# Homework Help: According to Mary,how fast is the ball moving?

1. Oct 10, 2008

### tanzl

1. The problem statement, all variables and given/known data
Ted and Mary are playing a game of catch in frame S', which is moving at 0.600c with respect to frame S, while Jim, at rest in frame S, watches the action. Ted throws the ball to Mary at 0.800c (according to Ted) and their separation (measured in S') is 1.8*1012m.
According to Mary,how fast is the ball moving?

3. The attempt at a solution
I am confused by this question. Ted and Mary are moving at the same speed (0.6c relative to rest frame), so whatever observed by Ted should be consistent with Mary right? Since Ted observes that the ball is moving at 0.8c so Mary should observes that the ball is moving to her direction at -0.8c.

Last edited by a moderator: Jan 7, 2014
2. Oct 11, 2008

### gabbagabbahey

Re: Relativity

Yes, your correct; perhaps there is a typo in the question and you were meant to determine how fast Jim sees the ball moving?

3. Oct 11, 2008

### tanzl

Re: Relativity

Actually there are four parts in the question
(a) According to Mary, how fast is the ball moving?
(b) According to Mary, how long does it take the ball to reach her?
(c) According to Jim, how far apart are Ted and Mary, and how fast is the ball moving?
(d) According to Jim, how long does it take the ball to reach Mary?