175
Discussion Questions
A. In the game of crack the whip, a line of people stand holding hands, and
then they start sweeping out a circle. One person is at the center, and rotates
without changing location. At the opposite end is the person who is running
the fastest, in a wide circle. In this game, someone always ends up losing their
grip and flying off. Suppose the person on the end loses her grip. What path
does she follow as she goes flying off. (Assume she is going so fast that she
is really just trying to put one foot in front of the other fast enough to keep from
falling; she is not able to get any significant horizontal force between her feet
and the ground.)
B. Suppose the person on the outside is still holding on, but feels that she may
loose her grip at any moment. What force or forces are acting on her, and in
what directions are they. (We are not interested in the vertical forces, which
are the earth's gravitational force pulling down, and the ground's normal force
pushing up.)
C. Suppose the person on the outside is still holding on, but feels that she may
loose her grip at any moment. What is wrong with the following analysis of the
situation. "The person whose hand she's holding exerts an inward force on
her, and because of Newton's third law, there's an equal and opposite force
acting outward. That outward force is the one she feels throwing her outward,
and the outward force is what might make her go flying off, if it's strong
enough."
D. If the only force felt by the person on the outside is an inward force, why
doesn't she go straight in.
E. In the amusement park ride shown in the figure, the cylinder spins faster
and faster until the customer can pick her feet up off the floor without falling. In
the old Coney Island version of the ride, the floor actually dropped out like a
trap door, showing the ocean below. (There is also a version in which the
whole thing tilts up diagonally, but we’re discussing the version that stays flat.)
If there is no outward force acting on her, why does she stick to the wall.
Analyze all the forces on her.
F. What is an example of circular motion where the inward force is a normal
force. What is an example of circular motion where the inward force is
friction. What is an example of circular motion where the inward force is the
sum of more than one force.
G. Does the acceleration vector always change continuously in circular
motion. The velocity vector.
Discussion question E.
Discussion questions A-D.
Section 9.1Conceptual Framework for Circular Motion