Newtons first Law

Discussion in 'Physics' started by braddy, Sep 17, 2005.

  1. braddy

    Thread Starter Well-Known Member

    Dec 29, 2004
    83
    0
    Please ,
    Explain me this situation
    A spider is attached to two threads of equal length that make a 120o angle with each other. The spider is motionless and its mass is m=30mg.
    What is the tension in each thread?

    According to the Newton first Law, the sum of the force must be 0.

    T1+T2+Fspider=0
    since the threads are the same length and symetrical, T1=T2.

    Fspider=-3.0 *10-4N

    so I found T1=1.5 *10-4N

    I am surprised to see that the answer is 3.0 *10-4N on each thread

    please now I am confused! help
     
  2. Brandon

    Senior Member

    Dec 14, 2004
    306
    0
    Remember you are dealing with forces in 2 axes. X and Y not in 1. I think that is where you made ur mistake.

    Fyspider=mspider*gravity

    Since spider is not moving back and forth then Fx=0
    which means that |Fxthread1|=|Fxthread2|

    Since the angle is 120 between them and symectrical we have 30-60-90 triangles created for each thread.

    Fyspider-2Fythread=0
    therefore Fythread=Fyspider/2

    Now we need Fxthread, since both at equal, we solve for either. Knowing Fythread, we use the law of sines.

    Fythread/sin 30 = Fxthread/sin 60
    Fxthread=Fythread*sin60/sin30

    Fthread=sqrt(Fxthread^2+Fythread^2)

    Hope that helps.
     
  3. braddy

    Thread Starter Well-Known Member

    Dec 29, 2004
    83
    0
    Thanks I got it now
     
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