What is your displacement if you walk 100 m north, 20 m east, 30 m south, 50 m west, and then 70 m south?

Displacement isn't total distance traveled, it's the distance from the original starting point. So the 70 meters South in addition to the 30 meters South would decrease the distance to the North. It would then, instead of being 100 meters North and 100 meters South, there would be zero displacement. Then you take 50-20 to get 30 meters West. Your final displacement is 30 meters.

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