Tuesday, 20 March 2012

How many kg in a kN?

Quick physics lesson today.  Often we see carabiners, rings, rope, etc. rated for a certain number of kilo Newtons (kN).  What does that number mean and how does it affect me?

The quick and dirty answer, working with normal gravity, is that 1 kN = 101 kg = 224 lbs.  These are conservative masses, meaning I have rounded down.  To convert kg to kN, multiply by 0.00981, to convert kN to kg, multiply by 101.97.

Why is this so?  Glad you asked.  While a kg is a unit of mass, a kN is a unit of force.  The unit of measurement for kN is the Newton, defined as the force of earth's gravity required to move 100g (0.1 kg).   Since force is defined as mass multiplied by acceleration, we must apply some sort of acceleration to our unit of mass (kg) in order to get kN.  Usually, this acceleration is gravity (9.81 m/s).  With this in mind, we can see that a 1 kg mass under gravity is equal to:

  • F = m * a
  • F = 1 kg * (9.81 m/s)
  • F = 1000 g * 9.81 m/s
  • F = 9.81 N
  • F = 0.00981 kN
So, as an example, that climbing item you have rated for 20 kN would not be able to take more than 2039 kg under gravity. 

10 comments:

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Anonymous said...

Shouldn't units for acceleration be m/s^2?

mpc said...

So impressed. Many many thanks for this. Plain and simple language, great explanation and exactly what I wanted to know. No time now but I'll bookmark your blog and return to browse at a more leisurely pace.

Seafarer said...

Thanks! Informative.

Anonymous said...

Excellent post, exactly the answer I wanted to know, was very confused before this, thanks.

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