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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