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Normal reaction, is this a concept we really understand?

  1. Jun 24, 2012 #1
    Hi guys ^^ im still at A level physics, so dont flame me for my lack of knowledge
    Now i have been taught that for ex:

    We have a block of wood named A at rest on a perfectly smooth surface.

    If we analyse a free body diagram, we have a weight mg downwards acting on the block and a normal reaction force acting at the boundary of the block and surface. Also we have the same normal reaction, by the block on the ground at the boundary too.
    So why do we have the misconception (maybe its me only XD) that the weight and the normal reaction cancel out.
    Is'nt it really the weight and the force the block attracts the earth(laws of gravitation) which cancel each other.
    I dont know if its "my" misconception, but i've seen many books very unclear about this.

    Feel free to comment. Thank you :)

    Edit: o.o if we assume what i said is true, then how do we assign a magnitude to normal reaction force? D: im getting confused

    Edit 2 : thanks a lot everyone, that cleared many doubts for me :)
    Last edited: Jun 24, 2012
  2. jcsd
  3. Jun 24, 2012 #2


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    That is not a misconception. The blocks weight and the normal force on the block cancel out when computing the net force on the block, which zero so the block remains static.

    The foces of gravity acting on block and earth act on two different objects. So they don't cancel out when computing the net force on any object. They do cancel out when computing the net momentum-transfer for the entire system, which must be zero for a closed system.

    Fnet = 0 so Fnormal = -Fweigth
  4. Jun 24, 2012 #3


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    You should always remember what object the forces you are looking at are acting on. The weight force and the normal force both act on the block of wood itself. The gravitational force that the wood attracts the Earth with acts on the Earth and not the wood block.
  5. Jun 24, 2012 #4


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    No. Those forces are indeed equal and acting in opposite directions, but they're acting on different things. One (the weight of the block) is acting on the block so wants to accelerate the block and the other is acting on the earth so wants to accelerate the earth.

    Suppose the block were not resting on the ground; we pick up and then let go of it so it falls. While it's falling, both of these forces are still at work and the block is accelerating (so is the earth, but the same force acting on much more massive earth produces no measurable acceleration), so it's pretty clear that these two forces don't cancel.

    When the block is resting on the surface of the earth, it's no longer accelerating, so something has to be opposing the force of its weight; that something is the normal force, which isn't present until the block and the earth are touching.
    Last edited: Jun 24, 2012
  6. Jun 24, 2012 #5


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