Transcript 05. RotationalReg
•Circular motion occurs about an axis
–Rotation: object spins about an internal axis
•Earth rotates about its polar axis once a day.
–Revolution: object moves about an external
•Earth revolves once about the sun each year.
Speeds involved in Circular Motion
• Linear speed (v): distance covered per unit
time by a point on the object. (m/s) (Also
called tangential speed)
• Rotational speed: amount of angle swept
out per unit time. (revolutions per minute
(rpm) or radians per second (1/s))
• On a rigid rotating object:
– Is rotational speed everywhere the same?
– Is linear speed everywhere the same?
For a Rigid Rotating Object
Linear velocity is proportional to rotational velocity
r = distance from axis of rotation.
“~” means “proportional to”
v = rω
if ω is in radians per second
Suppose you get a flat tire while
• You put on a “toy spare” tire that came with
• The toy spare tire is smaller than your other
• How does this affect your driving?
Period, Frequency and Speed
• Period (T) : Time for one full rotation or
revolution (one round trip).
– units are seconds (s)
• Frequency (f) : the number of rotations or
revolutions (number of “cycles”) per second.
– Units are 1/s also called “hertz” (Hz.).
• Relationship: T=1/f or f = 1/T
• For constant speed (v), circular motion:
– Speed = 2π(radius)/period or v = 2πr/T
1. I spin in a swivel chair at a frequency of .5 Hz.
What is the period of my spin?
2. A record has a scratch 12 cm from the center
that makes the record skip 45 times each
minute. What is the linear speed of the
What causes Circular Motion?
Suppose I swing an object at constant speed in a
circle. (“uniform circular motion”)
• Does the object have constant velocity?
• Does the object accelerate?
• Does the object feel a force?
• If so, what causes the force?
• In what direction is the force?
• How does the object move if I cut the rope?
• To keep an object in circular motion, we must
constantly exert a force
– Perpendicular to the object’s velocity
– Directed inward toward the center of the orbit.
• This direction is called the centripetal direction.
• The force is called the centripetal force.
• Examples of centripetal force:
– Tension in string, keeping ball in orbit.
– Sun pulling on Earth, keeping it in orbit.
– Earth pulling on Moon, keeping it in orbit.
Fictitious Forces: Centrifugal Force
• When a car turns left (inward, “centripetal”), why do you
feel pushed to the right (outward, “centrifugal”)?
• Do you feel like you can stand on the wall of the car?
• Can we simulate gravity by standing on the wall of a
Fictitious Force: Coriolis Effect
• A force we see due to the rotation of the Earth
and how things on Earth move.
– Foucault Pendulum: proved that Earth Rotates
– Affects projectile motion
– Affects flight plans of pilots.
• Coriolis Effect in Action:
Movie From Nasa
Wiley Animation and Discussion
Effects on Airplane Flight
Coriolis Effect on Wind Direction
(a closer look)
• There is a relationship between
– Centripetal Force (FC) and
• Speed of object (v)
• Radius of orbit (r)
• Mass of object (m)
• FC = mv2/r
• Let's see a movie of it.
• Now let’s test it in action.
• Circular motion requires the velocity to
constantly change direction.
• The acceleration causing this change of
direction is always directed towards the
• The term “centripetal” means “towards the
• Centripetal Acceleration: ac= v2/r
• Centripetal Force: Fc = mac= mv2/r
3. Missy’s favorite ride at the fair has a radius of
4.0 meters and takes 2.0 seconds to make one
a. What is Missy’s linear speed on the ride?
b. What is Missy’s centripetal acceleration on the
4. The captain of a 60500 kg plane flies in a
circle of radius 50.0 km. He completes a
circle every 30.0 minutes. What is the
centripetal force exerted by the air on the
wings of the plane?
Yet More Examples
1. A space station is in a circular orbit about the
Earth at an altitude h=500 km. If the station
makes one revolution every 95 minutes what are
its orbital speed and centripetal acceleration?
2. The Conical Pendulum
A mass, suspended
from a pivot, swings
in a circle as shown.
Find its period as a
function of r, m, θ,
A Lab We’ll Do.
3. A mass (m1) attached to one end of a
string is swung in horizontal circular
motion. At the other end of the string,
another mass (M2) is suspended. The
apparatus is as shown.
Find the period of the motion in terms
of m1 and M2.
• Attractive force
between two masses
• r = distance between
their centers of mass.
1. What is the Gravitational Force on
an object at the Surface of the Earth?
•Object has mass (m)
•Radius of the Earth: RE=6.4*106 m
•Mass of the Earth: ME=6.0*1024 kg
•Big G: G = 6.67*10-11 Nm2/kg2
2. An Object in Circular Orbit
• What is the condition for a stable orbit?
