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: Velocity of image in plane mirror

  1. Dec 8, 2018 #1
    1. The problem statement, all variables and given/known data
    upload_2018-12-8_17-39-28.png
    A mirror is moving to the right with a velocity 15 m/s, and an object is moving to the right with a speed of 5m/s. All speeds are wrt ground. Find the velocity of image wrt ground.

    2. Relevant equations


    3. The attempt at a solution
    I present to you two probable solutions:
    1) Using formula: VIMAGE=2VMIRROR-VOBJECT
    Now, its a mere substitution problem. Using right as positive direction, we get the required velocity as 25m/s to the right.
    2) This is a controversial one. I thought about this as i like to explore all the different correct of incorrect methods of a problem.
    the above problem is equivalent to: the mirror is moving to the right with a velocity of 10m/s and the object is at rest. Now, substituting in the formula used in the previous method, VIMAGE=20m/s to the right.

    Now, my book says that that the correct answer is 10m/s, but its not matching with any of my answers. Please check it and let me know why both are incorrect.
     
  2. jcsd
  3. Dec 8, 2018 #2

    CWatters

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    You could try making two scale drawings, one showing the starting position of the object, mirror and image, and another showing their positions after say 1 second. Figure out how far the image moved in that time.
     
  4. Dec 8, 2018 #3
    Can you please tell me the correct answer?
    Yeah, i'll try by that method.
     
  5. Dec 8, 2018 #4

    CWatters

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    PS: Book answers are sometimes wrong.
     
  6. Dec 8, 2018 #5

    CWatters

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Forum rules discourage us from giving the answer.
     
  7. Dec 8, 2018 #6

    gneill

    User Avatar

    Staff: Mentor

    Sometimes changing your frame of reference can be helpful to clarify what is happening. Suppose you were an observer riding along with the mirror. What is the motion of the object from that point of view? Is it approaching or receding from you? How quickly?
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted