Transcript Circular Motion

Circular Motion and
Forces
Newton’s 1st Law tells us that an object in
motion will keep moving at the same speed
in a straight line unless an unbalanced
force acts upon it.
r
m
F
v
So…for an object to travel in a circular
path, that means that the object must have
a force applied to it.
That unbalanced force (or net force) must
be directed toward the center of the
circular path in order to cause the object to
move in a circle.
The object is trying to move forward in a
straight line at all times, BUT it is being
pulled into a circular path by that
unbalanced force.
What kind of forces can cause
an object to move in a circle?
•Tension
•EX: spinning an object over your head on a string
•Friction
•EX: friction between the road and your tires causes
your car to turn
•Gravity
•EX: The moon orbiting the earth, OR the sun orbiting
the moon, OR a satellite orbiting the earth
•Normal Force
•EX: The wall of the washing machine pushing against
your clothes during the spin cycle, OR the wall of a
spinning amusement park ride.
What is Centripetal Force ?
the general name given to any force or
combination of forces that causes an object
to move in a circle.
Since it is a force, it is a vector with the
direction toward the center of the circular
path.
So, any of the forces listed previously
(tension, friction, normal, or gravity) can
be considered centripetal forces.
What is centripetal acceleration?
Newton’s 2nd Law tells us that if there is a force
there must be an acceleration.
The acceleration of an object in circular motion is
called the centripetal acceleration.
It is ALWAYS directed toward the center of the
circular path.
It depends on the speed of the object and the
radius of the circular path.
What is this “centrifugal” force I have heard
of all my life?
•Actually there is no such thing as a centrifugal force! It is fake!
•If a force is defined as a push or pull, then we can clearly identify
the force pulling on the object toward the center of the circular
path, the centriPETAL force.
•Centrifugal force, however is commonly thought to be the force
pulling the object outward from the center. Think about it! There is
nothing on the outside of the circle pulling on the object.
•Newton’s third law tells us that “every action force has an equal
and opposite reaction force” and since we can clearly feel
something pulling against us when we twirl an object attached to a
string over our heads, there must be an outward force…right?
NO!
•What you are feeling is the effect of the inertia of the object that is
trying to cause the object to continue moving in a straight line path
at all points. This gives the effect of a force, but in reality is not a
push or pull!
For an object traveling in a
horizontal (flat) circle…
•Gravity affects the object at all points on the circle the
same, there fore it does not have any effect on the
circular motion itself.
•Centripetal force will usually be caused by a single
force (ex: tension, gravity, friction, or normal)
•Speed of the object will usually remain constant which
means that the centripetal force remains constant for all
parts of the circular path.
For an object traveling in a
vertical circle…
•Gravity is definitely a factor!
•Speed changes as the object moves from
the top of the circle to the bottom.
•Speed is greatest at the bottom of the
curve and slowest at the top as a result of
gravity acting on the mass.
•Therefore tension in the string is less at the
top than it is at the bottom of the path.
•The centripetal force in this case is a combination
of the tension and gravity.
Force Diagram for a vertical circle
Remember: Fc is the
net inward force!
T
v
v
Top: Fc = Fg+T
Fg
r
Sides: Fc = T
T
T
Fg
Fg
T
Fg
T
v
v
Fg
Bottom: Fc =T-Fg
For a mass on a string moving in a vertical circle,
the centripetal force is due to different forces in
different locations.




At the top of the circle, Fc = T + Fg (tension plus
weight or gravity)
At the bottom of the circle, Fc = T - Fg (tension
minus weight or gravity)
On the outermost side, Fc = T
Anywhere other than above, you would need to
find the component of gravity parallel to the
tension and either add or subtract tension
depending on the location on the circular path