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Topic Date User

Rope - static v dynamic for top-roping and leading 30-Jan-2012 At 11:11:56 PM Olbert

Message

On 30/01/2012 Dan Theman wrote:
>On 30/01/2012 Olbert wrote:
>
>>Ummmm - no. No and no. Isaac Newton had a good deal to say about simple
>>motion, the forces involved in climbing are just an application of his
>>theory.
>
>I was joking. Playing on the way OGH worded the sentence.

It wasn't a very good joke. It implied that Newtonian motion shouldn't be applied to a climber falling and getting caught by the rope situation - this is false. I guess you did it because you thought ODH was wrong...he wasn't.

>>
>>Dynamic ropes absorb energy.
>>Dynamic ropes do not absorb force.
>
>humm you sure about this? Force = mass * acceleration. when you (mass)
>fall and accelerate due to gravity you generate a force. A large amount
>of this force is then absorbed by the rope (depending on how much is out
>of course) and is stored as potential energy in the rope.
>

Yes. I am sure. Thank you for the highschool level demonstration of physics. You are correct in saying that F = ma. You are, however, wrong in applying it. Objects, at any speed, do not have a property known as 'force'. They have 'energy' in various forms - heat, kinetic, chemical etc.

Let me repeat. When an object falls, it doesn't have a 'force'. Gravity is a force, it has been acting on the falling object, making it fall.

When the rope becomes taut and the object starts the slow down, a force is applied to both the object and the rope. The anchor at the top of the rope experiences the exact force in the opposite direction as the object which is being caught by the rope. This is an application of Newton's Third Law - for every action there is an equal and opposite reaction.

The reason the force is less on the top anchor point, as well as the object being caught, with a dynamic rope is because the acceleration is smaller. You can use simple motion physics formulas to relate the distance, acceleration and velocity: v^2 = u^2 + 2as. The final velocity, v, is zero, and the initial velocity, u, is the same with a static or dynamic rope. With a dynamic rope the stopping distance, s, is longer and so the acceleration, a, is smaller. With a static rope, the stopping distance, s, is shorter, so to keep the equation balanced, a must be larger.

Now to get back to the original equation - F = ma. So with a dynamic rope has a smaller acceleration, a, and the same mass, m, as a static rope. The force, F, the falling object, and the anchor, experiences is therefore smaller as well with a dynamic rope.

Nowhere in this situation is 'force absorbed by the rope'. That is completely and utterly wrong. Seriously, it's wrong. Even if you can't understand that it is wrong, please just trust me. It's wrong. Actually, the statement 'force is absorbed by a' is just nonsensical in the physical sense (ie when talking about physics (when the news reader says "and the wall absorbed much of the force of the blast" he actually means energy)).

The rope, does absorb energy. Energy does not equal force. They are different things.