A woman stands in the middle of a perfectly smooth, frictionless, frozen lake. She can set herself in motion by throwing things, but suppose she has nothing to throw. Can she propel herself to shore ?without? throwing anything?
Solution 20DQ As the surface is frictionless, she cannot move herself by standing on it. If air would be there then she can propel her hands backwards to generate some momentum in the forward direction. Anyway, the momentum must be conserved as there is no external force. If the air density is negligible and she has nothing to throw, she can never make it to the shore. Because by throwing stones, she has added momentum to the system which includes herself and the stones. The total momentum must be zero as it was zero in the beginning. So, she could move in the opposite direction of the motion of stones. But without it she can’t.