I am getting confused with the 3 formulas for time, mass and length.
The part that is confusing is the Tv and To (in this case time dilation) (. How do I know which value to put in)
I thought with To was the observer but in length contraction, Lo seems to be the opposite.
You can actually practically do any HSC Physics relativity calculation Q without memorising which frame of reference the subscripts v and o refer to.
HSC Physics relativity calculation Q's will basically give you one time value, then ask you to calculate how long this time would be in another frame of reference. E.g. something like "A man in a spaceship moving at 0.8c waves to an observer on Earth for 3 seconds in his frame of reference. How long does the observer on Earth see him wave for?"
HSC Physics questions will just require you to either multiply the given time/length/mass (in this example, 3 sec) by
.
To know whether to divide, or multiply by this, all you need to know is what happens to time, length and mass when moving.
Time slows down for someone when they are moving (relative to you), lengths contract, and masses increase.
Therefore, when someone is moving relative to you, an event (e.g. a man's wave) takes less time in their frame of reference than in your frame (since time was slower for them, so 1 second for them is MORE time for you).
So you know that in the given example, your answer should be GREATER than 3 seconds, hence you DIVDE 3 seconds by
(since
 )
, and dividing by something between 0 and 1 makes the thing bigger )
So for any calculation relativity Q, know whether your answer should be LESS than, or GREATER than the given time (or length/mass) value, and then divide or multiply appropriately by
. (And v will almost always be given to you)
