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: Relative Motion/Velocity?

  1. Feb 4, 2015 #1
    A police car is chasing a stolen car ahead of it. If the police car is traveling at 150 km/h (N) and the stolen car is traveling at 120 km/h (N), then determine:
    a) velocity of police car relative to ground b) vel. to the pol. car relative to the stol. car c) vel of stol car relative to the pol. car.

    2. oVe= oVm+ mVe

    3. I'm confused about relative motion or just motion in general, but this is what I came up with:
    pVe=150 and sVe=120 (e being earth)
    So, A=150km/h B) pve=pvs+sve which when you plug in you get pvs=30km/h (I'm assuming the positive answers mean the direction is North?? and C) sVp = -pVs= -30km/h ....but that doesn't make sense...direction would be South?

    I'm confused, can someone please explain?
  2. jcsd
  3. Feb 4, 2015 #2


    User Avatar
    Homework Helper

    You're right. The direction of the stolen car relative to the police car would be South.

    When we say "relative to the police car" all we are saying is "from the police car's point of view." So if the police car is moving faster than the stolen car, in what direction would the stolen car appear to move? (It may be easier to imagine it in space, so that you're not distracted by all the trees and such moving at you at 150 km/hr)
  4. Feb 4, 2015 #3
    If you look at b and c, they are equal but opposites essentially. For (b), if you were in the stolen car and pretended you were stationary, you would see the police come at you with +30. For (c), If you were in the police car and you imagined that you were stationary, it would seem like the car was moving backward at - 30 (when an observer really just sees you catching up).
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted