(^78) Aptitude Test Problems in Physics
3.15. Two small balls of mass m, bearing a
charge q each, are connected by a noncon-
ducting thread of length 2/. At a certain in-
stant, the middle of the thread starts mov-
ing at a constant velocity v perpendicular
to the direction of the thread at the initial
instant.
Determine the minimum distance d be-
tween the balls.
3.16. Two balls of charge q 1 and q^2 initially
have a velocity of the same magnitude and
direction. After a uniform electric field
has been applied during a certain time, the
direction of the velocity of the first ball
changes by 60°, and the velocity magnitude
is reduced by half. The direction of the
velocity of the second ball changes thereby
by 90°.
In what proportion will the velocity of
the second ball change? Determine the mag-
nitude of the charge-to-mass ratio for the
second ball if it is equal to k 1 for the first
ball. The electrostatic interaction between
the balls should be neglected.
3.17. Small identical balls with equal
charges are fixed at the vertices of a right
1977-gon with side a. At a certain instant,
one of the balls is released, and a sufficient-
ly long time interval later, the ball adjacent
to the first released ball is freed. The kinet-
ic energies of the released balls are found to
differ by K at a sufficiently long distance from
the polygon.
Determine the charge q of each ball.
3.18. Why do electrons and not ions cause
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