Strain energy as half force times elongation

Steps:

• Strain energy U in a tendon equals (1/2) × tension force F × elongation δ for linear elasticity.
• F = 400 N from the problem.
• Elongation δ = 2 mm = 0.002 m (given in problem context).
• U = (1/2) × 400 × 0.002 = 0.4 J.

Why A is correct:

• Matches the formula U = (1/2) F δ, yielding exactly 0.4 J for the given values.

Why the others are wrong:

• B: Assumes δ = 4 mm, double the actual elongation.
• C: Assumes δ = 10 mm, overestimating deformation.
• D: Assumes δ = 35 mm, far exceeding realistic tendon stretch.

Final answer: A