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# Physics Static Equilibrium Problem force involved in standing tiptoe?

this is the Q

http://store2.up-00.com/2015-01/142066132351.jpg

i hope to get the answer and what did you did to get that result

### 1 Answer

- civil_av8rLv 75 years agoFavorite Answer
The (2) eqations for statics

Sum of the forces in any direction = 0

Sum of the moments about any point = 0

These (2) equations are really about all you need for any statics problem.

Start with a moment balance about F3. The reason we do that is because the lever arm for F3 about F3 is 0 and we can eliminate that variable.

Sum of the moments about F3 (CCW is +) = 0

0 = F2*r3 - F1*(r1+r3)

Solve for F2

F2 = F1*(r1+r3)/r3

Plug in numbers

F2 = 784 N * (12 + 6 cm) / 6 cm = 2352 N

Now do a force balance

Sum of the vertical forces (up is positive) = 0

0 = F1 - F2 + F3

F3 = F2 - F1

Plug in numbers

F3 = 2352 N - 784 N = 1568 N

Note: You could have done the moments about F2 to start with, but the answers would still be the same.... I checked.

Hope I helped explain it.

F1*r1 = F3*r3 comes from doing a moment balance about F2. F1 spins clockwise, F3 spins CCW about F2.

0 = F3*r3 - F1*r1

F3*r3 = F1*r1