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Friction on horizontal rough surface (finding tension)

  1. Mar 5, 2013 #1
    Please refer to the image attached.

    For 20(b), I thought the tension can be calculated just by taking the weight of B, which is 0.05 x 9.8 = 0.49N? How come the answer is 0.42N?
     

    Attached Files:

  2. jcsd
  3. Mar 5, 2013 #2
    Since the system is accelerating.
     
  4. Mar 5, 2013 #3
    But since the whole thing is accelerating together, why would it affect the tension?
     
  5. Mar 5, 2013 #4
    If the weight of B was equal to the tension of the string, then the net force on B would be 0 and it would not accelerate.
    Look at your free-body diagram for B and apply Newton2 to it to calculate the tension.
     
  6. Mar 5, 2013 #5
    If GB=T then acceleration would be zero.
    Sorry ap123 you already said this.
     
  7. Mar 5, 2013 #6
    Ok, the acceleration of the whole thing is 1.4 and the pull of gravitational force is 9.8.
    So I take the mass of B times net acceleration = 0.05 (9.8-1.4) = 0.42N.

    Is it all about the "net acceleration"?
     
  8. Mar 5, 2013 #7
    New question

    This question uses the same diagram as the previous one.

    I don't know how to do (c).
    When B hits the floor, A should stop accelerating and start to decelerate. But I don't know the deceleration and now it wants me to find speed. The time can't be known either.

    Now the only information I have is u=2.33m/s and s= 0.5. What equation can I use?
     

    Attached Files:

  9. Mar 5, 2013 #8
    When B hits the floor, what are the horizontal forces on A?
     
  10. Mar 5, 2013 #9
    Friction. I got the answer, thanks.

    I want to ask, is this related to inertia? Is inertia partly because of air resistance/frictions?
     
  11. Mar 5, 2013 #10

    haruspex

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    No. Inertia is the fact that it requires a net force to accelerate a mass. Quantitatively, it's the force divided by the acceleration.
     
  12. Mar 7, 2013 #11
    So actually does inertia cause an object to require more force to stop/start moving?

    For example, if I were to push an object of 10kg across a frictionless table, the force required for the object to start moving is due to its big mass only, or does it include inertia?
     
  13. Mar 7, 2013 #12

    haruspex

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    It's not to do with starting and stopping as such. It is natural to think in those terms because our everyday experience is that there there is static friction to be overcome. It is to do with how much force is needed to produce a given change in speed. In principle, you could push a supertanker away from the edge of the dock, but the acceleration would be very low.
    Not sure what you mean by "due to its big mass only, or does it include inertia?". Mass has two properties: masses experience gravitational attraction, and they exhibit inertia. For the purposes of your question, the inertia is the mass.
     
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