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This is my guess. The force that you are using to push yourself off the boat and onto the dock pushes the boat away from the dock. This is due to the direction of the force that you applied. I'm not sure if this is what you're asking for, but I tried :)
i need to explain how this is relative to newton's third law of motion which is that for every action force, there is a reaction force equal in magnitude, but opposite in direction.
The action is you stepping off the boat onto the dock. The reaction is the boat being forced away from the dock.
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so then technically I'm applying the same force stepping on the dock as i am stepping off the boat? that's why the boat accelerates the opposite direction I'm going towards.
The same force that is propelling you forward off the boat, towards the dock, is also propelling the boat away from the dock.
You could think if it in terms of rowing as well. The force of you pushing the water backwards with your oar propels the boat forwards. Likewise, pushing the boat backwards in the water (by jumping or stepping off of it), will propel you forward.