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Physics/Regarding Netwon's 3rd law-followup



Sorry to bother you again in this.

If earth accelerates and there is no equal and opposite reactionary force, will not the newton's 3rd law become invalid?

Also, in case of flights when it flies, where is the reactionary force acting?


ANSWER: No it will not.  It accelerates because of the force of the tires on the Earth, the reactionary force is on the tires from the Earth.  The net applied force to the Earth is equal to its mass multiplied by its acceleration.  There's no violation of any law there.


QUESTION: Thanks, Could you please answer the last part of the question

"Also, in case of flights when it flies, where is the reactionary force acting?"

Also planets go around the sun due to the gravitational pull of the Sun, where is the reactionary force acting on them in this case?

Right.  I take it to mean airplane flights?  Those are in a steady state of being dragged on by air as they push on the air.  Air provides all the forces involved through molecular collisions.  The reactionary force is between the colliding air molecules and the part of the plane they collide against.


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Dr. Stephen O. Nelson






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