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How do I find Tension of a suspended object, when given only weight and acceleration?

  1. Dec 7, 2008 #1
    1. Pulling up on a rope, you lift a 4.00-kg bucket of water from a well with an acceleration of 2.40m/s2. What is the Tension in the rope?



    2. I know the basic equations like net force = mass X acceleration. Is the tension my net force? Or is it my normal force? I basically have no clue what equation to use.



    3. I tried Fnet=ma, assuming Tension is the net force. With that, I got 9.6N. Is this correct?
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
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  3. Dec 7, 2008 #2

    PhanthomJay

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    Re: How do I find Tension of a suspended object, when given only weight and accelerat

    correct
    you have the right equation, but you need to identify the net force. The tension force is just one of the forces acting on the bucket. What's the other one?
     
  4. Dec 7, 2008 #3
    Re: How do I find Tension of a suspended object, when given only weight and accelerat

    The only information I was given was the acceleration and mass. I don't know what other force there would be except maybe normal force..?
     
  5. Dec 7, 2008 #4

    PhanthomJay

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    Re: How do I find Tension of a suspended object, when given only weight and accelerat

    Normal forces act perpendicular to the objects and are generally pushing type forces. The bucket's hanging, so there is no normal force. But you are missing a very basic force which acts on all objects due to the force of the earth on the object. It's a gravitational force. What is it??
     
  6. Dec 7, 2008 #5
    Re: How do I find Tension of a suspended object, when given only weight and accelerat

    Weight? So do I take the mass given (4kg) and multiply by gravity (9.81)? What do I do with this weight then? Subtract from the net force (9.6)?
     
  7. Dec 7, 2008 #6
    Re: How do I find Tension of a suspended object, when given only weight and accelerat

    Waitt! I figured it out, do you ADD the weight to the net force?
     
  8. Dec 7, 2008 #7

    PhanthomJay

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    Re: How do I find Tension of a suspended object, when given only weight and accelerat

    The tension force, T, acts up (tension forces always pull away from the object). Weight always acts down. So you have T up, and mg down. The net force must be up, since you are lifting and accelerating the bucket upward. Solve for T. (Note: For sample, if you pull right at 50N and I pull left at 20N, the net force is 30N right. Don't overthink the net force. If you pull up at T newtons and the weight pulls down at 40N, the net force is T-40, up.)
     
    Last edited: Dec 7, 2008
  9. Dec 7, 2008 #8
    Re: How do I find Tension of a suspended object, when given only weight and accelerat

    So does Fnet=T+W? Since the net force is the total of all the forces acting on an object? Like if the T is the same amount as the W, it would cancel out and give me 0 for Fnet. But since the bucket is accelerating upwards, the T must be greater than weight. But if that is true, wouldn't W have to be negative to be able to cancel out when you have an object that doesn't move?
     
  10. Dec 7, 2008 #9

    PhanthomJay

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    Re: How do I find Tension of a suspended object, when given only weight and accelerat

    Well, yes, that's right. T acts up, so it is positive. The weight acts down, so it is negative. F_net is T - W.
     
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