Difficulty: Medium
Correct Answer: w L^2 / (2 A G)
Explanation:
Introduction / Context:
Total deflection of beams consists of bending (flexural) and shear components. For short/deep members, shear deflection may be significant. This question isolates the shear part for a cantilever with uniformly distributed load (UDL).
Given Data / Assumptions:
Concept / Approach:
For shear deflection, the incremental rotation due to shear is γ(x) = V(x)/(A G). The deflection at the free end equals the integral of γ(x) along the beam from the fixed end to the free end.
Step-by-Step Solution:
Shear force along the cantilever: V(x) = w (L − x).Shear slope: γ(x) = V(x) / (A G) = w (L − x) / (A G).Shear deflection at free end: δ_s = ∫_0^L γ(x) dx = (1 / (A G)) ∫_0^L w (L − x) dx.Evaluate integral: ∫_0^L (L − x) dx = [L x − x^2/2]_0^L = L^2 − L^2/2 = L^2/2.Therefore, δ_s = w L^2 / (2 A G).
Verification / Alternative check:
Dimensional check: w has units force/length; dividing by A G (force/area) yields length; multiplying by L^2 gives length → consistent.
Why Other Options Are Wrong:
w L^3 / (3 E I): that is a bending-deflection form (flexural, not shear).Other choices misplace coefficients or powers of L.
Common Pitfalls:
Mixing shear deflection with flexural deflection, or forgetting the linear variation of V(x) in a cantilever under UDL.
Final Answer:
w L^2 / (2 A G).
Discussion & Comments