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Homework Help: Forces - Finding Net Force

  1. Sep 30, 2007 #1
    1. A ball is flying through the air, and at a given point in the flight, the ball is submitted to a gravitational force: F(g) = 1,5 N [downwards] and air resistance: F(air) = 0,50 N [32° above the horizontal]. Calculate the net force on the ball.

    2. F (nette) = m x a

    3. I thought I might have to find the vertical and horizontal components of the air resistance, so I found:
    sin32 x 0,5N = 0,26N = x
    cos32 x 0,5N = 0,42N = y

    I also tried calculating the mass of the ball, in case it might prove useful:
    Fg = m x g
    1,5N = 9,8m
    0,153 kg = m

    But after calculating these values, I'm at a loss of what to do next. How do you find out the net force without having the other force on the horizontal axis? How can I use this data to calculate the net force? The answer key in my book says it should be 1,3N [71° under the horizontal], but I have no idea how this answer was achieved. I would be grateful for any help in solving this problem.
  2. jcsd
  3. Sep 30, 2007 #2
    You have the x and y components interchanged.

    Why should it have a non-zero horizontal component?

    Think of it this way: "Downwards" is 270 degrees away from the positive x-axis. Can you find the components of the gravitational force now?
  4. Sep 30, 2007 #3
    If the ball is in movement, shouldn't the horixontal component be non-zero?

    And what does it being 270 degrees away from the positive x-axis have to do with it?
  5. Sep 30, 2007 #4
    To find the net force just add the two force vectors. You know what their components are along each direction.

    I was just trying to show you that the horizontal component of the gravitational force is zero.
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