# consider a force of 72.7 N

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reshown Sep 26, 2014

Consider a force of 72.7 N, pulling 3 blocks of identical masses, where each mass is 0.8 kg, along a rough horizontal surface. The coefficient of kinetic friction between the blocks and the surface is given by 0.8. The acceleration of the blocks is $a$. The acceleration of gravity is 9.8 m/s2 . Find the acceleration a.
Treat the three blocks as a single body with $M=0.8\times 3 (kg)$. The total normal force is $Mg$. The total frictional force is $f=\mu Mg$. The net force is then $F_{net}=72.7-f=Ma$, which gives the acceleration.
The horizontal forces applied on block m2 include $T_1$, $T_2$, and $f=\mu m_2g$. Define the "right" as the positive direction, one can find the net force.