When a 2.90-kg object is hung vertically on a certain light spring described by Hooke's law, the spring stretches 2.68 cm. (a) What is the force constant of the spring? N/m (b) If the 2.90-kg object is removed, how far will the spring stretch if a 1.45-kg block is hung on it? cm (c) How much work must an external agent do to stretch the same spring 6.50 cm from its unstretched position? J