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λ=v/f
v=λ•f
T=m₀•v² = m₀• (λ•f)²,
T=M•g,
m₀• (λ•f)²=Mg,
M= m₀• (λf)²/g =
=8•10⁻³(1.8•52)²/9.8 =7.15 kg