The correct value for the rocket speed measured by an observer on earth would be = 14c
To calculate the speed of the rocket we should observe that the rocket and the missile are in a relative motion which are moving on the same direction.
The speed of the rocket = 12c
The speed of the missile = 12c
The individual speed of earth =0c
But, the relative speed of the missile and the Earth is the sum of individual speeds.
Thus, the speed of rocket as measured by an observer on the Earth would be 0 + 14c = 14c.
Therefore, the correct value for the rocket speed measured by an observer on earth would be =14c
Learn more about relative motion here:
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