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Homework Help: High school physics forces help

  1. Nov 28, 2014 #1
    I'm trying to understand forces a bit more, but i have questions that i've been searching online for and can't find an answer. My questions:
    Question 1. is Fnet = ma? Because i'm doing some questions my teacher gave us and the explanations say to use F=ma, not Fnet. F and Fnet aren't the same things, so how come they have the same formula?

    Question 2. If the static friction of an object is 10N, would applying 10N of force move the object at a constant speed, or will it still stay at rest?

    Question 3. If every action causes an opposite and equal reaction, why do objects move? If i apply a 20N force on an object, shouldn't it apply a 20N force back, which would mean the Fnet is 0 and it shouldn't move.

    Question 4. One of my questions from homework: A 20N object is sliding from left to right and the force of friction is 2.4N. The object stops moving after traveling 10 meters. Find the acceleration of the object.
    The answer is: acceleration is equal to 1.2m/s^2. The explanation given was:
    Fnet = 2.4N
    Fnet = ma
    2.4N = (20N/9.8m/s^2) * a
    a = 1.2m/s^2

    I understand everything about the problem, but one thing. Why can i assume that the Fnet is 2.4N? It says that the Force of Friction is 2.4N, not the Fnet. The force of friction could be 2.4N, while the Fapplied could be anything less than 2.4N. So why do we assume that Fapplied is 0N?

    Relevant Topics/Formulas:
    Fnet = ma

    3. The attempt at a solution
    Really confused about all 4.

    Thank you for reading this, any help is appreciated.
  2. jcsd
  3. Nov 28, 2014 #2


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    Staff Emeritus
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    Homework Helper

    At PF, we have rules governing HW questions. You should have read these upon joining.

    One of these rules is that PF prefers its users post one HW question per thread, not an entire list of unrelated questions. Having one question per thread keeps replies on point and is less confusing. PF also requires posters to show an attempt at answering posted questions. Saying "I don't know." or similar is not sufficient.
  4. Nov 28, 2014 #3

    Doc Al

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    Staff: Mentor

    It should be Fnet = ma, but if there's only one force acting, then Fnet = F.

    Generally, static friction will be whatever is needed to prevent motion up to some maximum value. If that maximum value were 10 N, you would have to apply a force greater than 10 N to get it to move.

    Those forces act on different objects.

    It does.

    The Fnet on which object?

    Since no applied force is even mentioned, it is reasonable to assume there is none.
  5. Nov 28, 2014 #4
    Thank you Doc for all your help.
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