I find myself difficultly tired when I walk on smooth surface but on rough surface, I am easily tired. Is it because of friction force?
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If you mean rough like carpet versus smooth like tile…. Pick up your feet.
Otherwise a flat surface uses less muscles, sloped will engage more muscles. Rocky, snow, and such terrain would involve more or less muscles depending on the terrain.
On rough surface your body muscles are doing a lot more work balancing your weight that’s why you become slower and it becomes somewhat a bit more tiresome to walk.