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 problem.

  1. Sep 26, 2011 #1
    1. The problem statement, all variables and given/known data
    A plane traveling horizontally to the right at 100 m/s flies past a helicopter that is going straight up at 20 at 20 m/s. From the helicopter's perspective, the plane's direction and speed are:
    a) Right and up, less than 100 m/s.
    b) Right and up, 100 m/s.
    c) Right and up, more than 100m/s.
    d) Right and down, less than 100m/s.
    e) Right and down, 100 m/s.
    f) Right and down, more than 100 m/s.

    2. Relevant equations
    [itex]\vec{v} = \vec{v}' + \vec{V}[/itex]
    [itex]\vec{v}[/itex] = velocity of the object in the helicopter's reference frame.
    [itex]\vec{V}[/itex] = the relative velocity measured between two reference frames
    [itex]\vec{v}'[/itex] = velocity of the plane relative to the helicopter's reference frame.
    3. The attempt at a solution
    Just a conceptual question that I do not have the answer to that appeared at the end of the chapter of my text. If the helicopter's reference frame is reference frame S, then the plane would have a vertical velocity component of -20 m/s and horizontal velocity component of 100 m/s. Using the equation:
    [itex]\vec{v} = \vec{v}' + \vec{V}[/itex]
    [itex]\vec{v} = (100\hat{i} - 20\hat{j}) m/s[/itex]
    [itex]\vec{v} =\sqrt{(100^{2}) - (20^{2})}m/s \approx 102 m/s[/itex]
    Therefore, relative to the helicopter's reference frame, the plane's velocity would be f) right and down, more than 100 m/s. Correct?
  2. jcsd
  3. Sep 26, 2011 #2
  4. Sep 26, 2011 #3
    Great. Thank you. :biggrin:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook