A spring is hanging down from the ceiling, and an object of mass m is attached to the free end. The object is pulled down, thereby stretching the spring, and then released. The object oscillates up and down, and the time T required for one complete up-and-down oscillation is given by the equation T = 2π√m/k, where k is known as the spring constant. What must be the dimension of k for this equation to be dimensionally correct?