- #1

- 20

- 0

## 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

*m*2. (Use the following as necessary:

*m*1,

*m*2,

*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 [Broken]

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

Last edited by a moderator: