Assume that, when we walk, in addition to a fluctuating vertical force, we exert a periodic lateral force of amplitude 25 n at a frequency of about 1 hz. given that the mass of the bridge is about 2000 kg per linear meter, how many people were walking along the 144-m-long central span of the bridge at one time, when an oscillation amplitude of 75 mm was observed in that section of the bridge? take the damping constant to be such that the amplitude of the undriven oscillations would decay to 1/e of its original value in a time t=6t, where t is the period of the undriven, undamped system. express your answer numerically to three significant figures.