Uniform circular motion can be described as the motion of an
object in a circle at a constant speed. As an object moves in a
circle, it is constantly changing its direction. Because of this
direction change, you can be certain that an object undergoing
circular motion is accelerating (even if it is moving at constant
speed). And in accord with Newton's second law of motion, an
accelerating object must be acted upon by an unbalanced force. This
unbalanced force is in the same direction as the direction of the
acceleration. For objects in uniform circular motion, the net force
and subsequent acceleration is directed inwards. It is often said
that circular motion requires an inward (or "centripetal")
force.
Without a centripetal force, an object cannot travel in circular
motion. In fact, if the forces are balanced, then an object in motion
continues in motion in a straight line at constant speed. This can be
demonstrated by carrying a tennis ball upon a flat, level board. Once
the tennis ball and the board are in motion, they will continue in
motion in the same direction at the same speed unless acted upon by
an unbalanced force. This is in accord with Newton's first law of
motion. But if an unbalanced force is applied to the flat board, then
the flat board will accelerate. If the force is continually directed
towards a point at the center of the circle, then the flat board will
round the corner in a circular-like path. The ball on the other hand
will continue to move in the same direction since there is no
unbalanced force acting upon it. The board will move out from under
the tennis ball. This is illustrated in the animation on the left
below.
Now if a block is secured to the board in such a manner that the
block applies an unbalanced force to the ball that is directed
towards the center of the circle, then quite another phenomenon will
be observed. With the block providing a normal force directed inward,
the ball can round the corner in a circular-like path. The block
supplies the centripetal force required for circular motion. With the
centripetal motion requirement met, uniform circular motion can
occur. This is illustrated in the animation on the right below.
Without a
centripetal force, an object in motion continues along
a straight-line path.
With a centripetal
force, an object in motion will be accelerated and
change its direction.
The tendency of a moving object to continue in a straight line in
the absence of an unbalanced force and to turn in a circle in the
presence of a inward-directed force (i.e., centripetal force) has
been experienced by any passenger in an automobile. When the car
makes a sudden turn, the passengers tend to continue in their
straight line path. This straight line motion continues until the
presence of a side door or another passenger pushes upon the
passenger in order to accelerate him/her towards the center of the
turn. The force experienced by the passenger is an inward force;
without it, the passenger would slide out of the car.
For more information on physical descriptions of motion, visit
The
Physics Classroom. Specific information on circular motion
will be available soon at The Physics Classroom. Currently, specific
information is available about the following topics:
