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Abid Rizvi
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Homework Statement
What horizontal force must be applied to a large block of mass M shown in the figure below so that the tan blocks remain stationary relative to M? Assume all surfaces and the pulley are frictionless. Notice that the force exerted by the string accelerates m2. (Use the following as necessary: m1, m2, M, and g.)
Homework Equations
F = ma
The Attempt at a Solution
So I found the right answer to be (M+m1+m2)* ## \frac{m1g}{m2}##, but I don't understand why. My way was:
T = ##m_1##a
##m_2##g-T = ##m_2##a
##m_2##g = ##m_2##a + ##m_1##a
so therefore a = ##\frac{m_2 g}{m_1+m_2}##
now we know that F = ma, and I know that the entire system must have the same acceleration as the acceleration of the two blocks so that they don't move relative to the big block. Therefore (M+m1+m2)* ##\frac{m_2 g}{m_1+m_2}## was my answer. So what is going wrong here is my acceleration. But why? Also I found two pdf's from two different universities containing the solution to the problem. Both have different solutions...
http://www.physics.oregonstate.edu/~mcintyre/COURSES/ph211_S04/PHpdfs/hw5_solns.pdf
http://opencourses.emu.edu.tr/pluginfile.php/6813/mod_resource/content/1/tutorial_The_Laws_of_Motion.pdf
Interestingly enough my solution was more similar to the first pdf (which was wrong) but both pdf's say ##m_2##g-T = 0 where I had ##m_2##g-T = ##m_2##a. The only difference between the two are that the fraction at the end of the answer is flipped
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