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Homework Statement
A rectangular box is being pulled by a rope with a force T on a horizontal surface, with a friction coefficient μ. What is the minimum angle α at which it will lift of the surface? See attached image.
Homework Equations
\begin{equation*}
\tau = rF\sin\alpha
\end{equation*}
\begin{equation*}
F_y=F_g-Tsin\alpha
\end{equation*}
\begin{equation*}
F_x=F_f-Tcos\alpha
\end{equation*}
The Attempt at a Solution
OK, I am slightly confused here. Should I look at this as a torque problem where the y component of force T will cause rotation around the lower left corner until the angle the box makes with the horizontal is equal to the angle of the force T? In the key the answer is expressed as
\begin{equation*}
\alpha=tan^{-1}(\frac{a}{b}+\frac{1}{\mu})
\end{equation*}