Determine the length that a lead shield should have in order to to stop 99% of a beam of γ-rays of 0.5 MeV. (λ1/2 (γ, Pb)= 0.42 cm.)

Why does the equilibrium position of the spring change when a mass is added to the spring? will the mass oscillate around the new equilibrium position of the spring or the previous position without a mass attached to the spring? if the equilibrium position of the spring changes by 20 cm (assuming no initial mass) when a mass is added to the spring with constant 4.9 kg/s^2, what is the mass of the object attached to the spring?