Centrifugal Force:
The Fictitious Force
Daniel J. Reichard
Definition
• Commonly known to be the outward force on
an object away from the center of rotation
– Examples:
• Water staying in a bucket when it is quickly flipped
upside down
• An object moving to the right side of a car when making
a sharp left turn
• Amusement park ride where passengers are stuck to
the walls as they spin rapidly
• However, this force does not actually exist
Why, then, does water stay in the bucket?
Why does an object
move to the right when
a car turns to the left?
Why does a
person stick to
the side of
rapidly
spinning ride?
The explanation for this phenomenon is simple
and does not involve a centrifugal force!
Connection: Circular Motion
• In order for an object to move in a circular
path, there must be an external force,
according to Newton’s first law of motion (law
of inertia).
• This force must be in the direction of the
object’s acceleration (towards the center),
according to Newton’s second law of motion.
• This force is called the centripetal force.
Centripetal Force
vt
• According to Newton’s law of inertia, an
object in motion will stay in motion with
the same speed and same direction unless
it is acted on by an external force.
vt
vt
Fc
Fc
Fc
vt
Fc
Fc
vt
Fc
Fc
vt
Fc
• Therefore, an object in circular motion
wants to continue moving in the direction
tangential to the circular path.
• However, a centripetal force pulls the object
perpendicularly to the object’s velocity.
• This creates a circular path.
vt
vt
Fc = centripetal force
Vt = tangential velocity
Explanation of Water Bucket Example
Why, then, does water stay in the bucket?
• When a bucket of water rotates in a vertical
circular path, the water appears to be stuck
to the bucket.
• The water wants to continue moving at a
velocity tangential to the circular path (vt).
• The sides of the closed bucket prevent the
water from doing so.
• Therefore, the water is forced to move in
the same path as the bucket.
• There is no force pushing the water away
for the center (no centrifugal force).
Instead, the bucket is pushing the water
toward the center of rotation.
Explanation of Car Example
• The object in the car wants to continue
to move in a straight line even though
the car is turning.
• The object initially does continue to
move in a straight line, but it eventually
is stopped by the right side of the car.
• As the car continues to move in a
circular path, the object remains
against the right side of the car, just like
the water in the bucket.
• Once again, there is no centrifugal
force acting on the object. Instead, the
side of the car is pushing the object in a
circular path.
Explanation of Spinning Ride Example
• Like the other two examples, the object,
in this case a person, wants to continue
moving in a straight line due to inertia.
• However, the walls of the spinning ride do
not allow the person’s body to move in a
straight line.
• Therefore, the person becomes “stuck” to
the wall even though there is no force
pushing the person against the wall.
Conclusion
• In conclusion, there is no such thing as a
centrifugal force.
• Circular motion of any kind can be explained
using the law of inertia and centripetal force.
• There is no force that pushes an object away
from the center of its rotation. The reason the
object moves in a circle is because it wants to
continue to move in a straight line tangent to
the circular path.
New Insight
• After research on this topic, I gained a lot of
information on how and why objects move in a
circular path.
• It is interesting that a centrifugal force is a
common explanation for circular motion when it
is not even true.
• I can now easily visualize what really happens in
the real world with regards to inertia, centripetal
force, and circular motion.
• This new information is beneficial when solving
difficult circular motion problems.
Works Cited
• http://regentsprep.org/regents/physics/phys0
6/bcentrif/centrif.htm
• http://csep10.phys.utk.edu/astr161/lect/histo
ry/newton3laws.html
• http://en.wikipedia.org/wiki/Centrifugal_force