# Homework Help: Min force to open the water gate

1. Feb 26, 2016

### werson tan

1. The problem statement, all variables and given/known data
why Fr(Lp) = FB / 2 ??? can someone help to explain ?

2. Relevant equations

3. The attempt at a solution
shouldn't it be 2FB = Fr(Lp) ? since FB act on 2 sides of the gate ...

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2. Feb 26, 2016

### haruspex

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.

3. Feb 26, 2016

### BvU

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 ?