By Newton's second law, the net force on the object is
∑ F = T - mg = - ma
where
• T = 25 N, the tension in the string
• m is the mass of the object
• g = 9.8 m/s², the acceleration due to gravity
• a = 2.0 m/s², the acceleration of the elevator-object system
Solve for m :
25 N - m (9.8 m/s²) = - m (2.0 m/s²)
==> m = (25 N) / (9.8 m/s² - 2.0 m/s²) ≈ 3.2 kg
