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When I push a an object, why does it move?

  1. Jun 12, 2012 #1
    A discussion my maths teacher brought up today arguing that the Newton's Third Law pair of pushing on the table and the table pushing against the person should cancel eachother out.

    So there should be no resultant force and it should not move. I argued it was to do with F=ma so if the object weighs less there will be greater movement as opposed to the greater mass of a person pushing on it. He argued that the forces would still cancel and would still not move as Newton's First Law says that unless unbalanced force is acting it will stay at rest.

    I also thought that as the weight of the table and the pushing force are perpendicular, they are independent. So you are only doing work against the friction. But does this change anything?

    Confusing.........
     
  2. jcsd
  3. Jun 12, 2012 #2

    Fredrik

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    A massive object will change its velocity if and only if the sum of the forces acting on it is non-zero. Newton's 3rd is telling you about something else. It says that if you push the table, it will push back. Now we're talking about a force that the table exerts on you, so this force doesn't tell you anything about how the table will move. It tells you something about how you will move.
     
  4. Jun 12, 2012 #3
    Ahhhh I see so they're both kind of independent and each object will act accordingly depending on its mass etc... Funny how my Maths teacher is making me think about Newton's laws more than my Physics one. So are the forces exerted on each equal? If so can you work out how it moves with F=ma?
     
  5. Jun 12, 2012 #4
    The person is dentin body and the table is mechanical body——how can you put F and F' together?
    If both are holded in a container back-to-back :no one can move.

    You can only say the synthetic force of the container received is F+F'=0
     
  6. Jun 12, 2012 #5
    Why does it move when I push it then?
     
  7. Jun 12, 2012 #6
    Well, it doesn't really move, it accelerates. But I'll assume that's what you mean. If you're just asking a simple "why does a table move when I push it," then here's the answer.

    The unsatisfying answer: because that's what we observe. In many (but not all) cases, asking why something happens in physics won't get you anywhere, since no theories will generate any predictions that can be tested and disproved, or at least at the time at which the theory is proposed. Note that while the "satisfying" answer appears to remedy this, this is still going to be a consequence of some "unsatisfying" answer.

    The satisfying answer: when the atoms in your hand get close to the atoms on the table, the electrons, having the same charge, will repel each other, and the table will experience a net force. But why do like charges repel? And here we get to the same question, and we keep going until we get to an argument that's a simple "that's what we see happening."

    Or, if you mean why won't the forces cancel out, yep, see Fredrik's post. Your question seems a bit unclear to me.
     
  8. Jun 12, 2012 #7
    sum of the forces being the key word here.

    If you were on a frictionless surface, either nothing would move, or you'd both move (say, if you punched it).

    Since you are not living in a frictionless world, there is friction between your shoes and the floor, and the table legs and the floor. when you push on the table, it will not move (and nor will you) as long as the force of friction at the table legs is not overcome. Once it is, the table will be experiencing more force from your push than it is experiencing friction from the floor. When you push a table, it isn't the table that is pushing back, it is the static friction of the table against the floor.

    Put some felt on your shoes and push against the table, this time it is likely you that will move.
     
  9. Jun 12, 2012 #8
    I think its beginning to come clear to me now...

    So when I put felt on my shoes I move because the static friction between the table and the floor is greater than that of the friction of my shoes on the floor?
     
  10. Jun 12, 2012 #9
    The question is unclear to me. I can see why there is always going to be an 'unsatisfying' answer. What is supposed to be asked then? How? :biggrin:

    I think I was confused with Newton's third law mostly by thinking that the two forces cancel eachother and therefore give no net force on either object.
     
  11. Jun 12, 2012 #10

    Fredrik

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    The "a" in "F=ma" is the acceleration of one of the objects involved in this interaction. The "F" is the sum of all the forces acting on that object. The total force acting on the table can be thought of as the sum of four forces: You, friction, gravity and the normal force. The last two cancel each other out (this is obvious since there are no other forces, and we know that "a" is parallel to the floor). So the "ma" of the floor is equal to the sum of the remaining two forces. One of them is equal in magnitude and opposite in direction to the force that the table exerts on you.

    If you know the magnitudes of the two relevant forces acting on the table, then yes, you can calculate how the table moves. But you would usually not know the friction. You can however determine the friction by measuring the acceleration, if you know how hard you're pushing.
     
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