1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Special Relativity (time/length)

  1. Jan 30, 2017 #1
    1. The problem statement, all variables and given/known data
    Spacecraft moving at 0.6c, length 180m with someone sitting at the back. He throws a cage with a mouse towards the front also at a speed of 0.6c. A stationary observer watches.

    2. Relevant equations
    What is length of spacecraft from observers POV? Length according to guy sitting at back? Length according to little mouse? How long does it take for cage to reach fron of ship according to mouse and observer? How fast (according to observer) does mouse travel

    3. The attempt at a solution
    According to Galilean velocity, I would say speed according to observer was 1.2c. However, as this is not possible then my answer would be c.
    The length according to observer would be 144m ie 180 m x sq route of (1 - v^2/c^2)
    Struggling to work out time/length as seen by mouse and how long it takes for mouse to reach the front.
    Cheers
     
  2. jcsd
  3. Jan 30, 2017 #2

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    This is impossible as well.

    It is a question about special relativity. You'll have to use special relativity to answer it.

    144 meters is correct.
    What did you try?
    The different parts can be calculated in every reference frame, but often some frames are easier than others. If you get stuck, try a different reference frame.
     
  4. Jan 30, 2017 #3
    Many thanks. Will take another look this evening and post what I have. Cheers :)
     
  5. Jan 30, 2017 #4
    Thanks mfb. I have been through my notes properly and found the equation: u + v/1 - uv/c^2

    With u and v = 0.6c, my answer is now 0.88c. So the observer views the speed travelled as 0.88c

    Also, I take it from my notes that the astronaut on board will view the length as 180m. I now need to work out how long the craft is as seen by the flying mouse.

    Cheers
     
  6. Jan 30, 2017 #5

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Correct so far.
     
  7. Feb 3, 2017 #6
    Morning

    Still so confused as to what I need to do.
    So far

    Space craft (190m long at rest) moving at 0.6c so observer at rest will see the length as proper length / Lorentz constant. In this case the constant is 1.67 so:

    Lenth = 190m / 1.67 = 113.78m

    Therefore observer sees the length as 113.78m and sees the speed as 0.88c

    The astronaut at the back of the craft will see the length as 190m

    The astronaut fires an object also at 0.6c relative to the rocket. To calculate the time taken for object to reach the front,

    Astronaut:

    t = 190 / 0.6c = 1.06 x 10^-7 seconds

    Observer:

    He/she will calculate the time taken as length/velocity PLUS the the time taken according to astronaut, so t = 113.78m / 0.88c = 4.31 x 10^-7 PLUS 1.06 x 10^-7 seconds. Is this correct?



    How long will the spacecraft be according to the object being thrown? Will this be the same as the observer's calculated length, as the object also moving at 0.6c??

    I am finding this so difficult to grasp.

    Thanks
     
  8. Feb 3, 2017 #7

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Apart from the rest length of the spaceship changing from 180m to 190m, you were right on the first part.

    Regarding the object moving within the spaceship: yes, it is an important and valid point that to them the spaceship is moving at 0.6c backwards, so their observations will agree with the "stationary" observer in terms of length contraction.

    The final point is the time for this object to travel the length of the ship according to the stationary observer. The trick here is to note the following fact:

    In SR (as in classical physics), if you stay within a single IRF, you may apply the normal addition/subtraction of velocities (for velocities in your frame).

    If ##v## is the velocity of an object in your frame and ##u## is the velocity of another object in your frame, then the "separation" velocity of these two objects in your frame is simply:

    ##v_s = u - v##

    In other words, SR requires no changes to this natural, classical calculation, as long as everything is measured in your reference frame. It's the same with distances. SR only requires changes when you consider velocities (or distances or times) measured in two different reference frames.

    Note that the velocity addition formula relates velocities as measured in different frames, as follows.

    If ##V## is the velocity of frame S' relative to your frame and ##u'## is the velocity of an object (as measured in frame S'), then the velocity of the object as measured in your fame is:

    ##u = \frac{V + u'}{1 + Vu'/c^2}##
     
  9. Feb 3, 2017 #8

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    I forgot to say. That's not valid in any form of physics. In classical physics, the time would be the same in both frames. In SR you can get the answer two ways:

    1) Forget the astronaut's frame, which is irrelevant, and simply do the calculation in the observers frame (assuming you have all velocities and distances in the observer frame).

    2) Have you studied the Lorentz Transformation yet?
     
