Question

A jet airplane travelling at the speed of 500 km h^–1 ejects its products of combustion at the speed of 1500 km h^–1 relative to the jet plane. What is the speed of the latter with respect to an observer on ground?

Answer 1

Speed of the jet airplane, v_jet = 500 km/h

Relative speed of its products of combustion with respect to the plane, 

v_smoke = – 1500 km/h

Speed of its products of combustion with respect to the ground = v′_smoke Relative speed of its products of combustion with respect to the airplane,

v_smoke = v′_smoke – v_jet 

1500 = v′_smoke – 500 

v′_smoke = – 1000 km/h

The negative sign indicates that the direction of its products of combustion is opposite to the direction of motion of the jet airplane.