1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

A problem with Newton's mechanics

  1. Aug 14, 2016 #1
    [Mod note: Thread moved from forum Classical Physics so no template shown]

    This set is in equilibrium and I can't find out what will the scale write down in newtons, any help?
    Capture.JPG
     
  2. jcsd
  3. Aug 14, 2016 #2

    Doc Al

    User Avatar

    Staff: Mentor

    What do you think? To start with, what's the weight of those masses?
     
  4. Aug 14, 2016 #3
    Everyone weights 49N
     
  5. Aug 14, 2016 #4

    Doc Al

    User Avatar

    Staff: Mentor

    Good. So what must be the tension in the rope?
     
  6. Aug 14, 2016 #5
    The tension T is T=49 N
     
  7. Aug 14, 2016 #6

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Yes. The tension is the same whether there are two counter balanced masses or one mass and one end of the scale tied off to a large static object.

    If there was only one mass, the mass would simply fall, pulling the scale with it and there would be effectively no tension.
     
  8. Aug 14, 2016 #7
    In every rope, there's also a tension which is equal to 49N
     
  9. Aug 14, 2016 #8

    Doc Al

    User Avatar

    Staff: Mentor

    Exactly. So what must the scale read?
     
  10. Aug 14, 2016 #9
    I know what you mean, but there are two masses, if we simply put them at one end it'd read 98 N, and now by putting one at every end it will read 48 N, I just can't understand why :/
     
  11. Aug 14, 2016 #10

    Doc Al

    User Avatar

    Staff: Mentor

    Careful here. If you put both masses at the the same end of the scale, the scale would go flying.

    What you need to understand is that the scale (assuming it's not accelerating) must be in balance. Equal forces pulling it in both directions. And the scale simply reads the tension with which it is being pulled.

    Try these. Say two people were playing tug of war with ropes attached to each end of the scale. Say they each pull with a force of 49 N. What will the scale read? Now have one of the ropes tied to a tree. If one person pulls on the other rope with a force of 49 N, what force must the tree be exerting? What will the scale read?
     
  12. Aug 16, 2016 #11
    The tree will exert 49 N just to balance the force exerted
     
  13. Aug 16, 2016 #12

    Doc Al

    User Avatar

    Staff: Mentor

    Right. And the tension in the ropes will be 49 N. Which is what the scale will read.
     
  14. Aug 16, 2016 #13
    Oh, that was easy I just couldn't see that, thanks Doc!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted