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Homework Help: Kinematics-Review help

  1. Jul 29, 2009 #1
    Hey guys, I reallly don't understand how to do this question. i'd really appreciate it if you can help me..thanks :)

    1. The problem statement, all variables and given/known data
    A person applies a force of 15 N [E] on an object of weight 20 N. If the force of friction has a magnitude of 5.0 N, what is the acceleration of the object?
    a) 7.5 m/s^2 [E]
    b) 0.13 m/s^2 [E]
    c) 0.50 m/s^2 [E]
    d) 5.0 m/s^2 [E]
    e) 10 m/s^2 [E]

    2. Relevant equations

    3. The attempt at a solution

    Weight= 20 N = F = Fg
    Fapp= 15 N
    Ff=5 N

    Fnet= 15+5 = 20 N
    I don't know what to do next ..thanks
  2. jcsd
  3. Jul 29, 2009 #2
    Re: Kinematics-Review

    Friction opposes motion. So should you be adding the 5 N to 15 N? Or substracting?

    Once you have the correct Fnet, then use Fnet = m * a. You get "m" from the weight.
  4. Jul 29, 2009 #3
    Re: Kinematics-Review

    Yea i just realized that thanks..so it would be 15+(-5) = 10 N

    But weight isn't only the mass, from what i know weight is equivalent to net force that's why it's measured in newtons..that's where i'm getting confused!

    oh btw the answer at the back of the textbook says d) 5m/s^2
  5. Jul 29, 2009 #4
    Re: Kinematics-Review

    Ohhhhhh nvm! i just realized what you meant!

    20/9.8= 2.0kg

    therefore a= 5m/s^2

    Thanks :D
  6. Jul 29, 2009 #5
    Re: Kinematics-Review

    I agree with that answer. I'm assuming the applied force and friction is horizontal and that the block is moving horizontally. That means that the vertical forces are not affected by the horizontal forces. How is weight force defined? Like any other force it is F = mass * accel. However, the accel in this equation is not an acceleration due to motion but due to gravity. You should know what the acceleration of gravity is and you know what the force is. Solve for mass.

    Edit: Yep you got it before I finished writing. Good Job!
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