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I was considering how a kayak travels through water, and the thought processes I came up with drove me to this question. Here's what went through my head (keep in mind, what went through my head about how a kayak works could be totally wrong, but nonetheless it's what I thought):
A person with two oars in his/her kayak can be considered as one collective mass.
Since this is a collective mass, if the person turned the boat with his left oar, there is a restoring force pushing the system in the exact opposite direction of the oar's force vector, which is towards the person, so that the water's restoring force vector is away the person.
- Since the system is a collective mass, this causes a torque on the mass, causing it to rotate in the positive direction. This makes sense, since the boat turns to the right if you drive the left oar.
This made sense to me, since the farther you drive the oar from you, the easier you rotate (which should make sense since torque is directly proportional to the length of the radial vector).
However, this, in my view, shouldn't drive the boat forward, but merely cause it to rotate. Even when you use the left and right oar one after the other like one would do to go forwards, in my mind this would merely make it turn left, then right all in place, each time an oar is driven.
In my view, only a force that is placed on the object's center of mass can cause it to move linearly instead of causing a torque and rotate it. However, this seems to not be true in reality. How should I expand my view of this, and my knowledge of torques, to explain how applying a torque can accelerate an object linearly (albeit not with as much force as it would if it were applied at the center of mass, I'd assume)?
Oh, and just as an aside, when I say "drive the oar" it's just a for-lack-of-a-better-term of using the oar to push the boat forward. I guess "row" is synonymous.