They seem like very similar concepts to me, both inferring that all actions have consequences. It seems Karma is roughly like an extension of Newton's Law, it goes beyond the current moment and includes the statistical probabilities of actions that are out in the world. If I agitate the world, there is more agitation as a consequence. If I calm the world, there is more peace for me to enjoy. If I strike an object with my fist, it strikes me back. If I gently nudge and object, it gently nudges me back. If I send anger and violence out into the world, anger and violence are my consequence. If I send joy and happiness out in to the world, joy and happiness are my reward.
There is a clear difference between the two in that Newton was dealing with physical matter and their motions, while karma deals with the moral dimension of our actions and experience. But there also seem to be some similarities between the two concepts.
I know this is a bit of a stretch, but my general theme is that it seems to me one doesn't need a "magical" force to understand Karma, and it is not a stretch of the imagination to realize that consequences always come around sooner or later. I know sometimes in life it feels like you can "get away with" something, but it seems like there is always some consequence, even if it's just guilt, shame, and lost night's sleep, or a hit to one's reputation. More often than not, we really do face the direct consequences of our actions. I never seem to "get away with" anything!!

(Edits in red)