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Why do forces make an object move?

  1. May 24, 2010 #1
    Hello everyone,

    May be the question is when something has energy and there is no resistance why does it move? Thanks!!
     
  2. jcsd
  3. May 25, 2010 #2
    A force by definition is a change in velocity of some object with mass m. Therefore by definition a force moves an object.
     
  4. May 25, 2010 #3
    Forces can make an object to accelerate only if the resultant force (vector sum of all forces) is non-zero.
    If you ask why, it is the effect in order to neutralize the cause. For example, if there is a potential difference between two points, electric current flows from high potential to lower one, provided there is a path, to bring both the points at same potential. It seems that nature favors equality and does action whenever wherever possible to attain that.
     
  5. May 25, 2010 #4
    I can take a stone and push it uphill, thereby making things "unequal" if your definition of equality is all things are at an equal potential. Objects move when they experience a "force" because that's what we define a force to be. Forces don't have to be real things.
     
  6. May 25, 2010 #5
    Forces can also stop movement. For example, a sliding block on a horizontal surface will eventually stop due to the friction force acting on it.
     
  7. May 25, 2010 #6
    Well...change in velocity, as already discussed. Stopping and starting is all potato potarto. xD

    Sort of going out on a tangent...The concept of a force is kind of weird. I don't know what teachers expect to teach kids when they talk about forces.
    Especially since acceleration is generally discussed year(s) later. To be honest, I never really understood anything about forces until I did some F=ma examples. And then the confusion begins again with F=mv^2/r.
    I guess it's confusing because forces don't really exist per se, but acceleration does.
    I think people get mixed up with all the force/force field stuff without relating them back to their affect on stuff's accelerations.
    /o\ just rambling.
     
  8. May 25, 2010 #7
    Thanks for all the answers. My question is more however, let's say I push a box on the floor, it gains energy and moves. Why does an object decide to change in position when energy is gained. Is this something we know because of observation. I think this question is more philosophical.
     
  9. May 25, 2010 #8
    Why does energy behave the way it does?

    Hello everyone,

    I don't know I have got really confused. Now if an object has energy, it can move. Is their any scientific explanation in molecular level or something why does energy make an object move. Also how do net forces work. If their is a box at rest and two people are pushing it with equal force in opposite directions. The object stays at rest. My question is what happens to the energy provided by two people. Does it cancel (how?), or does the object keep gaining energy without moving? I don't understand how forces give energy in a certain direction, what determines the direction? Thanks!! :smile:
     
    Last edited: May 25, 2010
  10. May 25, 2010 #9
    Forces do really exist in nature and they are measurable. As we all know its SI unit is Newton.
    I would like to say, when an object is disturbed by a force and with no resistance in the path, it will move (accelerate) in order to show its opposition to the source which is creating force. The source has to impart some energy to it, thereby it is weakened.
    Force can be viewed as a disturbance and the objects react to eliminate/de-magnify it.
     
  11. May 25, 2010 #10

    Doc Al

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    Re: Why does energy behave the way it does?

    The people are not providing any energy to the box.
     
  12. May 25, 2010 #11
    Re: Why does energy behave the way it does?

    Thanks for the reply :smile: When one person pushes on the box, due to momentum collision energy is transferrred to the box. Then the other person who pushes in the opposite direction also transfers energy to the box. I don't understand why no energy is transferred?
     
  13. May 25, 2010 #12

    russ_watters

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    Re: Why does energy behave the way it does?

    No. Force is not energy. Energy (work) is force times distance, so if there is no disance, there is no energy.
     
  14. May 25, 2010 #13

    Doc Al

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    Re: Why does energy behave the way it does?

    Are you talking about pushing the box or colliding with the box? If you push something and it doesn't move, then you are not transferring any mechanical energy to that something.
     
  15. May 25, 2010 #14

    russ_watters

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    The question isn't philosophical, it is just a misunderstanding of what energy is: you're looking at the issue backwards. Energy is the biproduct of the movement, not the cause. The force is the cause of the motion.
     
