Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Two Dimensional Motion

  1. Sep 30, 2011 #1
    1. The problem statement, all variables and given/known data
    A car is parked on a cliff overlooking the ocean on an incline that makes an angle of 25.0° below the horizontal. The negligent driver leaves the car in neutral, and the emergency brakes are defective. The car rolls from rest down the incline with a constant acceleration of 3.97 m/s2 for a distance of 30.0 m to the edge of the cliff. The cliff is 50.0 m above the ocean.
    (a) Find the car's position relative to the base of the cliff when the car lands in the ocean
    (b) Find the length of time the car is in the air.

    2. Relevant equations
    Not sure

    3. The attempt at a solution
    Not sure what to do I made pictures in all ways but cant see what I should do.
  2. jcsd
  3. Sep 30, 2011 #2
    Start with the basic equations of motion.
    [itex]x = x_0 + v_0t + \frac{1}{2}at^2[/itex]
    [itex]v = v_0 + at[/itex]
    Find the speed at the end of the cliff. (Do the work yourself)
    [itex]v = a\sqrt{2s/a} = \sqrt{2sa}[/itex]

    Now this vector is pointing down in an angle [itex]\theta[/itex]. Divide this vector into x-component and y-component using trigonometry. If air resistance can be neglected, x speed will remain the same for the whole time but v speed will keep increasing due to gravity. Try and ask for more help if necessary.
    Last edited: Sep 30, 2011
  4. Sep 30, 2011 #3
    Alright can you explain this that has no numbers to put into equations.

    A home run is hit in a baseball game. The ball is hit from home base into the center field stands, along a parabolic path. What is the acceleration of the ball at each of the following points? (Ignore air resistance.)
    (a) while it is rising
    (b) at the highest point of the trajectory
    ___ m/s2
    (c) while it is descending after reaching the highest point
  5. Sep 30, 2011 #4


    User Avatar
    Homework Helper
    Gold Member

    Hello Adam900,

    Welcome to Physics Forums!
    In the future, please start a new thread for a new question.

    But I'll give you a hint anyway. At each point, what is(are) the force(s) acting on the ball? (You get to ignore air resistance, so forget about that one.) Is(are) any particular acceleration(s) associated with said force(s)?

    (Remember Newton's second law. An object isn't going to accelerate unless forces are acting upon it. :wink:)
  6. Oct 1, 2011 #5
    Hmm so Vi would be 0 and acceleration/gravity would be -9.8? I still dont see what else I can find from this information in the problem like a distance or angle it was hit at.
  7. Oct 1, 2011 #6
    [PLAIN]http://img37.imageshack.us/img37/5674/carpic.png [Broken]

    You know [itex]v = \sqrt{2sa}[/itex]
    Thus [itex]v_x = vcos(\theta) = \sqrt{2sa}cos(\theta)[/itex]
    and [itex]v_y = vsin(\theta) = \sqrt{2sa}sin(\theta)[/itex]

    There's some more help. Use [itex]v_y[/itex] to find the time it takes for the car to fall to the ocean.
    Remember the equations of motion.

    Oh and just to clarify, s = the distance the car rolls down the hill = 30m.
    Last edited by a moderator: May 5, 2017
  8. Oct 3, 2011 #7


    User Avatar
    Homework Helper
    Gold Member

    Well, no, the velocity isn't zero. But the question isn't asking about the velocity.
    There ya go. :approve:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook