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**“proper” length**

A spaceship travels past a planet at a speed of 0.80 c as measured from the planet’s frame of reference. An observer on the planet measures the length of a moving spaceship to be 40 m.

a) How long is the spaceship, according to the astronaut?

b) At what speed would the spaceship have to travel for its relativistic length to be half its “proper” length?

a) L=L0 sqr 1-v2/c2

b) the previous equation where c=3*10^8

is that right?