Forces up are greater than forces down. However, the masses on the table obviously don't start flying upwards. Am I missing a force here?
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2$\begingroup$ There are too many forces in this picture. If you want to draw it like this, yes, you are missing the N3L pair of $N_2$, which is why it is not balanced. Ideally, you should be drawing just for the $m_1$ mass alone, so that you can see precisely how the things are balanced. $\endgroup$– naturallyInconsistentCommented Jul 19, 2023 at 9:53
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$\begingroup$ What does the L stand for in N3L? $\endgroup$– photonCommented Jul 19, 2023 at 10:06
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$\begingroup$ @photon If you sum all forces with direction , then all forces cancels out. $(M_1+M_2)g$ is canceled by $N_1$ and $M_2g$ is canceled by $N_2$. Try to balance forces on each mass seperately. $\endgroup$– AlvCommented Jul 19, 2023 at 10:24
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$\begingroup$ Newton's Third Law $\endgroup$– naturallyInconsistentCommented Jul 19, 2023 at 10:36
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$\begingroup$ N2 is an internal force of the two block system and should not be added to the external forces, (m1+m2)g down and N1 up which sum to zero for equilibrium $\endgroup$– Bob DCommented Jul 19, 2023 at 11:04
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1 Answer
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Given that $N_{12}$ means the normal force on body $1$ due to body $2$ and the forces are colour coded to identify the Newton's third law pairs here are the free body diagrams for the three systems under consideration.
You can then add the forces together for each of the systems and equate each of them to zero as there is static equilibrium.
