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!

A question about speed of light

  1. Aug 21, 2015 #1
    If there is a person walking along the road at the speed of 5km/h and a car traveling in the same direction as the person at 60km/h relative to the person. How to calculate their the distance between them?

    Also the velocity of light is 186 000 miles per second and what if both object X and Y can travel at 186 000 miles per second but object Y has an head start of 186 000 miles. How far apart would they both be in an hour? Or will there be no difference between them?

    Thanks also for your help.
  2. jcsd
  3. Aug 21, 2015 #2


    User Avatar
    Gold Member

    Insufficient information. They could be adjacent, they could be a mile apart.

    You could calculate how far apart they are after a given duration since they passed each other. Is that what you're asking?

    They are separating at 55km/h. That's 55,000 metres every 3600 seconds.
    Convert that to units of your choice, say metres per second, and multiply by the number of seconds of your desired duration.

    c = 186,262mi/s.

    Not nitpicking - this will matter in your scenario:
    They cannot travel at the speed of light (unless they're photons). But at 186,000mi/s (99.86c), they are still 262mi/s short, so it's OK.

    They are travelling at the same speed. After one hour, they will be the same distance - 186,000 miles - apart. Is there a reason you would think otherwise?
    Last edited: Aug 21, 2015
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook