00:01
In this problem, we're going to talk about length contraction.
00:03
So consider that we have a reference frame, i'm going to call it s, and assume that in this reference frame we have an object at rest, and the length of this object is l.
00:16
We call this length, the proper length, and it has the special property that is the length measured, that is the greatest possible length measure for this object, and it can be identified, as the length measured in the reference frame, the object is at rest.
00:36
Now consider a second reference frame.
00:39
This time i'm going to call it s prime.
00:43
And according to this reference frame, the first one is moving with the speed v.
00:52
Now, the object is not at breast anymore, and the length measured by the s prime reference frame to the object is equal to l, the proper length, divided by gamma, where gamma is is 1 over the square root of 1 minus v squared over c square.
01:14
Okay, so in our problem, what we have is a railroad car that has a proper length of 20 meters.
01:23
The exercise tells us that this is the length measured at rest, and then it passes rachel, who records a time of 120 nanoseconds for the passing of this railroad car...