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: Mirror thingy

  1. Sep 6, 2006 #1
    hello everyone, i saw this two problems on a sample oxford admission test and i was wondering what the answer is ...

    If you run at 5 m/s towards a plane mirror, at what speed does your
    image approach you?
    A 5 m/s
    B 2.5 m/s
    C 10 m/s
    D 15 m/s

    i think the right answer is A... am I right?

    A baby's bath should be at a temperature of 37C. The bath already
    contains 10 kg of water at 15C. Approximately how much hot water,
    with a temperature of 50C, should be added to achieve the desired
    A 10 kg
    B 15 kg
    C 17 kg
    D 20 kg

    well for this one, I have no clue. please help.. or point out what i should use. thanks!

  2. jcsd
  3. Sep 6, 2006 #2
    I think for the first one you are considering the velocity of your image from your point of view. Your image approaches the mirror at the same speed as you, but it approaches you at a different speed because you're moving.

    For the second one, I think you'd need to use conservation of energy, and the equation E=mcT. Don't worry about c, it cancels.
  4. Sep 7, 2006 #3


    User Avatar
    Homework Helper

    2) Heat lost by the added hot water = Heat gained by the bath water
  5. Sep 7, 2006 #4


    User Avatar

    What the hell? The image doesn't approach you at all, it is just a mirror. :confused:
  6. Sep 7, 2006 #5
    ok, so for the second one, i got the answer C.
    what about the first one? :confused:
  7. Sep 7, 2006 #6

    Doc Al

    User Avatar

    Staff: Mentor

    No. Figure it out. If you are 10 m from the mirror, how far away is your image? Then when you are 5 m from the mirror, how far away is it?
  8. Sep 8, 2006 #7
    i'm 10m, the image is 20m away.
    i'm 5m away, the image is 10m.

    it's moving with the same speed, i'm moving with 5m/s, the image should be moving with 10m/s

    thanks :)
    Last edited: Sep 8, 2006
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook