the velocity of the payload as seen by someone at rest at the inertial frame would be 2100km/hr-500km/hr, or 1600km/hr. So if we do not assume the direction, then how do we know which velocity of the payload is the correct one? The velocity of the payload could be 2100+500=2600.
Homework Statement
The payload of a spaceship accounts for 20% of its total mass. The ship is traveling in a straight line at 2100km/hr relative to some inertial observer O. When the time is right, the spaceship ejects the payload, which is moving away from the ship at 500km/hr immediately...