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When a rod falls on floor in such a way that the rod starts rotating. The work done by floor is zero as floor is not moving. But still, rotational kinetic energy is changing. Is it due to the fact that the only net work done is zero and Transitional negative work and rotational positive work cancel each other?

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  • $\begingroup$ What do you mean by translational negative work? $\endgroup$ – m__ Nov 15 '18 at 17:20
  • $\begingroup$ Having trouble visualizing what's going on with the rod. How does it fall and and in what manner does it rotate? $\endgroup$ – Bob D Nov 15 '18 at 17:22
  • $\begingroup$ The rod was vertically kept. It slid a little and tilted and started falling . With the floor tilting it more as it falls . $\endgroup$ – Aramaan meher Nov 15 '18 at 17:41
  • $\begingroup$ The rod interfaces to the floor with friction, there is a net force on the earth equal to the force on the rod. The earth is very large so the force is not noticed. $\endgroup$ – PhysicsDave Nov 15 '18 at 17:52
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The rotational kinetic energy changes (speed of the rod increases) due to the active gravitational force that acts (does positive work) upon the rod coupled with the slightly offset opposing force that the rod encounters at the contact point with the floor. Forces acting upon a tilted rod (excluding friction)

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  • $\begingroup$ I have doubt regarding the work performed by each of the forces . $\endgroup$ – Aramaan meher Nov 16 '18 at 4:49
  • $\begingroup$ Both rotational and Transitional and net work done by each force. $\endgroup$ – Aramaan meher Nov 16 '18 at 4:49
  • $\begingroup$ @Aramaanmeher Gravity does work on the rod. Or we could call that work potential energy. So: potential energy is converted into translational kinetic energy during the fall - and when in touch with the ground (see the sketch in the answer) potential energy is converted into both translational and rotational kinetic energy (because there is now a restriction at one end, being the normal force). The normal force still does no work, as it does not cause any displacement (no vertical displacement of the contact point it pushes at). $\endgroup$ – Steeven Nov 16 '18 at 12:35
  • $\begingroup$ @steeven in a book , it was said that the floor did positive rotational work . Was it wrong 😅. Just asking ? $\endgroup$ – Aramaan meher Dec 3 '18 at 12:55
  • $\begingroup$ @Aramaan The contact point isn't moving. In the frame of the Earth, no work is done on that point. Rotational work is torque times angular displacement: $$W=\tau\cdot\theta$$ and if you consider the centre of rotation as the contact point, then indeed there is no angular displacement. If you instead consider the point of rotation somewhere else, for example in the centre of mass, then you've chosen another frame and it does indeed look like normal forces cause angular displacement seen from this point. You can do this for mathematical purposes. But in generel normal force still does no work. $\endgroup$ – Steeven Dec 3 '18 at 16:07

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