ELI5: Why does running your finger down the back of a hall make it roll forwards then backwards?

  1. When you put your finger on top of the ball and push down, you are creating a force and the direction of that force starts at your finger and runs through the center of that ball into the ground, going no where.

  2. You're simultaneously shoving the ball forward, and spinning it backwards. It skids forward from the shove, then starts moving backward when the shove has lost its momentum and the spin takes over.

  3. I would first define rolling as moving along a surface without slipping (this is how a physicist wouldefine rolling). So if you take, say, a toilet paper tube and stop it from rotating by gripping it and slide it along the ground, it's not rolling, it's slipping. If you open your palm and push it along the ground it is rolling, not slipping (in real life, everything slips, even if it's microscopic.

  4. You're creating forward momentum, with lots of back spin. When the forward momentum dissipates, the back spin can gain traction, which causes backwards momentum.

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