A Russian submarine and a US submarine move toward each other during maneuvers in motionless water in the North Atlantic. The Russian sub moves at speed v_R = 60.0 km/h and the US sub at v_US = 72.0 km/h. The Russian sub sends out a sonar signal at 1110 Hz. Sonar waves travel at 5470 km/h.
a.) What is the signal's frequency as detected by the US sub?
b.) What frequency is detected by the Russian sub in the signal reflected back to it by the US sub?