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Old Jul 12th 2014, 12:00 AM   #1
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Normal force and gravity force are not action reaction pairs?

I know that Newton's third law is an action reaction on only two objects. In a situation with a box on the floor, is the reason why the normal force is not an action reaction pair with the force of gravity because there are more than two objects that are in play?

I was wondering what in the world is normal force and why does it point up from gravity. Is it because of another celestial body exerting its force?
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Old Jul 12th 2014, 09:20 AM   #2
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For a box sitting on the floor it is pulling pulled downward by the force of gravity between the box and the earth. And at the same time the Earth is being pulled up by that same force. The magnitude of these forces comes from Newton's Law of Gravity: GMm/r^2. So the box pulls on the Earth and the Earth pulls on the box.

Considering the reactions between the box and the floor: the box's weight (downward force) is equal to the force being applied upward by the floor of the exact same magnitude - that's the "equal and opposite" force, which we refer to as the normal force. So the box pushes on the floor and the floor pushes on the box.

And no - there are no other "celestial bodies" in play here.

Last edited by ChipB; Jan 28th 2015 at 08:12 AM.
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Old Jul 12th 2014, 03:01 PM   #3
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I quick point to make here.

Newton's third law pairs do not act on the same object. As in ChipB's comment above the third law pairs are the gravitational force on the object by the Earth (the object's weight) and the gravitational force on the Earth by the object. In the case of the normal force we know it cannot have to do with the third law pair to the weight because the weight and normal force are acting on the same body.

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Old Jul 12th 2014, 03:17 PM   #4
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Ooh thanks Dan and ChipB!

Lastly, say that the box is sitting on a table, would the earth be pulled up the force of the box or the combination of the box and table?
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Old Jul 12th 2014, 03:46 PM   #5
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Originally Posted by jlyu002 View Post
Ooh thanks Dan and ChipB!

Lastly, say that the box is sitting on a table, would the earth be pulled up the force of the box or the combination of the box and table?
You look at the problem one object at a time. For example, the box on a table: You have a normal force on the box by the table and you have a weight on the box by the Earth. For the table you have a normal force on the table (legs) by the floor, a normal force on the table by the box, and a weight on the table by the Earth. The normal force on the floor by the table incorporates the weight of both table and box through the normal forces.

Note how I listed these: The forces on the box always start out as "force on the box by the..." It's a good way to do the book-keeping.

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Old Jul 12th 2014, 08:32 PM   #6
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I had this same question. Thanks Dan this makes sense.

Last edited by hongiddong; Jul 12th 2014 at 08:35 PM.
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Old May 21st 2017, 09:21 PM   #7
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The normal force always formed perpendicular to the surface. You know that the normal force is calculated by mg cos theta. A box on the floor, its normal force is mg cos theta, but the angle between the mg and the normal force is zero. Therefore cos 0=1 and then normal force equals to the mg*1=mg. Therefore normal force equals to mg. The downward normal force is formed on the table, and the reaction of that formed on the box.(It form upwards). But it is same to the mg and the opposite direction. Therefore mg and the reaction of normal force is clear off and only the normal force to the table is balanced. (Note that when we keep something on a scale, it shows the normal force as the mat.reading)

Summery

The reaction of mg- formed by the center of the earth.(Don't think the reaction is normal force. Normal force is change when the angle is change{cos theta})

The reaction of the normal force- The upward force by the table, floor etc...
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