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Newton's Second Law Question

  1. Dec 13, 2006 #1
    1. A 2.0 kg otter starts from rest at the top of a muddy incline 85 cm long and slides down to the bottom in 5.0 s. What net force acts on the otter along the incline.

    2. Relevant equations- F=ma, maybe some more too.

    3. The attempt at a solution
    I attempted this question for one and a half hour, but I couldn't get it. This is coming in my exam TOMORROW, but I just don't get it.
    The answer is 14.0 N, but I just don't know how to get it. And it's too late to ask help from a school teacher. I've done this question before, but someone took my physics copy to copy all the assignments and never gave it back...
    Now I don't know what to do.
  2. jcsd
  3. Dec 13, 2006 #2
    you can calculate the acceleration along the incline using

    [tex]x-x_i = v_i t + \frac{at^2}{2}[/tex]

    [tex]x_i[/tex] is the initial position along the incline : 0 m
    [tex]v_i[/tex] is the initial velocity along the incline : 0 m/s

  4. Dec 13, 2006 #3
    The initial x position isn't 0 m, it's an incline :S
  5. Dec 13, 2006 #4


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    You are looking for the acceleration and the net force in the direction of the incline; therefore, you can choose the slope of the incline to be the x axis, and set x_initial = 0, as previously noted. Also, recheck the value of the correct answer.
  6. Dec 13, 2006 #5
    Yes it is. Check how i defined the reference frame (Like PhantomJay explains).

    Again just take the origin at the starting point. The x axis is along the incline. You can chose the X and Y axis anyway you want.

  7. Dec 13, 2006 #6

  8. Dec 13, 2006 #7
    I still don't get it
    Suppose i take the incline as the x axis...
    but vi=0 m/s
    and the acceleration in the x direction is 0 m/s... or not??
    This is the part i'm getting confused in
    There's no accelration in the x direction.
    Suppose i take the condition as it is.
    According to my frame of reference the height of the incline is on the y axis.
    the incline is a horizontal line with a negative slope...
    then the acceleration in the x direction is 0 so is vi...
    I don't understand. how can 0=0.85....

    can someone please explain!
  9. Dec 13, 2006 #8


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    What makes you think there is no acceleration in the x direction? You are looking for the net force acting on the otter.. this force will provide the acceleration. If you consider, like marlon says, taking x_i = 0 and v_i = 0, then there must be an acceleration in order for the otter to move!

    Using Marlon's equation

    work out the acceleration. Then you should be able to find the net force
  10. Dec 13, 2006 #9

    Doc Al

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    Why in the world would you think there's no acceleration down the incline? If there were no acceleration, the otter would just sit there. But he slides down!

    Taking x to be along the incline, use the given kinematics equation to solve for the acceleration.
  11. Dec 13, 2006 #10
    I think i'm confusing it a bit with projectile motions....
    i was thinking that that the otter slid down the incline because of gravity. and since there is an incline, he slides down it, but... errr.. I don't know, I think I"m confusing myself. Let me see if I can solve this now.
    EDIT: can someone explain why the acceleration in a projectile motion without air resistance is zero, and why the acceleration here isn't zero?
    Last edited: Dec 13, 2006
  12. Dec 13, 2006 #11

    Doc Al

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    The acceleration of a projectile without air resistance is 9.8 m/s^2 down. Not zero!

    The only force on a free projectile is gravity, and that acts in such a way that all objects experience the same acceleration--called the acceleration due to gravity. :wink:
  13. Dec 13, 2006 #12
    Yeah so the accelration along the y axis is 9.81 m/s^2, and the acceleration along the x axis is 0!...
    So why wasn't the only force acting on the otter here gravity? Please explain, I hope this answer would remove all my confusions.
    Thanks a lot
  14. Dec 13, 2006 #13

    Doc Al

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    For a free projectile, where the y-axis is vertical and the x-axis is horizontal, this is correct.

    But in your problem:
    (1) Gravity is not the only force acting on the otter. The incline exerts a force on the otter! That makes a big difference: something sliding down a ramp is not a projectile.
    (2) The x-axis is not horizontal, it is parallel to the incline. This is done for convenience, since we know that the only motion will be parallel to the incline.
  15. Dec 13, 2006 #14
    Thanks a lot! I think I'm clear about all this now. :)
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