- #1
BOAS
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- 19
Homework Statement
On a winter’s day in Maine, a warehouse worker is shoving boxes up a rough plank inclined
at an angle α above the horizontal. The plank is partially covered with ice, with more ice
near the bottom of the plank than near the top, so that the coefficient of friction increases
with the distance x along the plank: [itex]µ = Ax[/itex], where [itex]A[/itex] is a positive constant and the
bottom of the plank is at [itex]x = 0 m[/itex]. (For this plank the coefficients of kinetic and static
friction are equal: [itex]µ_{k} = µ_{s} = µ.[/itex]) The worker shoves a box up the plank so that it leaves
the bottom of the plank moving at speed [itex]v_{0}[/itex].
Show that when the box first comes to rest, it will remain at rest if [itex]v_{0}^{2} \geq \frac{3g \sin^{2} (\alpha)}{A \cos (\alpha)}[/itex]
Homework Equations
The Attempt at a Solution
[/B]
Essentially, unless the box has large enough initial velocity, it will not reach far enough up the plank so that the static friction prevents it from sliding back down. It is therefore necessary for [itex]f_{s} \geq mg \sin (\alpha)[/itex], the component of the box's weight down the slope.
[itex]f_{s} \geq \mu_{s} N \geq xAN[/itex]
[itex]x \geq \frac{mg \sin (\alpha)}{AN}[/itex]
[itex]x \geq \frac{mg \sin (\alpha)}{Amg \cos (\alpha)}[/itex]
It feels like this is a relevant step, but I'm struggling to see where to go from here.
I don't know how to relate this to initial velocity...
Thanks for any help you can give.