Set gravitational force equal to centripetal force.
3. Kepler’s Laws of Planetary Motion
(consequence of Newton’s Laws)
1. Planets move in elliptical orbits with the Sun at
2. A line from the sun to a planet sweeps out equal
areas in a given period of time.
3. The square of the orbital period of a planet is
proportional to the cube of its average
distance from the sun. (T2 ~ r3)
(can be derived for circular orbit)
*You are not responsible for laws 2 and 3.
4. Planet in Elliptical Orbit about the Sun.
• Where does a planet have greatest kinetic
• Where does a planet have greatest potential
• Where is the planet moving fastest?
• (HINT: Total Energy is Conserved)
5. Suppose we drill a hole through
the Earth and drop someone in.
• What is the person’s initial acceleration?
• What is the person’s acceleration at center of the
• What happens to his acceleration as he falls
towards the center?
• What happens to his acceleration after he passes
• Does he reach the other side? If so, then what?
Center of Gravity (CG)
• Position through which gravity acts on an object if
the object were condensed to a particle.
•CG of an object must be supported to avoid
•Stable Equilibrium: Object balanced such that any
displacement will raise its CG. (CG will then fall
back to lower P.E.)
•Unstable Equilibrium: Object balanced such that
any displacement will lower its CG. (CG will then
continue to fall lower P.E.)
Translation, Rotation, and Rolling
• Translation: motion of the CG only.
• Rotation: Spinning about the CG.
• Rolling: A combined motion
– Object rotates about a constantly moving axis between
object and surface.
– Velocities of points on object are proportional to their
distances from the axis.
Rotational Mechanics: Torque
• Torque causes things to rotate about an axis (just as
________ causes things to ________________).
• Types of Torque we see everyday:
– Torsion or twisting: Torque applied about the length of
– Bending: Torque applied about an axis perpendicular to
the object’s length.
What makes up a Torque?
• Do we need a force?
• Do we need a net force?
• Do we need anything else?
OR (put another way )
• Can I get a torque with no force?
• Can I get a torque with no net force?
• Can I apply a force to an object and get no
Requirements for a Torque
• A Force
• A Lever Arm, also called a “moment arm”, equals
distance from the axis of rotation.
The amount of torque (τ) we get depends on the
Amount of force we apply (F ┴)
• Length of lever arm (r)
• τ = r * F ┴ = Torque about the pivot point
A Balance of Torques?
• Can we apply a number of torques and have
• Can torques cancel out?
• A net torque causes rotation.
• Rotational Equilibrium: τnet= 0
– If torque produces counterclockwise rotation it
– If torque produces clockwise rotation it is (+)
• A meter stick is on a pivot at its center.
– If a 1 kg mass is placed 8 centimeters to the left of
the pivot, what is the torque produced about the
– Can I place a .2 kg mass to the right of the pivot and
balance the 1 kg mass? If so, where should the .2 kg
mass be placed?
– After placing the .2 kg mass, what is the force
exerted by the pivot on the meter stick? What torque
does this force produce?
• A meter stick is on a pivot at its center.
– A 1 kg. mass is placed .1 m to the right of the
pivot and a .5 kg. mass is placed .2 m to the right
of the pivot.
– Where must a .5 kg. mass be placed to balance
– After all masses are in place, what is the force
exerted by the pivot on the stick?
– What is the net torque about the right end of the
Torque exerted on a spool
• Which case of applied torque will cause the fastest
rolling (for the same applied force)?
Rotational Inertia (I)
• Resistance of an object to being rotated.
• It is more difficult to rotate an object about a point
if more of its mass is further from that point.
• It is easier to rotate an object whose mass is closer
in to the point of rotation.
• I ~ mr2
• For a small mass, a distance r from a pivot:
I = mr2 Ex: Pendulum
Rotational Inertias for various objects.
(Note heavy dependence on r)
• Measure of the resistance of an object to having its
rotational motion changed.
• L = I×ω
– L = angular momentum
– I = rotational inertia
– ω = rotational velocity (recall: v = rω)
• For a mass moving in a circle at speed v:
L= I × ω =(mr2) ×(v/r) = r × m v = r × p
• Applying an external Torque to an object or
– Increases ω and increases L but…..
Conservation of Angular momentum
• If the net external torque on a system is zero,
the angular momentum of the system is
– Example: L = mvr ; if r decreases with no net torque,
then v increases.
– Figure skaters spin faster when they pull in their arms.
– Swimmers curl their bodies inward to turn faster after
swimming a length.
• Angular momentum is a vector. If I reverse the
direction of spinning, the direction of L reverses.
Here we can see how translational vectors
(F and p) relate to rotational vectors (τ and L).