Min force to open the water gate

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The discussion centers on the equation Fr(Lp) = FB / 2, with participants questioning the reasoning behind it. One contributor clarifies that the force FB only acts on one side of the gate, hence the division by two. There is also criticism of the diagram's accuracy, particularly regarding the representation of the hinge and the horizontal line meant to indicate a sturdy structure. The drawing is described as potentially misleading, leading to confusion about the forces at play. Overall, the conversation emphasizes the importance of clear diagrams in understanding physics problems.
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Homework Statement


why Fr(Lp) = FB / 2 ? can someone help to explain ?

Homework Equations

The Attempt at a Solution


shouldn't it be 2FB = Fr(Lp) ? since FB act on 2 sides of the gate ...[/B]
 

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It's Fb/2 because F acts at b/2 from the hinge. It only acts on one side.
Fr acts at Lp from the hinge on the other side.
 
Author sure has a funny way of drawing a 45 degree angle !
And conspicuously does not draw a hint of a hinge at B.

Furthermore, the horizontal line at 3 m is supposed to designate something sturdy that has water above and air below ? Quite something different from the liquid surface at 4.2 m; more like the stuff the gate itself should be made of. Deceptive drawing, incompetence, sloppiness ?
 
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Yes, that makes sense. Thank you for pointing that out.
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