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21. A horizontal spring attached to a wall has a force constant of k = 8.50 X 102 N/m. A block of mass m = 1.00 kg is attached to the spring and rests on a frictionless, horizontal surface as in Figure P1321. (a) The block is pulled to a position x; = 6.00 cm from equilibrium and released. Find the potential energy stored in the spring when the block is 6.00 cm from equilib rium. (b) Find the speed of the block as it passes through the equilibrium position. (c) What is the speed of the block when it is at a position sci/2 = 3.00 cm? l': l: NWNNWWWWM i T x=0 x=x-/2 x=xi Figure P13.21