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NSTA Web Seminar:
Force and Motion: Stop Faking It!
Thursday, January 25, 2007
7:00 p.m. to 8:00 p.m. Eastern time
NSTA Web Seminar
Force and Motion: Stop Faking It!
Bill Robertson
January 25, 2007
Circular Motion—Which of the
following are true?
Objects
The force that
moving in a causes objects
circle at a
to move in a
constant
circle is called
speed are not the centripetal
accelerating
force
The force that
causes objects
to move in a
circle is called
the centrifugal
force
Centrifugal
force doesn’t
really exist
Is an object moving in a circle at
constant speed accelerating?
Velocity is a quantity that
includes both the magnitude
(the speed) and the direction.
Any change in speed and/or
direction is a change in
velocity.
Newton’s second law
ΣF = ma
ΣF: represents the net force acting on an object.
m: represents the mass of an object, which is a
numerical measure of its inertia.
a: represents the acceleration of the object.
So, there must be a net force
acting on something moving in a
circle at constant speed.
Let’s investigate that force
So, circular motion at a constant
speed requires a force that acts
toward the center of the circle. You
might be aware that we call that
force the centripetal (center
seeking) force.
Centripetal force is…
A force that you
A fictitious force that Just a name applied
have to add to your doesn’t really exist to the net force that
diagram because
already exists
the object is moving
in a circle
The centripetal force is a name
given to forces that are already
present, that happen to cause
something to move in a circle. In
this case, the friction between
Einstein and the record is the force
causing Einstein to move in a
circle. Therefore, friction is the
centripetal force.
A centrifugal (center fleeing)
force is a force that pushes you
away from the center of the circle in
which you’re moving.
Is the centrifugal force a real force?
Yes
No
Now look at things from Einstein’s
point of view
When you view things from a
rotating frame of reference, a
centrifugal force appears. This
force is very real to someone in
that frame of reference.
Another example
As viewed in the rotating frame of
reference
Centrifugal force does not exist if
you are not in a rotating frame of
reference. Centrifugal force does
exist in a rotating frame of
reference.
You can choose a frame of
reference in which the force of
gravity does not exist
True
False
In a freely-falling frame of reference
(think falling elevator), there is no
force of gravity. So, centrifugal
force is every bit as real as the
force of gravity.
Know of any other forces that come
into being in a rotating frame of
reference?
In a rotating frame of reference,
any moving object experiences a
sideways force known as the
coriolis force.
Think hurricanes.
Newton’s second law
ΣF = ma
ΣF: represents the net force acting on an object.
m: represents the mass of an object, which is a
numerical measure of its inertia.
a: represents the acceleration of the object.
image
The rotational analog to F=ma is
 = Iα
•  is the net torque acting on the system
• I is the moment of inertia of the system
• α is the rotational acceleration of the
system
Push on a door and try to make it
swing. First try pushing near the
hinge and then try pushing as far
from the hinge as possible.
In which case is it easiest to make
the door swing?
Near the hinge
Farthest from the hinge
Torque includes not just the
applied force, but the distance the
force is from the point of rotation.
Realizing that the net torque acting on an
object that’s not rotating is zero gives us
the “law of the lever.”
National Science Teachers Association
Gerry Wheeler, Executive Director
Frank Owens, Associate Executive Director
Conferences and Programs
Al Byers, Assistant Executive Director e-Learning
NSTA Web Seminars
Flavio Mendez, Program Manager
Jeff Layman, Technical Coordinator
Susan Hurstcalderone, Volunteer Chat Moderator
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