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Circular motion does not produce an outward force
My younger self’s understanding of circular motion was partly right and
partly wrong. I was wrong in believing that there was a force pulling me
outward, away from the center of the circle. The easiest way to understand
this is to bring back the parable of the bowling ball in the pickup truck
from chapter 4. As the truck makes a left turn, the driver looks in the
rearview mirror and thinks that some mysterious force is pulling the ball
outward, but the truck is accelerating, so the driver’s frame of reference is
not an inertial frame. Newton’s laws are violated in a noninertial frame, so
the ball appears to accelerate without any actual force acting on it. Because
we are used to inertial frames, in which accelerations are caused by forces,
the ball’s acceleration creates a vivid illusion that there must be an outward
force.
In an inertial frame everything makes more sense. The ball has no force
on it, and goes straight as required by Newton’s first law. The truck has a
force on it from the asphalt, and responds to it by accelerating (changing
the direction of its velocity vector) as Newton’s second law says it should.
(a) In the turning truck’s frame of reference, the
ball appears to violate Newton’s laws, display-
ing a sideways acceleration that is not the re-
sult of a force-interaction with any other object.
(b) In an inertial frame of reference, such as the
frame fixed to the earth’s surface, the ball obeys
Newton’s first law. No forces are acting on it, and
it continues moving in a straight line. It is the
truck that is participating in an interaction with
the asphalt, the truck that accelerates as it should
according to Newton’s second law.
Chapter 9Circular Motion