching spot. Despite being the least able, it’s Brigitta that always goes first. She’ll crouch, then spring into the air – then out come the wings and its manic flapping and squarking all the way to the roof of the shed. If she misjudges it, she’s left grabbing at the grape vine and runner beans that climb up the side of the shed, and she hauls her way the final few centimetres to the top.

Once Brigitta’s settled down, Hyacinth has a go, usually rather more safely, but still far from elegantly.

While their abilities in locating safe, quality housing are somewhat questionable, their understanding of Newtonian mechanics is impeccable. To reach the top of the shed, they need to do work against the force of gravity. This is first done by the initial jump – they give themselves initial kinetic energy by pushing from the ground. By Newton’s third law, as they push downwards, the ground pushes back at them, and they are propelled upwards. The greater the velocity obtained in the initial jump, the more likely they are to be successful.

The next stage is the flap. Here they are using their wings to provide an upward force, in opposition to the downward force provided by gravity. Force is the rate of change of momentum. To provide a force now, they are propelling air downwards, rather like a helicopter does.  The more downward momentum they can give to the air, the correspondingly greater the upward force they are going to experience, and, again, the greater their chances of making the roof.

 I don’t think even Hyacinth is yet capable of generating enough upward force with her wings to balance the force of gravity. That means that, while she is in the air, her net acceleration is downwards. If she’s moving upwards, because of the initial jump, she will find that her upward velocity reduces. If she didn’t land on the shed, but kept flapping, eventually that upward velocity would drop to zero, then it would become a net downward velocity, and she’ll land on the ground again. That means there’s a limit to how high she can get.

Evidence for this comes from watching them get down from the shed in the morning. Again, its frantic flapping as they try to control the speed of their descent. Both end up hitting the ground harder than I’m sure they’d like to. Even still, it’s the shed that gets their vote for a wet and windy night. Strange things.




Leave a Reply