Cause & effect
Friction, $f_s$ or $f_k$, and the normal force $n$ are obviously reaction forces, since they are never there unless something else pushes.1 Other forces are also obvious to recognise as either cause or reaction:
- When sitting on a sleigh and throwing a snowball, the snowball flies forward and you are yourself pushed backwards. Obviously, it makes sense to think about the throwing force as the cause. The snowball then reacts by pushing you backwards.
But sometimes it is not obvious at all which that is the action and which that is the reaction. It might be impossible to figure out:
- With gravity, the Earth and moon pull in each other. They both want to move closer together – not only the moon and not only the Earth. Both. It turns out that when Earth pulls in the moon, the moon reacts by also pulling in Earth.
Or is it the other way around: the moon starts out pulling in Earth, and Earth then reacts by pulling in the moon?
There is no answer to this. Sometimes it feels obvious which that is the action and which that is the reaction; other times not. But it also doesn’t matter. In all cases they both appear simultaneously – they both appear instantly; one never exists without its other force-pair half, not even for a brief moment.
‘Which is the cause? Which is the effect?’ That’s an ambiguous question for an endless bar discussion, much like ‘which came first: the hen or the egg?’.