4–86. The metal strap has a thickness t and width w and is subjected to a temperature gradient T 1 to T 2 ( T 1 < T 2 ). This causes the modulus of elasticity for the material to vary linearly from E 1 at the top to a smaller amount E 2 at the bottom. As a result, for any vertical position y , E = [( E 2 − E 1 ) > w ] y + E 1 . Determine the position d where the axial force P must be applied so that the bar stretches uniformly over its cross section.