The Physics of Climbing Trees

To climb the tree, first learn to climb the rope
To climb the tree, first learn to climb the rope

One of the appeals of teaching in a sixth form college was the fact I no longer had to teach physics. I could remain within the boundaries of my biological specialism, my main interests and passion. Physics was often too mathematical and abstract for me to find inspiration and excitement in. Translocate to eight years later and I find myself in the breezy National Trust gardens of Stourhead, Wiltshire learning about angles, loads and forces and I am engrossed. My aim? To competently set a rigging system of ropes, slings and karabiners that will allow me to safely climb into the upper most branches of a beautiful old Beech tree. Physics will enable me to explore the splendour of the biological world found above my head. Not only do I have to get my head round the equipment jargon of Maillons, Jumars, Gigris, Crolls and Ducks (not the feathered type) perhaps more importantly I need to get my head around the Physics that will keep me safe. Canopy Access Limited are enlightening me to a new take on Physics through their Basic Canopy Access Proficiency Course. Climbing trees has become a lot more technical than I remember. As a child I would seek out what I called the ‘monkey trees’, haul myself up into the branches and stretch out carefree amongst the leaves without a second thought to safety. This time however I am aiming for slightly higher branches.

The climbing frame of a beautiful Beech tree
The climbing frame of a beautiful Beech tree

First, I must consider the angle at which to catapult my rope over the chosen fork in the tree. The combination of the strength of the rubber, the stretch of the rubber and the weakness of my biceps makes holding this catapult steady at the correct angle tricky to say the least. Second, I need to decide how much I should weight the rope. It cannot be so heavy that it never reaches the fork but yet not too light that it doesn’t fall to the ground on the opposite side. Third, I must consider that the force applied to the rope at the fork (the top anchor) will be twice the load (the climbers’ weight) due to the ‘Pulley Effect’. The branch supporting this top anchor can also be considered as a lever, introducing the idea of pivot points and mechanical forces. Basically aim to keep that pivot point close to the trunk so there are no nasty surprises.  Fourth, consider all rigging angles at ground anchors. An anchor basically secures the end of my climbing and safety rope. I must not have any rigging angle greater than 90o. Why? Obtuse angles greater than 120o will result in the generation of forces greater than the load. This can be calculated through force vector analysis. The load being me hanging on the end of a rope and the forces being those exerted on the equipment that is stopping me from hitting the ground. Important. Furthermore, I should consider the effect of angles on the ability of a rope or sling to retain strength. Fundamentally know my knots (figure of eight, double fisherman’s, alpine butterfly, barrel and clove hitch), some of which can reduce the strength of the rope by up to 30%. Whatever I do, I must avoid ‘Lark’s feet’, the formation of acute angles on a single rope or sling by re-threading. This will dramatically decrease the strength of the rope, a bad habit picked up through rock climbing.

Slings, karabiners, rigging angles, Grigris and knots securing the rope at the ground anchor point
Slings, karabiners, rigging angles, Grigris and knots securing the rope at the ground anchor point

Although many of the rules I am now being taught are general rules of thumb rather than exact calculations of force vector analysis, the concepts to understand remain the same. Of course the ropes, slings and karabiners that I am reliant on all have very high loading parameters. Stamped on to the side of the steel karabiner is the minimum breaking load. As the load (i.e. the climber) is not a static weight but a moving dynamic load the minimum breaking load is given in Kilo-Newtons (kN). Imagine jumping up and down on a scale and seeing the needle swing wildly from one extreme to another rather than showing your actual weight and you get the idea of the load on the end of the climbing rope. So this karabiner I am using may break above 30 kN of force. Protocol (sensibly called the Factor of Safety) states to only use a tenth of this so as to work within safe limits, so 3 kN which translates to 300 kg of weight. I doubt my 64 kg is going to cause many problems. Even when I calculate the Peak Impact Force of around 1.3 kN that I am able to generate by bouncing up and down on the rope it is way below the safe limits. Yet with feet dangling into the air, hanging from a rope slung over a fork in the tree and 20 m above the ground it is best to be secure in the knowledge that I have Physics on my side.

Suspended in the harness, reliant on the strength of the tree, the equipment and the physics
Suspended in the harness, reliant on the strength of the tree, the equipment and the physics

So I ask the question: why do so few students wish to progress with Physics into either further or higher education? Because considerable amounts of classroom Physics can be boring: mathematical formulas, diagrams and calculations. Teach the wider applications. Inspire the students, show them the adventures and fun that can be had through grasping Physical concepts. Illustrate to students the thread connecting the sciences, the link between Physics and Biology. It is Physics that is enabling me to explore and understand the arboreal world which I found peace in as a child. Giving me a new perspective on life forms we are generally familiar with only from ground level. Allowing me to discover life and biological laws that formulate this remarkable arboreal environment. And the next stop? The huge Dipterocarp trees dominating the rainforests of Borneo.  If only I could take the classroom with me.

Stourhead is home to an amazing woodland with an eclectic mix of old and new trees
Stourhead is home to an amazing woodland with an eclectic mix of old and new trees

 

One thought on “The Physics of Climbing Trees


  1. Spot on Stephanie! A great read and I’m so glad you got so much from the course. Good luck out in Borneo and would love to see some photos when you get back. All the best and stay safe, James

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