# Help! (1 Viewer)

#### Alchemist K

##### IOI
An astronaut set out in a spaceship from Earth orbit to travel to a distant star in our galaxy. The spaceship travelled at a speed of 0.8 c. When the spaceship reached the star the on-board clock showed the astronaut that the journey took 10 years.

An identical clock remained on Earth. What time in years had elapsed on this clock when seen from the astronaut's spaceship?
A 3.6
B 6.0
C 10.0
D 16.7

The answer is B... but why??? How do you explain this in terms of the time dillation formula? Isn't the astronaut measuring proper time? Then shouldn't the clock be measuring dilated time..? By the formula the dillated time is more.. ? Help pls.

BTW this is from 2003 HSC Question 5

#### serge

##### Member
Alchemist K said:
Isn't the astronaut measuring proper time? Then shouldn't the clock be measuring dilated time..?

BTW this is from 2003 HSC Question 5
There's your problem, the astronaut is not measuring proper time

(the word 'proper' isnt very scientific because time is relative)

time dilation means time slows down... making the spaceship a slower
reference point.

earth time= (observed time) /[(1-v^2)^.5]

which when subbing in all the values becomes 6 years

#### Alchemist K

##### IOI
Thanks.. it was really pissing me off