Download Circular Motion - Effingham County Schools

Circular Motion
http://www.scicomics.com/uploads/centripetal_acceleration.jpg
Rotation vs Revolution
• Rotation occurs when an
object rotates around an
internal axis
– ex: the earth rotates around its
axis (internal) every day
• Revolution occurs when
an object rotates around
an external axis
– ex: the earth revolves
around the sun (external
axis) every year
Rotational Speed
• also called angular
speed
• equals the number of
rotations per time unit
– All parts of the earth
rotate about its axis in the
same amount of
time…thus all parts of the
earth have the same
rotation …or the same
number of rotations per
time (revolutions per
minute or RPM)
When a twirling ice skater
extends her arms outward,
her rotational speed
decreases.
Tangential Velocity
• While the speed of
the object is
constant, its
velocity is
changing.
Velocity vector and position
vector for an object executing
uniform circular motion
Tangential Speed
• Tangential speed is directly proportional
to rotational speed and the distance
from the axis
Tangential ~
Speed
Radial x
distance
Rotational
Speed
-at the center of the earth…there is no
tangential speed, (zero distance) but there
is rotational speed
What type of speed is changing –
rotational or tangential?
fast
twice as
fast
Mathematical Relationship
between tangential and rotational
speed
v=r
v = tangential speed
r = radial distance
 = rotational speed
Accelerometer
• The deflection of the
flame will be in the
direction of the
acceleration. This is
because the hot gases
of the flame are less
massive and thus
have less inertia than
the cooler gases which
surround
Centripetal Force
Fc = mv2 / r
• the force required to
change the direction
of a moving object
http://webpages.uah.edu/~wilderd/resources.html
http://www2.ignatius.edu/faculty/decarlo/Centripetal%20force.htm
Centripetal Force
• center–seeking or toward the
center
• any force that causes an object
to follow a circular path
– think about the force you feel when riding a
fast circular ride at an amusement
park…without that force you would
continue to move in a straight line…being
“launched” form the ride
Centripetal Force
As a car makes a turn, the force of friction acting
upon the turned wheels of the car provide the
centripetal force required for circular motion.
As a bucket of water is tied to a string and spun
in a circle, the force of tension acting upon the
bucket provides the centripetal force required for
circular motion.
As the moon orbits the Earth, the force of
gravity acting upon the moon provides the
centripetal force required for circular motion.
Centrifugal Force…False Force
• center-fleeing or away from the center
• An apparent outward force that acts on a
rotating object…it is due to inertia…an
object’s desire to continue moving in a
straight path.
• The sensation of being thrown outward is
attributable to the idea of inertia, rather than
the idea of force.
• Centrifugal force holds people on walls in
Round Up ride
The Fictitious
Force
http://www.scigolf.com/scigolf/myths/myth2.htm
• When a ball swings on a
string, many people
would say that
"CENTRIFUGAL
FORCE" keeps the
string taut.
– There is actually no force
pulling the ball out.
– There is nothing out there
pulling the ball!
– All the sideways and back
and forth forces you feel
while riding in a car are
FICTITIOUS, they are the
results of your inertia
resisting acceleration
Tether Ball
• Twirl a ball, at a constant speed,
at the end of a rope.
change of direction = acceleration
http://www.hoopsplus.com/nssfolder/outdoorequipment/tether.jpg
http://www2.ignatius.edu/faculty/decarlo/Centripetal%20force.htm
http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/circmot/rht.html
http://www.coolschool.ca/lor/PH12/unit4/U04L01.htm
• Gravity: force of
attraction between two
objects.
• Newton noted that the
moon was falling
toward the Earth.
• The moon falls along a
curved path. It acts as
a projectile.
• The moon has a
component of velocity
parallel to the Earth’s
surface known as
tangential velocity.
• The force of gravitational attraction
decreases as you move further from the
center of the Earth.
• The moon orbits the Earth, just as the Earth
& other planets orbit the Sun.
• The Sun is the center of the solar system.
Universal Gravitation
• Law of Universal
Gravitation: Every object
attracts every other object
with a force that for any
two objects is directly
proportional to the mass of
each object.
• Ocean Tides are due to the
differences in the gravitational pull of
the moon on opposite sides of the
Earth. Variations in sun, moon and
earth alignment causes low and high
tides.
• http://oceanlink.island.net/oinfo/tides/tides.html
• http://home.hiwaay.net/~krcool/Astro/moon/moontides/
Spring & Neap Tides
• the sun and moon are aligned,
there are exceptionally strong
gravitational forces, causing
very high and very low tides.
• The sun and moon are not
aligned, the gravitational forces
cancel each other out, and the
tides are not as dramatically
high and low.
Center of Gravity
• Located at the object’s
average position of weight
(or balance point).
• For a symmetrical object
such as a ball. The CG is
located in the center.
• For an irregular shaped
object (a bat), the CG is
located towards the more
massive end.
Toppling (falling over)
• If the CG is located above the area of
support, the object will not fall. If it is
outside the support area, the object will fall
over.
• Neutral, Stable and Unstable Equilibrium.
Torque
• The turning force used to turn a door knob or
tighten a bolt produces torque.
• Torque is defined as force times length of lever
arm. For greatest torque, force should be
applied perpendicular to the lever arm.
Torque = Force x distance
• Notice that the unit of measurement for torque
contains a distance (meter) and a force (Newton).
• To calculate the torque needed to turn something,
multiply the force by the distance from the center of
the object you are trying to turn.