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25.69. Model: The charged ball attached to the string is the point charge. Visualize: The charged ball is in static equilibrium in the external electric field when the string makes an angle θ with the vertical. The three forces acting on the charge are the electric force due to the electric field, the weight of the ball, and the tension force. r r r r r Solve: In static equilibrium, Newton’s second law for the charged ball is Fnet = T + w + F3 = 0 . In component form, ( Fnet ) x = Tx + 0 N + qE = 0 N ( Fnet ) y = Ty − mg + 0 N = 0 N These two equations become T sin θ = qE and T cosθ = mg . Dividing the equations gives tan θ = −9 qE (25 × 10 C)(200,000 N / C) = = 0.255 ⇒ θ = 14.3° mg (2.0 × 10 −3 kg)(9.8 N / kg)