I came upon this question as I was going through the concepts of tension. Well according to Newton's third law- every action has an equal and opposite reaction. Here my question is that if the tension at point B balances the tension at point A then which force balances mg as it can't be balanced by the reaction force of mg which attracts the earth towards the mass as it is not in contact with the mass. Then why doesn't the mass fall?
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$\begingroup$ Be careful of third-law reaction forces. They act on different bodies. The tension force at A acts on the block, the tension force at B acts on the ceiling. $\endgroup$– electronpusherCommented Nov 16, 2019 at 22:52
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$\begingroup$ What keeps the ceiling from falling? $\endgroup$– Adrian HowardCommented Nov 16, 2019 at 23:53
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$\begingroup$ Please someone draw the diagram of forces which balance each other $\endgroup$– user214279Commented Nov 16, 2019 at 23:59
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$\begingroup$ Is B attached to something that is falling, or the ceiling of a building, or what? $\endgroup$– Adrian HowardCommented Nov 17, 2019 at 0:06
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$\begingroup$ @AdrianHoward it's a fixed platform $\endgroup$– user214279Commented Nov 17, 2019 at 0:09
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2 Answers
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It is the normal force ( https://en.wikipedia.org/wiki/Normal_force ) of the Earth against the supports that hold the platform up. Normal force is what keeps our feet from sinking into the Earth, due to our weight, it keeps a book on a table from falling through the table. It will keep the supports from falling into the Earth, the supports hold the platform, which holds the string and mass.
answered Nov 17, 2019 at 0:25
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There are two forces acting on the mass are gravity and tension from the rope. This is seen in your free body diagram on the right side of your image. The mass does not fall (or rise) because these forces are of equal magnitude and opposite direction, thus the net force acting on the mass is $0$.
Yes, if we look at the rope the force pulling it down at $A$ is equal and opposite to the force pulling it up at $B$, but this doesn't make the force at $A$ suddenly nonexistent for the mass. There is a downward force acting on the rope at $A$, and by Newton's third law this means there is an upward force acting on the mass at $A$. This is the upward force discussed above. Note that the forces at $A$ and $B$ acting on the rope do not form a Newton's third law pair.
answered Nov 17, 2019 at 0:39
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$\begingroup$ Can you please share a diagram of all action reaction forces. $\endgroup$– user214279Commented Nov 17, 2019 at 13:52
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$\begingroup$ @Janstew If you reverse the directions of the vectors on your diagram on the left then you are good to go. $\endgroup$ Commented Nov 17, 2019 at 14:45
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$\begingroup$ Could you please draw it i can't visualize $\endgroup$– user214279Commented Nov 17, 2019 at 22:33