Most arborists know about blocks, pulleys, rigging lines, SRT, and rope wrenches but how much do they know about physics? When we ask climbers coming to the Bartlett Arborist Supply open house they tend to laugh and snicker or roll their eyes. What many arborists don’t understand, is that we are confronted with physics right in front of our faces every day!
First off, let’s take a look at some rigging forces. According to Newton’s Third Law of Motion, Newton states that all forces exist in pairs. When one object exerts a force upon another object the opposing object exerts a simultaneous force upon the first. When written out, this can tend to be a bit daunting so let’s take a look at a diagram.
As you can see, the groundsman has lowered the log easily until it has come to a stop. The downward force of the log is equal to its weight at 100lbs. According to Newton’s third law the groundsman is holding an equal amount of downward force on the rope. If the groundsman is holding 100lbs and the downward force of the log is 100lbs, how much force is amounted at the pulley? 200lbs!
By realizing that the forces are doubled can make some workers think back about some trees or rigging points and wonder how in the world they held what they did! Many things factor into how much force is exerted at the rigging point.
Obviously this is in a completely perfect scenario with no friction at the rigging point. This is never going to be the case when it comes to rigging down trees. With a natural crotch rigging point our loads will be roughly 130 - 150% of the weight of the piece being lowered. With a highly efficient block the loads could be more around 175% or greater. This is still a great thing to know when looking for the ideal rigging point!
Check our next blog for ways to mitigate risk when choosing rigging points in a tree!