- #1
matineesuxxx
- 77
- 6
Homework Statement
You are holding a special pendulum with two masses [itex] m_1 [/itex] and [itex] m_2 [/itex], instead of one, connected by a rope as shown in Fig. You lower the pendulum such that the tension in the rope between the two masses is half the weight of the bottom mass. Find the acceleration with which you lower the pendulum.
Homework Equations
[itex]\sum \hat{\text{F}} = m\hat{a}[/itex]
[itex]\text{w} = m\hat{g}[/itex]
The Attempt at a Solution
I thought that I simply needed to consider the bottom mass, [itex]m_2[/itex] and so since the forces acting on it are gravity and the tension between the two masses, I set up my equations as such:
[itex]\sum \hat{\text{F}}_\text{y} = \hat{\text{T}}_{1,2} - m_2\hat{g} = -m_2\hat{a}[/itex], where [itex]\hat{\text{T}}_{1,2}=\frac{1}{2}m_2 \hat{g}[/itex]
so I end up with [itex]\hat{a} = \frac{\hat{g}}{2}[/itex], however the book gives an answer of
[itex]\hat{a} = \frac{m_2\hat{g}}{m_1 + m_2}[/itex] but I can only get that answer if I start with
[itex]\sum \hat{\text{F}} =\hat{\text{T}}_{1,2}- m_2\hat{g} = -(m_1 + m_2)\hat{a}[/itex]
Can anybody explain to me why they are using the sum of the masses as the net force acting on [itex]m_2[/itex]?