Climbing Mag physics lesson

What are they teaching the children in the schools these days?! From the "What's in a Quickdraw?" article:

In case it’s been years since your last physics class, here’s a quick lesson: A kilonewton is a measurement of force, which is the product of mass and acceleration, and it should not be equated to weight. If you’re hanging motionless from a draw, you have zero acceleration and therefore zero force.

So as long as I don't move it, I can hang Jupiter off my QD?! Damn those things are strong!!

They aren’t wrong. It’s just that, as in most things, it’s more complicated than the simple explanation implies. 

This author should probably revisit physics class...

Oh I see your problem. you read a garbage climbing mag article.

anonymous cowardwrote:

They aren’t wrong. It’s just that, as in most things, it’s more complicated than the simple explanation implies. 

That might be true sometimes, but in this case they are unambiguously wrong.

Anonymous Cowardwrote:

If you’re hanging motionless from a draw, you have zero acceleration and therefore zero force.

Can I get some of that antigravity please?  Sure would help my grade.

Yeah, the latest articles have gone from not-great to really terrible. It's like they are being written by 12 year olds.

Hard to tell if it is misunderstood physics or just bad writing. Maybe they need English class more than Physics class.  Saying "you have zero force" makes it sound like the force in question is an intrinsic property of the suspended person, a condition requiring intervention by Yoda.  If said person were to clove hitch a lock of hair to the draw and suspend from that, they'd get a different impression of what it means to have zero force.

I can’t wait until the next article where they tackle potential energy…

Edit: just to make it abundantly clear, an object does not need to be in motion in order to have a force acting upon it. Author needs to go back to statics 101. 

or politics

If you pick at a scab, it will bleed. The weird thing is, some people like doing that.

I'll take "Magazines that used to be cool," for 100, Alex.

Sweet, when I need to take I can just keep my body really still and viola, levitation 

What exactly did the author get "wrong"?  The article reads, "A kilonewton is a measurement of force, which is the product of mass and acceleration, and it should not be equated to weight."  This is true.  Then the author addresses the weight issue by saying, "Many people want to know how much weight a carabiner can support, but manufacturers typically won’t give you that info because it’s not applicable to climbers..." That's also true because we're rarely hanging static loads on our gear, we're falling. That's why everything in our gear shop is rated to KN and not lbs and kgs.  What did I miss, other than everybody hating on Climbing Mag?  

When you are hanging off something gravity is accelerating you down. When you hang from a draw using the rope, the draw supports both the force from your mass accelerated by gravity and the force from your belayer that prevents gravity to pull you down. So a draw probably sees between around 2 kN from hanging off it. 

Surely, since we only climb on Earth, we can leave gravity out of it and just accept that our weight is equal to our mass.  Then we can accept that the author is just differentiating between static weight and dynamic force.  

Or you can actually try to understand high school physics and the world be less of a magical place? The force on a draw is not zero if you are hanging from it. 

I guess your bathroom scale measures your weight in newtons.  Most folks can understand the intent of the article and the fact that Climbing Magazine is not a scientific journal.  

Yes, bathroom scales measures force and not mass, but who cares? It is however important to understand that you create force on gear that you hang from. It is also good if you understand taking on a draw is a bigger force that hanging direct from it.

Todd Jenkinswrote:

 That's also true because we're rarely hanging static loads on our gear, we're falling.

Speak for yourself 

If the body isn't moving or is moving at constant velocity (no change in speed or direction), then the net force on the body is zero, and I assume that was what the author was trying to say.  In the case of the climber hanging from a draw, gravity is pulling in one direction (accounting for the climber's weight) and the draw is pulling in the opposite direction with the same magnitude.  This is zero net force on the climber, and zero net force on the draw as well.  But the tension in the draw caused by the equal but opposite opposing loads is going to equal the climber's weight and could, in the case of, say, defective material, break the draw.  So there's zero net force but the loads break the draw.  If there's a moral, it's that the net force doesn't tell you about what the object is experiencing in terms of loads, it only tells you that the loads balance each other out.

This may be "high-school physics", but there are real subtleties involved, and a bit of humbleness might be called for.  I've seen professional nuclear physicists make mistakes (in conversation) about it.