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Homework Help: Fnet Question

  1. Jun 15, 2006 #1
    Hey there all of you.

    I am doing some studying for my final for physics and i have come across this question. I thought it was easy but it is not working that way. The question is. A 4kg block (held by a block) is accelerating down at 3 m/s/s, what is the tension of the rope?

    So I am thinking to do this:
    Fnet = ma
    T - Fg = ma
    T = ma + mg
    T = 4kg(-3m/s/s) + 4kg(-9.8m/s/s)
    T = 51N

    So I look at the answer and it is wrong, it says it is: 27N

    So now I try making the opposite end negative.

    Fnet = ma
    Fg - T = ma
    - T = ma - mg
    T = -ma + mg
    T = -4kg(-3m/s/s) + 4kg(-9.8m/s/s)
    T = 27N

    Now I am getting the right answer. However I can not get the correct answer doing the same question the other way, so my question is what am I doing wrong? Thank you.
  2. jcsd
  3. Jun 15, 2006 #2
    u need to draw the Free-body Diagram. Once drawn, u with find that the tension is in an upwards direction, the weight downwards.

    since it is accelerating downwards, there is a netforce in the downwards direction. thus mg-T = ma allows acceleration downwards. ( this means mg>T)

    However, T-mg = ma give a netforce in the upwards direction, the object is not accelerating in the upwards direction. ( this means T>mg, but is this true ? )
  4. Jun 15, 2006 #3
    I am sorry, I do not quite follow. Isn't the netforce up AND down?
    Last edited: Jun 15, 2006
  5. Jun 15, 2006 #4
    nope, Net force = Resultant force

    A net force (also known as a resultant force) is a vector produced when two or more forces act upon a single object. It is calculated by adding the force vectors acting upon the object.

    for example, i put a 10N object on a table what is te net force?

    the object has a weight of 10N, and by newton 3rd law, the table exert a 10N force on the object also.

    the force exerted by table is in a upwards direction and the weight is in a downwards direction.

    thus the net force is [tex]F_net = 10 - 10 = 0[/tex] thus the object is not accelerating, as a netforce will cause it to accelerate
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