Physics is Used to Take Down a Tree

Today we took down the second big oak of the week.  This one was larger than yesterday's and caused us a little trouble.  One of the principle difficulties felling large oaks in a forest is getting them to fall where you want.  Any logger can tell you the general technique for felling a tree: You cut a notch on the side of the tree towards where you want it to fall.  Then you back cut the tree, sometimes driving a wedge behind your saw blade on a large tree to help tip it in the right direction.  Ideally the tree falls towards the notched side.  This works nicely on straight vertical pine trees where the center of mass of the tree is located directly above the center of the diameter of the tree.  On forest oaks this is almost never the case.  The center of mass may be as much as a meter or more away from the center of the base.  No mere notch is going to help make that tree fall anywhere except towards its center of mass.   We have had to put physics to work in order to reliably fell an oak where we want.  Our technique is to provide a force in the direction we want the tree to fall in order to compensate for the off balance force of gravity due to the lean of the tree.  For most trees the force needed is not too difficult to achieve.  We tie a rope around the tree as high as we can, typically 20 to 25 feet (6 - 7.5 m) up is adequate.  We then set up a come-along on the rope attached to any convenient anchor point, usually another tree trunk.  We cut the usual notch in the tree in the direction of the rope and we tighten up the come-along.  It doesn't take as much force as you might think.  For example, a 3000 lb. tree leaning at 5 degrees is only producing a torque of 261 lbs x the vertical distance to the center of mass of the tree.  So if we attach our rope anywhere near the center of mass of the tree we only have to pull with about that much force to pull it over.  Of course this is more than you could pull by hand but the come-along can pull with many times this force limited more by the strength of the rope employed than the come-along.  In the case of today's tree this technique was essential.  The tree looked fairly vertical but when we started to back cut it the tree started to lean in the wrong direction.  We actually had to increase the initial force we had with the come-along to get it to fall in the right direction but physics worked again and the tree fell right where we wanted.  Archimedes would have approved.

The tree of the day with the rigging rope attached.

The trunk of the tree was fairly vertical
but the branches were concentrated on one side .

For this big tree we had to put the 20 inch blade on our Stihl chainsaw.

A close up of the rigging in the tree.

The come-along being set up to pull on the tree.

The notch cut is made. 

With a bit of a pull the tree comes down where we wanted it to.

The behemoth on the ground.

This trunk will make excellent timber.

We got four sections of trunk from this tree which pretty much filled our new log crib.