1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: Question about relativity

  1. Nov 9, 2009 #1
    Assume c=30 m/s then solution these question.
    1) As the head traffic engineer for the department of highways, you are in charge of setting a safe but fuel-efficient speed limit on cars. Calculate a speed limit using relativistic equations and explain your reason for your choice of speed limit. Keep in mind that collisions cause changes in kinetic energy.

    :I think width, pulse rate, number of atoms in your body and mass are chaged. basic on relativity equation, frame of refrence is hisself, the time is bigger then the proper time. so the pulse rate. because mass chage, so the numeber of atoms is change.

    2) If you wished to store your valuable 5.0-m-long Rolls Royce in your tiny 4.0-m-long garage,at what speed would you need to drive it into the garage? What paradox can arise from this situation involving relative motion ?

    : I think- I make L=5 , L0=4,then use L=L0*(1-v2/c2)1/2. But infact , L<L0, so this is a paradox.
    this is my solution.

    3)you resume driving and decide to pass the lady in the car ahead by speeding up to 20m/s ,her speedomether reads 15m/s,but you are closing in on her at 7.5m/s ! how is that possible?

    : I cannot solve this question. I think , the lady's speed should be constant, why it has two speed ?

    4)Aliens visiting your planet from another quadrant of the galaxy, where the speed of light is 3*10^8 m/s,say your food seems relatively cold. Explain.

    : it is so confused for me. I cannot find relationship between cold food and the relativity.
    I think , because aliens world ,light speed is so fast, so they look our world, is slowly. as we cook hot food , because their time is fast.so the food iscold.

    P.S. :I solution these question, but I am not sure is it right ? please help me check it , and help me solve it . Thanks very much.
    Last edited: Nov 9, 2009
  2. jcsd
  3. Nov 9, 2009 #2


    User Avatar
    Homework Helper

    First, number of atoms doesn't change; speeding up or slowing down doesn't suddenly make atoms appear out of nowhere. Width doesn't change either because length contraction occurs in the direction of travel, not perpendicular to it. Pulse rate changes, but only from the reference frame of somebody on the ground. Somebody in the car wouldn't detect any changes in pulse rate. However, this doesn't answer the question. Are you sure you didn't leave any details out of the question? It seems horribly ambiguous as is.
    I think you have the right idea, but why is L<L0 a problem? Hint: what does the garage see?

    Unless there's a trick to this, I really don't think it's possible.

    The "real" speed of light is 3*10^8 m/s. Perhaps you meant another number?
  4. Nov 9, 2009 #3
    Thanks very much , question 2 and 3 , i finished, question one and four because I am careless, the questions are wrong, I posta a new topic ,and add a question .
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook