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What is a force?

  1. Jul 19, 2010 #1
    I thought I understood what a force is. Recently I'm reading up on it more thoroughly and found several 'interpretations'.
    Definition: A force is a push or a pull; produces or tends to produce motion, and stops or tends to stop motion.

    Question: What type of forces are there?
    Answer 1: Two - push and pull
    Answer 2: Two - contact force (eg. friction) and non-contact force (eg. gravitational force)
    Answer 3: Two - contact force and force field (??)
    Answer 4: There are many types of forces, such as weight, friction, tension, resistance.

    Those are some answers I found. Does the phrasing of the question matter? "What type of forces are there?" vs "What are some examples of forces?"
    This may be a fundamental concept so I hope to get a clear picture of what it is to help me in my own tutoring. Thanks much!!
  2. jcsd
  3. Jul 19, 2010 #2
    Fundamentally, in physics, there are 4 types of forces: gravitational, electromagnetic, weak (something to do with radioactive decay) and strong force (binds the nucleus). That means that, other than gravity, all the forces we feel in everyday life are electromagnetic.

    Of course, it's not convenient to describe everything that way, so we talk about friction, and normal forces, surface tension (force per length), etc. In that case I don't think there's any limit to the number of distinct forces you can define.

    Contact and non contact would be one way of classifying forces. I'm sure you can find other ways to classify them as well. It just depends on the context of the question. Are you a physicist or an architect? Think about this, you wouldn't call the electromagnetic force a contact force. But if all "typical" forces are electromagnetic, then none of them are contact forces. Do the atoms of your hand "really" touch the atoms of the table? Fundamentally, the charges are just repelling eachother across a small distance.
  4. Jul 19, 2010 #3


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    Answer 4 is merely a restatement of push or pull. I.e: Weight is a pull of an object on another and vice versa. Friction is a push. Tension is a pull. Resistance is a push. No matter what wording is used, every force can be described as a push or a pull.
  5. Jul 19, 2010 #4
    But not all forces push *and* pull. Gravity for instance will only attract, while the EM, Strong and Weak field will attract and repulse. Strong and Weak fields become equally as strong as the EM field at extremely high energies. However Gravity remains on it's own and is separate of any of the 3 other forces. Gravity is by far the weakest force and is caused by curvatures in space-time. To connect all 4 forces of Nature is one of the Holy Grails in physics.

    The basic idea of Force is F = ma, where Force is equal to mass times acceleration. Acceleration may be positive or negative but mass is always positive. In other words, the more you accelerate (or de-accelerate) an object, the more force is being applied. Gravity can be thought of as acceleration, since the longer an object will free-fall due to gravity, the more it will be accelerated by gravity.
  6. Jul 19, 2010 #5


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    Newton's III laws together are probably the best operative definition of force. Fuzzy mentioned II.

    Classically, any time there's a change in motion, there's a force involved. Any interaction between two particles (or a particle and a field, if you like, though fields are generally generated by other particles) is governed by a force.

    The reciprocal is not always true: There can be forces present without change in motion (but this would mean that the sum of the forces are zero.) This is a result of Newton's third law: when two bodies interact, they exert equal forces on one another.

    Notice also, that there can be motion without force (constant velocity) as long as the motion (velocity) isn't changing. Formally, Newton's first law says that an object will continue on its path (constant velocity) until acted on by a force.
  7. Jul 20, 2010 #6
    I believe it is in Fabric of the Cosmos where author Brian Greene talks about a theory were, under certain conditions, gravity can be repulsive.
  8. Jul 20, 2010 #7
    If gravity can be repulsive, it's yet to be discovered. I have a belief that it could be repulsive. It's mesmerizing what could happen if gravity would be repulsive, or if mass had negative values, etc.
  9. Jul 20, 2010 #8


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    That book had an interesting discussion on Newton's Bucket in the beginning... but then he lost me.
  10. Jul 22, 2010 #9
    Thanks for the clarification.
    FYI i'm editing secondary school student study materials, so it has to be simple, educational and conceptually sound.