  10. Feb 3, 2017 #9
    Many thanks Perok. oops regarding the length of the ship :)

    The Lorentz Transformation is confusing me. Observer's POV:

    u=V+u′1+Vu′/c2 where V = 0.6c and u = 0.6c This is how I get my 0.88c

    time taken from astronauts POV is distance / velocity = 180m / 0.6c = 1.0 x 10^-6 seconds

    How do I start to work out the length of the craft from the object's perspective? And how can I work out how long it takes for the object to reach the front of the spacecraft from the astronauts perspective? I have been through my notes and I cannot find anything.

    I know I work out the Lorentz Constant, which at 0.6c is 1.67

    What should be quite easy is proving to be very troublesome. Thanks for any extra help you can provide. I have tried looking for examples online, but so far nothing

    Many thanks
     
  11. Feb 3, 2017 #10

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Okay, that's fine.

    Let's do one question at a time.

    1) From the object's perspective, describe the motion of the ship.
     
  12. Feb 3, 2017 #11
    1) - At rest
     
  13. Feb 3, 2017 #12

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Okay, I don't agree with that!

    So, from the ship's perspective, what is the motion of the object?
     
  14. Feb 3, 2017 #13
  15. Feb 3, 2017 #14

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Yes. So, you have a fundamental misunderstanding about relative velocity. If the velocity of the object in the ship's frame is 0.6c, then the velocity of the ship in the objects frame must be -0.6c.

    That, in fact, is also true in classical physics. For example, if someone is walking down a train, then in their reference frame the train is moving backwards at walking speed. In the reference frame of someone sitting in a train, the train is at rest. And, to someone on the ground outside the train, the train is moving at 100km/h (or whatever).

    For each pair of observers, their relative motion with respect to each other must always be equal and opposite. That's essentially a postulate of classical physics and SR.
     
  16. Feb 3, 2017 #15
    Thanks PeroK. I am really confused though. If the train is moving forwards at 10mph and I am also walking forwards in the train at 10mph, to the observer at the platform I would be moving at 20mph. But...If the ship is moving at +0.6c in the +x direction AND the object is thrown in the +x direction at 0.6c, then they are both moving in the +x direction. Why is the velocity of the ship in the objects frame -0.6c?

    Thanks
     
  17. Feb 3, 2017 #16

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    That's velocity addition involving velocities in two reference frames: one velocity in the ground frame (velocity of the train) and one velocity in the train frame (velocity of you relative to the train), which can be "added" to give the your velocity in the ground frame.

    This is where classical physics and SR depart from each other.

    In classical physics you have simple addition ##u + v##.

    In SR, you have the velocity "addition" using: ##\frac{u + v}{1 + uv/c^2}##

    But, whatever this adds up to, let's call it ##v_{ug}## for your velocity relative to the ground, then we have:

    ##v_{ug} = -v_{gu}##

    I.e. the ground's velocity relative to you is equal and opposite to your velocity relative to the ground. The train is irrelevant.
     
  18. Feb 3, 2017 #17

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    PS

    If ##u, v## are small compared to ##c##, as in the case of a train, then we have:

    ##\frac{u + v}{1 + uv/c^2} \approx u + v##
     
  19. Feb 3, 2017 #18

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    PPS This is probably the most fundamental postulate of SR. As a first step, you must dispel any doubts about this.
     
  20. Feb 3, 2017 #19
    Ah ok, I am seeing now that (+) velocity as seen by the ship, would be (-) velocity as seen by the ground?

    I think I am still failing to grasp the negative velocity on the ship. If I was in the ship sitting down and holding the object, then the object would be moving at 0ms^-1 relative to me and the ship. If I then throw this towards the front at 0.6c, then the object will be moving at + 0.6c away from me, and would use the equation above and I work that out to be 0.88c. But this is what the observer on the ground would see as he is at rest. So if ship is moving at 0.6c and the object within the ship moves at 0.6c, then I still get 0.88c. So does this mean that object moves at 0.88c relaltive to the ship?

    Cheers and sorry for dragging this out. I cannot grasp the difference between what the observer sees the ship moving at and what the object experiences relative to the ship
     
  21. Feb 3, 2017 #20

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    No, this is not right. Let me give you two arguments.

    First, the stationary observer is irrelevant to the ship and object. They know nothing about any "stationary" ground observer. The astronaut fires an object at 0.6c relative to him/her and the object measures the astronaut's velocity as -0.6c. The rest of the universe is irrelevant.

    Second, suppose we had another observer to whom the ship was moving at 0.5c. Then he would measure the velocity of the object as approx 0.8c (at a rough calculation). Now, would the speed of the object to the astronaut be 0.88c (as a result of the unseen influence of the first observer); or at 0.8c (as a result of the unseen influence of the second observer)?

    You could even have a third observer, to whom the ship was moving at -0.6c and to whom, therefore, the object was at rest. So, would the astronaut now measure the speed of the object as 0 as a result of this observers measurements?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted