1. The problem statement, all variables and given/known data It's attached. 2. Relevant equations F = m*a F_f = -μ_k*F_N // [Force of friction] = μ_k*[Normal force] 3. The attempt at a solution So, I think I managed to solve it -- at least, I got what the book got, but I don't understand part of what I did to get the answer. This is how I solved it and got the answer the book got: I drew a free-body diagram, and realized that F_n = 2Mg Which means F_f = -μ_k*2Mg, Block 2 is being pulled by an effective force to the right of Mg (Since -Mg + 2Mg = Mg). So the summation of all the forces in the x direction would be: ***: Mg + F_f = Mg - μ_k*2Mg = (5*M)a Plugging in for a and other known values and some algebra later, I get μ_k = 0.37 And that's the answer the book gives. My question is why is it m = 5M in Newton's second law (the *** equation). When I first tried to solve it I used the mass of the 2nd block (m = 2M) and I thought that was correct.. apparently not. Please explain this to me. Thanks in advance!