The initial ultimate BOD after mixing of the Bergelmir River is 12.0 mg · L−1. The DO in the Bergelmir River after the wastewater and river have mixed is at saturation. The river temperature is 10◦ C. At 10◦ C, the deoxygenation rate constant (kd) is 0.30 day−1 , and the reaeration rate constant (kr) is 0.40 day−1 . Determine the critical point (tc) and the critical DO. tc = 2.88 days. Critical DO = 7.53 mg · L−1

An engine, weighing 3000 n, is supported on a pedestal mount. it has been observed that the engine induces vibration into the surrounding area through its pedestal at the maximum operating speed. determine the stiffness of the dynamic vibration absorber spring in (n/m) that will reduce the vibration when mounted on the pedestal. the magnitude of the exciting force is 250 n, and the amplitude of motion of the auxiliary mass is to be limited to 2 mm note: in this question type-in right numbers, no decimals, no fractions, no unit. approximate to right number if needed