FAQ: Climbing Physics  What's a Kilonewton (kN)?
Simple answer: 1 Kilonewton = 224.809 Pounds
Smart ass answer  1 Kilonewton = 1000 Newtons
Confusing answer  A Kilonewton is a measure of force. So
is a Pound.
Why is this so confusing?
When we see a number stamped on our climbing gear in Kilonewtons
(kN), it gives us a measurement of what kind of forces the gear
can withstand, or how strong it is. Understanding Kilonewtons is
difficult for us because we get confused between the two common
ways scientists describe how things behave when forces are applied
to them. Juggling the concepts of mass, weight, and force is not
simple, especially when you try to jump back and forth between two
different systems of measurement. In order to understand them you
must factor in time, gravity, and distance. While mass, length,
and time are absolute qualities, force varies relative to mass and
velocity. (Huh?, Don't worry read on, I just wanted you to fully
appreciate how confusing this can seem).
To explore these concepts deeper is to enter the science of physics,
more specifically the branch of engineering known as mechanics.
Much of our confusion with Kilonewtons comes from the situation
that there are two common systems of measure  The International
System of Units, or SI, and the English Engineering System,
sometimes referred to as US Customary Units. Believe it or not,
it's easier to work with Kilonewtons (a measure of force in the
SI system) than it is to work with Pounds (a measure of force in
the English Engineering System) when we are talking about the strength
of our climbing gear and evaluation of forces in climbing systems.
That's why we see the engineers express force ratings on climbing
equipment in the SI or International System of Units.
Understanding the two systems  SI vs. English:
There are four fundamentals in the science of mechanics  length,
time, mass, and force. Three of these fundamentals are defined.
Mass is defined as how much of something there is. Length
is defined as the distance between two points. Time is defined
as the interval between two events. These three absolutes are common
to both systems. Where the two systems differ, is how they look
at the fourth fundamental  force. Force is not an absolute
fundamental like the other three  rather, it is derived from the
result of the actions caused on a body by the values of the other
three fundamentals. The unit of force in the International System
of Units is the Newton. The unit of force in the English
system is the Pound.
In the SI system, the three fundamental quantities are mass,
length, and time. The units are kilograms (kg) for
mass, meter (m) for length, and second (s) for time. The force unit
is called a newton (N), and is defined as the force required to
accelerate a mass of 1 kg at a rate of 1 meter/sec. Mathematically
expressed;
1 N = (1 kg)(1 m/s²) or 1 N = 1 kg.m/s²
Even if the math is a bit beyond you, you can note that the SI
system uses the three fundamentals of mass, length, and time to
derive force.
In the English System, the fundamental quantities are weight,
length, and time. The English system complicates things
by using the weight of an object instead of a more
simple mass of an object. The weight of an object
depends on the gravitational attraction on it at the location of
interest. The units are pounds (lb) for weight, foot (ft) for length,
and second (s) for time. It now becomes more obvious how using the
English System complicates the mathematics involved and is not ideal
for the simple force calculations we use to evaluate climbing situations.
Why Kilonewtons are easier than pounds:
I didn't give you any of the fancy math equations for the English
System (yet), so I can keep your head clear long enough to point
out the obvious. In the SI system, we used the three fundamentals
of mass, length, and time to derive our force (N). Relatively simple
math. But looking closely at the fundamentals used in the English
System, we see length and time, but no mass. We also remember that
a pound is a measure of force. What we need to derive here
is mass, so we have all four fundamentals The unit of mass
in the English system is the slug. A slug is defined as the
mass which would be accelerated 1 ft/s² by a force of 1 lb.
1 slug = 1 LB x s²/ ft
So to use the the English system we have to get comfortable with
slugs. This alone should be enough to lead you towards the much
simpler SI system and an embrace of the Kilonewton.
What is Weight?
Weight is a measure of force. The measured force is gravity acting
on a mass. It is expressed in Newtons (SI) or Pounds (English)
Force = mass x acceleration
Weight = mass x acceleration due to gravity or W=mg
Where: W is Weight, m is mass, and g is acceleration
due to gravity
Approximate values for gravity on earth are:
g = 9.807 m/s² (SI units)
g = 32.17 ft/s² (English units)
The weight of an object is the gravitational force which is exerted
on that object which causes it to accelerate downward at the acceleration
due to gravity, or g. So, we can write for the weight of a 1 kg
mass:
W = mg
W = (1 kg)(9.807 m/s²)
W = 9.807 N
The weight of a 1 kg mass is 9.807 Newtons (usually rounded off
to 9.81)
Much of our confusion with Kilonewtons is our common misunderstandings
of mass and weight caused by carelessly interchanging the two systems
of measurement. We commonly refer to how much we weigh in pounds
or kilograms. For example, I weight 167 pounds or 75.9 kilograms.
While we all understand what this means, we don't realize we are
comparing a measurement of force (pounds) to a measurement of mass
(kilograms). To be accurate in the IS system, we would state our
weight in Newtons as 744 N (0.74 kN).
I arrive at this by using the formula W=mg
W = 75.9kg x 9.807m/s²
Weight = 744 kg.m/s² or 744 Newtons
While my mass would not change, my weight would be different on
the moon (27.8 lbs.), Mars (63.293 lbs), or in the space shuttle
(0 lbs.). The missing or assumed value in our statements is the
attraction of the force of gravity on my body. (Just for the record,
my mass in the English system would remain constant at 5.202 slug)
What does the 25 kN rating on my carabiner mean?
1 kN (kiloNewton) = 1000 N
25kN stamped on a product would be equivalent to 25000 N
I kN = 224 LB
25 kN = 5600 lbs of force
Conversion Factors:
English to SI
Length: feet x 0.3048= meters
mass : slug x 14.59 = kilogram
force : pound x 4.448 = Newton
Velocity : ft /s x 0.3048 = m/s
Acceleration : ft/s² x 0.3048 = m/s²
SI to English
Length: meters x 3.281= feet
mass : kilogram x .06854 = slug
force : Newton x .2248 = pound
Velocity : m/s x 3.218 = f/s
Acceleration: m/s² x 3.218 = m/s²
More information:
http://www.personal.psu.edu/faculty/r/c/rce2/mcht111/111intro.html
