1. Not finding help here? Sign up for a free 30min 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!

Really simple problem - can't get right answer!

  1. Sep 25, 2004 #1
    Suppose a 50kg block slides along a
    horizontal surface where the coefficient of kinetic friction
    between the block and the surface is uk = 0.60. A force F =
    400 N is now applied, where the
    angle of the force above horizontal is 20°.

    What is the magnitude of the acceleration of the block?

    0.54 m/s2
    2.31 m/s2
    3.27 m/s2
    6.78 m/s2
    8.11 m/s2

    Here's what i did:

    F(x-net) = F(applied) - F(friction) = ma
    a = F(applied) - F(friction) / m
    = (400)(cos 20) - (50 * 9.8 * 0.6) / 50
    = 375 N - 294 N / 50 kg
    = 1.67 m/s2

    I checked my numbers quite a few times and keep getting
    1.67, which is not one of the answer choices. The only thing
    I saw was that 3.72 m/s2 is exactly double what I got, but I
    can't find an explanation for that.

    Thanks!
     
  2. jcsd
  3. Sep 25, 2004 #2
    The answer is 3.27.
    You gave the right anwser for the acceleration in the x-direction. Now when you look vertically in the y direction you still have the y-component of the 400 N-force that you forgot (gravity and normal force annihilate each other)

    This y-component = 400*sin(20)

    Then calculate a_y and you already have a_x =1.67 which is correct.

    The magnitude is sqrt((a_x)² + (a_y)²)

    marlon
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?