Finally- scientific proof that you need a lighter load

Finally- scientific proof that you need a lighter load

12 May 2010

You wouldn't dare argue with Sir Isaac Newton would you? I know he got bonked on the head with an apple but we all do daft things.

The main concern for us walkers is that Newton discovered the reason we need to lighten the load of our rucksacks.

This is the massive universal force that must be overcome by our poor little muscles to get from Seatoller up to Scafell Pike. It is also the force that makes our ligaments squeak on our return to Borrowdale.

Newton, being not only clever but kind, gave us a lovely equation to work out this force weighing us down:
Force = (persons weight + rucksack weight) x force of gravity (9.8m/s2)

Let's put some figures in:

I weigh 80kg, and assuming my 40 year old knees were 10 years younger and able to carry a 20kg rucksack, this is the force I would be subjected to if I was walking along a flat road (of which there are none in Cumbria)

Total force = (80+20) x 9.8 = 980 newtons

What if I throw the plasma tv and deck chairs out of my rucksack to get my pack down to 10kg:

Total force = (80+10) x 9.8 = 882 newtons

Big deal I hear you say. I've halved my pack weight and only got a 10% reduction in force. Wait, before you put the chairs and tv back in, I forgot to mention another important physics concept.
Centre of Gravity
The above force calculation assumes that my centre of gravity doesn't change with a loaded rucksack. But as you know, it does.

The centre of gravity of the rucksack and the centre of gravity of the hiker are different. This creates a rotational force on the hiker called a torque. This torque, as well as the normal gravity forces, must be dealt with by the hiker's body.

The torque is the force exerted by the body, against the rucksack, to remain stable. Essentially, this is the strain on the back and shoulders.

Because we are being scientific I feel the need to throw in another equation. Get ready… here we go then:
Torque on the body = rucksack weight x horizontal distance between the centre of gravity of the pack and the centre of gravity of the body
So, for a 20kg rucksack that has a centre of gravity 25cm away from the body's centre of gravity:

Torque on the body = 20 x 25 = 500 kg-cm

Let's try it again with the same centre of gravity but a 10kg rucksack:

Torque on the body = 10 x 25 = 250 kg-cm

This is 50% decrease in strain due to torque.

What if we were then to change the centre of gravity by using a thinner rucksack with the centre of gravity only 15cm from the bodies' centre of gravity:

Torque on the body = 10 x 15 = 150 kg-cm

This is 70% reduction in strain on the shoulders and back.

So, 300 odd years ago, Isaac proved that the effects of gravity and torque can both be reduced by lightening the load in your rucksack.
You're all torqueYou're all torque
So, listen to Isaac, he says clever things like:
'every particle in the universe attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them'

Which for us thickos means: take the heavy stuff out of your bag or it'll break your shoulders.

P.S. look at sexy Issac on the left, he's obviously spent a couple of hours before this portrait with Trinny and Susannah

Thanks for reading.

Ian Young

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