Two boards are joined by gluing along a scarf joint, as shown in the figure. For purposes of cutting and gluing, the angle α between the plane of the joint and the faces of the boards must be between 10° and 40°. Under a tensile load P, the normal stress in the boards is 4.9 MPa.
(a) What are the normal and shear stresses acting on the glued joint if α = 20°?
(b) If the allowable shear stress on the joint is 2.25 MPa, what is the largest permissible value of the angle α = 20°?
(c) For what angle α will the shear stress on the glued joint be numerically equal to twice the normal stress on the joint?

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