- #1
jazznaz
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Homework Statement
Anne and Joe are twins, happily living in an inertial frame. On their 20th birthday Joe decides
to take a rocket.
(a) According to Anne the rocket moves with constant speed [tex]v = \frac{3c}{5}[/tex]. For 6 months it moves away from Earth and then returns in time for Anne's 21st birthday. How old is Joe on Anne's 21st birthday?
(b) According to Anne the rocket is initially at rest and then accelerates such that its velocity is always proportional to [tex]\sqrt{t}[/tex], where t is the time elapsed since the start of the trip. After two years the rocket reaches a velocity of 3c/5. How old is Joe on Anne's 21st birthday?
(c) Joe is on his trip, according to (b). How old is Anne on Joe's 21st birthday?
Homework Equations
[tex]d\tau = \int^{t_1}_{t_0} \frac{dt}{\gamma\left(v\left(t\right)\right)}[/tex]
The Attempt at a Solution
For part a, I have,
[tex]d\tau = \frac{dt}{\gamma}[/tex]
[tex]\gamma = 5/4[/tex]
[tex]dt = 1\text{yr}[/tex]
[tex]d\tau = 4/5 \text{yrs}[/tex]
So Anne has aged one year, and Joe has aged 4/5 = 0.8 years. So Joe is about 20 yrs and 292 days old.
For part b I'm having trouble getting my head around how to incorporate the time dependence of the velocity into the integral (which is what we have been led into doing). I'm not sure whether to use the proper acceleration or to try to evaluate the integral as it is.
Any pointers would be greatly appreciated.
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