(^30) Aptitude Test Problems in Physics
of centres of the box and the washer. At an
instant to, the washer hits the box.
Determine the time dependences of the
displacement Xwash of the washer and of
its velocity Vwash relative to the table,
starting from the instant to and assuming
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Fig. 27
that all the impacts of the washer against
the box are perfectly elastic. Plot the graphs
Xwash (t) and /2, ash (t). The friction be-
tween the box and the washer should be
neglected.
1.57. A thin hoop of mass M and radius r
is placed on a horizontal plane. At the ini-
tial instant, the hoop is at rest. A small
washer of mass m with zero initial velocity
slides from the upper point of the hoop along
a smooth groove in the inner surface of the
hoop.
Determine the velocity u of the centre of
the hoop at the moment when the washer
is at a certain point A of the hoop, whose
radius vector forms an angle cp with the ver-
tical (Fig. 28). The friction between the
hoop and the plane should be neglected.
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