Jeff weighs 890 N. The shear strength of the wood is $9.50\cdot 10^6\, Pa$. The cross-sectional area of the plank is $5.16\cdot 10^{-3}\,m^2$. The plank is $20\, m$ long. Assume that the wall exerts two vertical forces on the plank.

The wall exerts an upward force at the point where the outside edge of the wall contacts the plank and a downward force where the top of the plank contacts the inner edge of the wall (the wall is $10\, cm$ thick) see figure. Ignore the mass of the plank.

How far can he safely walk on a plank that is rigidly attached to a wall?

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