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Section 7: Centripetal Acceleration
Student view
Centripetal force is a force.
If a force is applied perpendicularly to an
objects velocity, it will start moving in that
When an object traveling in a circle, has a
constant speed, its velocity is not changing
and therefore cannot be accelerating.
If a car makes a sharper turn, the radius is
Physicist’s view
Centripetal force is any force that causes
uniform circular motion. Centripetal force is
caused by many types forces - gravitational
force causes the Earth to go around the Sun,
electrical force causes electrons to be
attracted to the nucleus, frictional forces
cause cars to make turns on the highway, and
tension in a sting makes a toy car move in a
If a constant force is applied perpendicularly
to an objects velocity, the result will be
uniform circular motion. But if the force is
extremely large it will not be noticed as a
curved path because the change in velocity
will be so great that it will appear to now be
moving nearly in the direction of the applied
perpendicular force.
Since velocity is a vector quantity, an object
that is traveling in a circle is constantly
changing its direction and is constantly
accelerating. And if the change is constant
as in uniform circular motion (constant
speed), its acceleration is constant.
When you draw the diagram of the turn and
draw the radius of the turn, it can be seen
that a tight turn (e.g. a sharper turn) has a
small radius.
In order to keep an object moving at a
constant speed, a force must be applied.
When rounding a curve, a force pushes you
away from the center of the curve and into
the door on the outside of the curve = a
centrifugal force.
Newton’s First Law of Motion states that an
object in motion remains in motion and an
object at rest remains at rest unless acted
upon by an unbalanced force. This
misconception arises from student
experiences that objects slow down and stop
in the real world due to friction. In the next
chapter there will be many examples of
objects with extremely low friction that
continue to move until an unbalanced force
is applied.
The unbalanced force is the centripetal force
toward the center of the curve caused by
either the friction from the seat or the seat
belt or the inside of the door. What is
happening is the passenger in the car is
traveling in a straight line while the car
makes a turn. The passenger travels in a
straight line and the car door turns into the
passenger. Since students have not
discussed inertia and Newton’s First Law of
Motion this is a topic for the next chapter.