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Is there a reaction pair for $mg \sin \theta$ on a inclined plane?

It is there for the normal force on a block on a flat plane, so is it there for $mg \sin \theta$ also?

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  • $\begingroup$ $mg \sin \theta$ is the reaction force by the plane. It's equal to the vertical component of the gravitational force. See khanacademy.org/science/physics/forces-newtons-laws/… $\endgroup$
    – user191954
    May 19 '18 at 13:21
  • $\begingroup$ How are you defining $\theta$? $\endgroup$
    – garyp
    May 19 '18 at 16:12
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$mg\sin\theta$ is the component of the gravitational force which is directed down the ramp.   It's exerted by the Earth on an object which is on the ramp.   The reaction force is the equal and opposite gravitational force exerted by object on the Earth.

Force pairs are always interactions between two objects.   So you can always say that the force exerted by object A on object B is equal and opposite to the force exerted by object B on object A.   There are no exceptions.

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    $\begingroup$ One could argue that there are three force pairs here: gravity between the Earth/block; contact forces between the block/ramp, and contact forces between the ramp/Earth. $\endgroup$ May 19 '18 at 17:18
  • $\begingroup$ And let's not forget the gravitational force between the ramp and the Earth :) $\endgroup$
    – D. Ennis
    May 19 '18 at 17:35

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