    Fundamentally, there are 4?? This is new to me. Especially the weak and strong. If it's not weak, then it must be strong? Or are they specific to radioactive decay and atomic scale?
    I don't get this statement too: "...other than gravity, all the forces we feel in everyday life are electromagnetic." :confused:

    When we talk abt contact between hand and table, isn't it viewed at the scale of the object in question, ie. the hand and the table? Will it make a significant difference to look into that micro level of the atoms of the hand touching the atoms of the table? Even if it's true, do we need to go into that great a detail at this level?

    I'm looking at how to approach forces at introductory level. I thought of beginning by stating force is a push or a pull. And then classifying in general 2 types of forces - contact and non-contact. Then to cover specific examples: friction, weight, tension, resistance, electric force, magnetic force. Newton's 3 laws will also be covered.
    Thanks for the help! :redface:
  11. Jul 22, 2010 #10
    Please, Please don't.

    How many more pupils will be driven away from physics simple because the first force that they are introduced to is friction?

    I remember a physics teacher (a woman), years ago, who got the boys' interest by starting with a contest.
    She started with a spring balance and asked
    "who can pull it out with the most force?"

    Start with a direct, obvious force within everyones' experience. Introduce the invisible ones later.
    Last edited: Jul 22, 2010
  12. Jul 22, 2010 #11
    Oh I didn't know friction is that...repulsive. :uhh:
    Good idea, may I include that as a suggested classroom activity?

    The plan I mentioned earlier is just a general outline. I want to get the concepts right first (push or pull, contact, non-contact etc). Of course at the start of the topic, the teacher will demonstrate or have an activity to show forces around us. After the whole idea of what is a force is understood, then the other forces are taught in detail that is required in the syllabus. Hence, enter friction and friends. Besides, coefficient of friction is not even covered in the syllabus that I'm handling.
    Well, the order it is taught is just as important as the content itself.
  13. Jul 22, 2010 #12
    I can't imagine why friction would be the first force anyone would introduce, but this is exactly what UK secondary schools were doing a few years ago. I can't say about now.
    Friction is one of the most difficult forces to comprehend or describe and you need the idea of reaction as well.

    Start with something simple like a push or a pull.
    Ask how you would apply this a stick can do both a string can only pull.
    You can introduce reaction quite naturally by asking how do you actually apply a pull - eg a weight over a pulley.
    You can introduce Newton with this if you like.
    But this also leads to the idea of less obvious forces - gravity on the weight and their being the same as direct forces.
    You can then move on to other means of apply a force - magentism electricity and finally friction.

    I think other basic physical concepts are best treated in the same way.
    First introduce a direct, obvious form eg heat for energy
    Do a little manipulation eg specific heat
    Then introduce other forms aand show they are the same.
  14. Jul 23, 2010 #13
    Thanks for the ideas, Studiot.
    Friction may not be the first force but certainly one of the first (from the various text that I refered to). One of my reference text first mentions friction as the force that affects motion, right after Newton's 3rd law. Friction here is simply stated as a force. Friction is covered further in a later chapter abt how it is useful & ways to reduce friction.
    Surprisingly, (at least to me) this text also covers collisions before energy, which is worse cos I think we need to understand conservation of energy before collision, right?

    Anyway, the problem now is more on getting the build-up of (correct) concepts right. Am I on the right track here? :frown:
    Last edited: Jul 23, 2010
  15. Jul 23, 2010 #14
    I am not a teacher, but yes that is my opinion.

    It is debatable whether Friction is even just a force.
    Certainly forces may act when friction is involved but things are more complicated.
    Energy (heat) is generated whenever there is also movement.
    And what about internal friction in a fluid or sandpile or even a solid?

    But what about this situation.

    Set two bricks or blocks side by side on a table.
    Bring them slowly together so they just touch and allow them to stand there.

    What are the forces acting between the blocks?
    Is there any frictional force acting?

    I prefer to talk about the frictional force rather than friction and say that Friction is an effect or phenomenon that can occur between two physical surfaces, internal or external to a body.
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