After a 10-lb weight is attached to a 5-ft spring, the spring measures 7-ft long. The 10-lb weight is removed and replaced with an 8-lb weight and the entire system is placed into a medium offering a resistance numerically equal to the instantaneous velocity. Find the equation of motion if the weight is released ft below the equilibrium position with a downward velocity of 1 ft/s. State your answer using only the sine function.