Firstly, I'll explain what makes us fall so easily while we are simply standing
still in our skates before going on to show why we are more likely to fall
backwards.
Why do we fall?
Put basically, it is because your Centre of gravity (CoG) moves either behind
back wheel or in front of your front wheel. As a result, there is a resultant
force (in Newtons) due to gravity:
sin(A) * m * g
A = angle between (line of back wheel to CoG) and vertical
m = your mass (kg)
g = acceleration due to gravity (approx 9.8 ms^(-2))
This force acts at your wheels because this means the least change in
momentum at CoG.
As we all know, it doesn't take a lot of force to push a skate along, so
your foot accelerates forward quickly. This increases 'A', and therefor
the force acting at your wheels. Result - positive feedback loop, and a
fall.
The same could be said for falling forwards from stationary, but for 2 points:
When relaxing and standing up straight, your weight is more on your
heels than on your toes.
It is easier to correct if your CoG goes too far forwards, by lifting
and putting a foot further forwards. If done quickly enough it
will go far enough in front of your CoG to give you a chance to to
regain your balance. It is much harder to make this correction when
falling backwards.
