# The Naked Scientists Forum

### Author Topic: how fast would these objects approach each other?  (Read 1774 times)

#### thebrain13

• Sr. Member
• Posts: 442
##### how fast would these objects approach each other?
« on: 25/10/2011 06:36:39 »
If there was an object traveling .5c relative to earth and it was being approached by an object traveling .9c from the rear relative to earth how fast would the two objects approach one another from the frame of reference of the object traveling .9c?

#### imatfaal

• Neilep Level Member
• Posts: 2787
• rouge moderator
##### how fast would these objects approach each other?
« Reply #1 on: 25/10/2011 12:06:38 »
v_close = (v_1 - v_2) / (1- v_1.v_2/c^2)

and the pretty version

« Last Edit: 25/10/2011 12:15:18 by imatfaal »

#### thebrain13

• Sr. Member
• Posts: 442
##### how fast would these objects approach each other?
« Reply #2 on: 26/10/2011 03:55:48 »
thanks

#### damocles

• Hero Member
• Posts: 756
• Thanked: 1 times
##### how fast would these objects approach each other?
« Reply #3 on: 26/10/2011 08:27:59 »
Two comments:

(1) In using the formula given, it is important to remember that one of the velocities is negative, so in terms of speeds, we have the sum of the two speeds divided by a number greater than 1.

(2) Is the answer for the closing velocity frame independent? I think it might be, but not at all sure. Because I am fairly sure that the formula given is for the closing velocity in Earth frame of reference.

#### The Naked Scientists Forum

##### how fast would these objects approach each other?
« Reply #3 on: 26/10/2011 08:27:59 »

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