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I'm trying to model a rigid bar lifting from the floor. The bar has A and B extremes. Consider that I just lift totally vertical from A point. The point of my question relies on 'when does the bar lose the contact with the floor?'.

If the lifting force is low enough, I think I can model like the upper model of the drawing. However, if the bar is lifted fast, point B loses the contact with the floor before (lower case in the drawing). How can I calculate the moment when point B loses the contact with the floor, depending on the vertical velocity of point A? For simplicity, don't consider friction.

enter image description here

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    $\begingroup$ Any thoughts yourself? What have you tried so far? $\endgroup$
    – JMac
    Commented Jul 16, 2019 at 11:46
  • $\begingroup$ I think you can't ignore friction. On a frictionless surface, the point B moves until AB is completely vertical. Friction plays a role in the early lifting of point B. $\endgroup$
    – polfosol
    Commented Jul 16, 2019 at 11:52
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    $\begingroup$ @polfosol That's actually not necessarily true. There's no reason that point B has to remain fixed to the ground, friction or not. I know at least one way to look at it without friction; I just would like to see more input from OP so that I'm not just giving the answer away without critical thinking. $\endgroup$
    – JMac
    Commented Jul 16, 2019 at 11:55
  • $\begingroup$ I don't mind friction at this time. I just want to calculate how to calculate the lost of the contact point, depending on the lifting velocity of point A $\endgroup$
    – galtor
    Commented Jul 16, 2019 at 12:08
  • $\begingroup$ @galtor Do you have any ideas on what you might expect, or why you might expect point B to lose contact with the floor? It's an interesting question, I just don't like giving away all the answers unless it's clear you've given it some thought. Is there anything about it that you can't wrap your head around that an answer could help clear up? $\endgroup$
    – JMac
    Commented Jul 16, 2019 at 12:36

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Surely, with a vertical bar, at the point that the bar loses contact with the floor its full weight will be hanging. If you then lift it a nano-meter higher it should still weigh the same, (within reason) The bar loses contact with the floor when friction is nill, and this should be reflected in the weight. If the bar is lying horizontally friction will be much more and lifting it should take more energy, due not only to increased friction but to the fact that it is lower down to start with.

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    $\begingroup$ How does this help model a bar lifting off the floor? $\endgroup$
    – JMac
    Commented Jul 16, 2019 at 12:38

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