Bukhovtsev-et-al-Problems-in-Elementary-Physics

MECHANICS 29
(Fig. 39). What should the tension of a rope tied to the middle
of the ladder be to prevent its falling down?
108. A man climbs up a ladder leaning against 8 smooth
vertical wall. The ladder begins to slip only when the man
reaches a certain height. Why? .
109. A picture is attached to a vertical wall by means of string
AC with a length I forming an angle a with the wall. The height
of the picture BC = d (Fig. 40). The bottom of the picture is
not fastened. At what coefficient of friction between the picture
and the wall will the picture be in equilibrium?
110. Four homogeneous rods are pin-connected to one another
at points B, C and D (Fig. 41). The two extreme rods AB and
DE can freely revolve with respect to stationary points A and
E on a horizontal straight line. The lengths of the rod AB = ED
and BC= CD. The masses of the rods are the same. Show that
the angles a and ~ are related by the ratio tan ex, = 3 tan ~ when
in equilibrium.
111. What is the coefficient of friction between a floor and
a box weighing one ton-force if a minimum force of 600 kgf is
required to start the box moving? .
112. A weightless unstretchable string is wound around a cylinder
with a mass m (Fig. 42). With what minimum force F min
and at what angle