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Basic (very basic) Newtonian physics question

  1. Feb 19, 2015 #1
    1. The problem statement, all variables and given/known data

    Just my own personal questions concerning newtons laws.
    For the most part I grasp all three laws to adequately take care of my homework, but I have some conceptual questions that keep bugging me.
    when solving diagrams including net forces that are acting on an object, the sum of the forces (say acting in the y direction) are equal to zero. Take for example a block of Mass m at rest on a table, it has a F of gravity and a normal force acting on it, but since the object is at rest it has a force equal to 0.
    so, my question is can forces that are acting on an object be equal to zero even though they are acting on the object?

    It feels like a dumb question but its been a conceptual question eating at me for a while.
    Thank you guys
    2. Relevant equations


    3. The attempt at a solution
     
  2. jcsd
  3. Feb 19, 2015 #2
    The individual forces that act on the block are not equal to zero. If they were, then they wouldn't be forces. They wouldn't exist. What's important is that they cancel each other out and that the net force is zero. Am I interpreting your question correctly?
     
  4. Feb 19, 2015 #3
    haha hit the nail on the head there, thank you.
    I had myself a little "aha" moment.
    Its not that the individual forces (the normal force or force of gravity acting in the y direction) are equal to zero, but the the net result when you add the two is equal to zero.
    is that right?
     
  5. Feb 19, 2015 #4
    and the object is in equilibrium in that direction?
     
  6. Feb 19, 2015 #5
    That is correct. Remember, Newton's second law isn't that the force on an object is equal to mass times acceleration. It's that the NET force on an object is equal to the mass times the acceleration.

    You're probably sitting down right now, so you've got a nonzero gravitational force pulling down on you, and since you're most likely not falling down, there is an equal and opposite normal force on you, and these two forces cancel each other out.

    Or if I were to push a block with force F towards the right, and you push it with force F from the left, then the block no longer accelerates, as the forces are cancelled out (but there still are forces on it. It's the NET force that we're focused on).
     
  7. Feb 19, 2015 #6
    haha i feel better now that I understand that, I got so caught up on the homework questions I forgot to look at the sum of the forces as a "net", which could be positive, negative, or zero.
    thank you for taking the time to clarify this for me
     
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