  16. May 25, 2010 #15
    Re: Why does energy behave the way it does?

    Thanks again Doc Al. I have lots of misunderstandings in physics. When something is pushes I'm using the momentum theory for that, so it doesn't work? Why does force have a direction? Could you explain it to me when two opposite forces act, why do they cancel each other. When there is a box on the table. Box due to gravity pushes down on the table, then a normal reaction force occurs, isn't this due to momentum transfer.
     
  17. May 25, 2010 #16

    russ_watters

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    Re: Why does energy behave the way it does?

    You're not being very clear: When you say "when something pushes" do you mean that the object being pushed moves? If it moves, energy is transferred and momentum is generated. If it doesn't move, no energy is transferred and no momentum is generated.
    It's just a property of force - it is a vector. There is no "why".
    It's just math: 1-1=0
    No, momentum is mv. If v=0, then momentum is zero.

    I think you need to stop thinking in terms of the words and start thinking in terms of the mathematical meanings of the words. You aren't using the words correctly. You have the definitions wrong and that's why you are getting confused about how these concepts interact.
     
  18. May 25, 2010 #17
    When something is acted upon by a force. It gains energy. So doesn't energy cause movement. Also if something is not acted upon by a force it can still travel at constant speed, that is because it has energy. I know force is something that causes an acceleration. What exactly is a force physically though. Is this going in a circle.
     
  19. May 25, 2010 #18
    Re: Why does energy behave the way it does?

    How is a normal reaction force generated. I thought it was due to momentum, the box pushes on the table, the table pushes on the box, like a momentum collision.
     
  20. May 25, 2010 #19

    Doc Al

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    Re: Why does energy behave the way it does?

    Nothing's moving, so where does momentum come in?
     
  21. May 25, 2010 #20
    Re: Why does energy behave the way it does?

    The box is trying to move but each time it loses energy to the table. Ok then if this is not right, how is normal reaction force generated. Also Russ said earlier that movement is a by product of energy, I mean how can you be certain it is not otherway round, meaning no clear cut answer.
     
  22. May 25, 2010 #21

    russ_watters

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    Re: Why does energy behave the way it does?

    A force can be generated due to change in momentum (a=f/m=dp/dt where p=mv), but if there is no change in momentum, then the force isn't generated that way. In a box sitting on a table, the force is generated in a different way: by gravity.
     
  23. May 25, 2010 #22

    russ_watters

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    Re: Why does energy behave the way it does?

    No. Again, you need to stop thinking in terms of the words and start thinking in terms of what the math says those words mean. Without the math, the words have no meanings. Math is the language of physics.
    In the case of a book on a table, the pair of forces is created by gravity.
    No, you said [implied] movement is a biproduct of (caused by) energy. I corrected you and said you have it backwards, that energy is a biproduct of movement. And how do I know it isn't the other way around? Again, it's the math. You must look at and think in terms of the math!

    w=fd
    e=.5mv^2

    What do these equations say about work/energy?
     
  24. May 25, 2010 #23
    Re: Why does energy behave the way it does?

    Ok first of all are most newton's third law action reaction pairs due to momentum collisions. Then why is normal reaction not an action reaction pair. Ok then if it gravity. Since gravity is the attraction between 2 objects. How is gravity creating a normal reaction force? Ok I apoligize for the mistake in the previous, that was unintentional, however how do you know from maths it is the other way round. If a force provides energy for the object, why is energy bi product of movement. How can you be certain like that?
     
  25. May 25, 2010 #24

    Doc Al

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    Re: Why does energy behave the way it does?

    No. You don't need a collision to have an action-reaction pair.
    The normal force is part of an action-reaction pair, just like all contact forces are.
     
  26. May 25, 2010 #25
    Re: Why does energy behave the way it does?

    Then how is normal reaction force created by gravity Doc Al. Also if gravity is attraction between two objects, why am I not attracted to objects near me beside the earth. All this time I thought change in momentum and newton's third law is the same thing. Is it not in the normal reaction force case, is their no change in momentum?
     
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