I agree with Dave Looman, I will expand on some details in my description below.

Place the sandbag tangent to the enclosure using the CAD system (or Part Transform in Mechanical) and assign the sandbag an Initial Velocity equal to the impact velocity.

Calculate the impact velocity separately using either equations for pendulum motion, an online calculator, or build a model in Rigid Dynamics.  In Rigid Dynamics, use a single body the size of the sandbag (no enclosure). Make sure the density is adjusted so that the mass is equal to 15 kg. Place a Revolute Joint to ground at the location where the string would be anchored. Turn on Standard Earth Gravity to accelerate the pendulum from rest. Plot the velocity of the center of the body vs the angle of the Revolute Joint. Set the solver to take small time steps so you get plenty of results in the Tabular Data.  Interpolate the table for the angle at impact to obtain the impact velocity.