An alien spaceship is moving toward Earth at 0.6c. It fires a relativistic projectile at Earth. If the
velocity of the projectile relative to the spaceship is 0.85c, at what speed is it coming toward the Earth
according to a stationary observer on Earth?

Question

An alien spaceship is moving toward Earth at 0.6c. It fires a relativistic projectile at Earth. If the 
velocity of the projectile relative to the spaceship is 0.85c, at what speed is it coming toward the Earth 
according to a stationary observer on Earth?

anonymous · Answer

1.45c.  If the projectile is move in same direction as the ship and its speed is .85c relative to the ship than it is .85c faster than the ship.  Since the ships speed is .6c you add the two together and get 1.45c.

anonymous · Answer

1.45 c is faster than speed of light , do I make relatisivitis adjustment

anonymous · Answer

$$u' =\frac{ u+v }{ 1+uv/c ^{2} }$$   With u being the speed of the projectile and v being the speed of the ship and u' being the speed according to the observer. Hope I'm correct.