N 2000, a 20-year-old astronaut left Earth to explore the galaxy; her spaceship travels at 2.5 x 10^8 m/s. She returns in 2040. About how old will she appear to be?

she'll be 42.3 years old

Explanation:

The year she left is 2020

she returns in 2040

the time observed to have passed by on earth is 40 years

she traveled at a speed of 2.5 x 10^8 m/s

From relativity, the astronaut will experience a time dilation and will age more slowly.

the time that is observed to pass on her ship will be gotten from

t' =

where t is the time that is observed to pass by an earthly observer

v is the speed of her ship

c is the speed of light = 3 x 10^8 m/s

v/c = 2.5/3 = 0.83  

t' = = 40 x 0.5578 = 22.3 years would have passed for her

This means that she'll be 20 + 22.3 = 42.3 years old

it is the second option.

fgravity=10x10=100n case 1

fgravity=30x20=600n case 2

6 times more