Chapter 7 Circular Motion Newton’s Laws of Universal Gravitation Motion in Space.
Circular Motion and Other Applications of Newton’s Laws
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Transcript of Circular Motion and Other Applications of Newton’s Laws
Circular Motion and Other Applications of Newtons Laws
Unit 5 Circular Motion and Other Applications of Newtons Laws
Uniform Circular Motion
A force, Fr , is directed toward the center of the circle This
force is associated with an acceleration, ac Applying Newtons
Second Law along the radial direction gives Uniform Circular
Motion, cont
A force causing a centripetal acceleration acts toward the center
of the circle It causes a change in the direction of the velocity
vector If the force vanishes, the object would move in a
straight-line path tangent to the circle Centripetal Force The
force causing the centripetal acceleration is sometimes called the
centripetal force This is not a new force, it is a new role for a
force It is a force acting in the role of a force that causes a
circular motion Horizontal (Flat) Curve
The force of static friction supplies the centripetal force The
maximum speed at which the car can negotiate the curve is Note,
this does not depend on the mass of the car Banked Curve These are
designed with friction equaling zero
There is a component of the normal force that supplies the
centripetal force Non-Uniform Circular Motion
The acceleration and force have tangential components Fr produces
the centripetal acceleration Ft produces the tangential
acceleration SF = SFr + SFt Vertical Circle with Non-Uniform
Speed
The gravitational force exerts a tangential force on the object
Look at the components of Fg The tension at any point can be found
Loop-the-Loop This is an example of a vertical circle
At the bottom of the loop (b), the upward force experienced by the
object is greater than its weight Loop-the-Loop, Part 2 At the top
of the circle (c), the force exerted on the object is less than its
weight Top and Bottom of Circle
The tension at the bottom is a maximum The tension at the top is a
minimum If Ttop = 0, then Rigid Object A rigid object is one that
is nondeformable
The relative locations of all particles making up the object remain
constant All real objects are deformable to some extent, but the
rigid object model is very useful in many situations where the
deformation is negligible Angular Position Axis of rotation is the
center of the disc
Choose a fixed reference line Point P is at a fixed distance r from
the origin Angular Position, 2 As the particle moves, the only
coordinate that changes is q As the particle moves through q, it
moves though an arc length s. The arc length and r are related: s =
q r Radian This can also be expressed as
q is a pure number, but commonly is given the artificial unit,
radian One radian is the angle subtended by an arc length equal to
the radius of the arc Conversions Comparing degrees and
radians
Converting from degrees to radians [rad] = [degrees] Angular
Displacement The angular displacement is defined as the angle the
object rotates through during some time interval This is the angle
that the reference line of length r sweeps out Instantaneous
Angular Speed
The instantaneous angular speed is defined as the limit of the
average speed as the time interval approaches zero Angular Speed,
final Units of angular speed are radians/sec
rad/s or s-1 since radians have no dimensions Angular speed will be
positive if is increasing (counterclockwise) Angular speed will be
negative if is decreasing (clockwise) Instantaneous Angular
Acceleration
The instantaneous angular acceleration is defined as the limit of
the average angular acceleration as the time goes to 0 Angular
Acceleration, final
Units of angular acceleration are rad/s2 or s-2since radians have
no dimensions Angular acceleration will be positive if an object
rotating counterclockwise is speeding up Angular acceleration will
also be positive if an object rotating clockwise is slowing down
Angular Motion, General Notes
When a rigid object rotates about a fixed axis in a given time
interval, every portion on the object rotates through the same
angle in a given time interval and has the same angular speed and
the same angular acceleration So q, w, a all characterize the
motion of the entire rigid object as well as the individual
particles in the object Directions, details Strictly speaking, the
speed and acceleration (w, a) are the magnitudes of the velocity
and acceleration vectors The directions are actually given by the
right-hand rule Rotational Kinematics
Under constant angular acceleration, we can describe the motion of
the rigid object using a set of kinematic equations These are
similar to the kinematic equations for linear motion The rotational
equations have the same mathematical form as the linear equations
Rotational Kinematic Equations Comparison Between Rotational and
Linear Equations Relationship Between Angular and Linear
Quantities
Displacements Speeds Accelerations Every point on the rotating
object has the same angular motion Every point on the rotating
object does not have the same linear motion Speed Comparison The
linear velocity is always tangent to the circular path called the
tangential velocity The magnitude is defined by the tangential
speed Acceleration Comparison
The tangential acceleration is the derivative of the tangential
velocity Speed and Acceleration Note
All points on the rigid object will have the same angular speed,
but not the same tangential speed All points on the rigid object
will have the same angular acceleration, but not the same
tangential acceleration The tangential quantities depend on r, and
r is not the same for all points on the object Centripetal
Acceleration
An object traveling in a circle, even though it moves with a
constant speed, will have an acceleration Therefore, each point on
a rotating rigid object will experience a centripetal acceleration
Resultant Acceleration
The tangential component of the acceleration is due to changing
speed The centripetal component of the acceleration is due to
changing direction Total acceleration can be found from these
components Rotational Motion Example
For a compact disc player to read a CD, the angular speed must vary
to keep the tangential speed constant (vt = wr) At the inner
sections, the angular speed is faster than at the outer sections
Moment of Inertia The resistance of matter to rotational
motion.
The dimensions of moment of inertia are ML2 and its SI units are
kg.m2 We can calculate the moment of inertia of an object more
easily by assuming it is divided into many small volume elements,
each of mass Dmi Moments of Inertia of Various Rigid Objects Torque
Torque, t, is the tendency of a force to rotate an object about
some axis Torque is a vector t = r F sin f = F d F is the force f
is the angle the force makes with the horizontal d is the moment
arm (or lever arm) Torque, cont The moment arm, d, is the
perpendicular distance from the axis of rotation to a line drawn
along the direction of the force d = r sin Torque, final The
horizontal component of F (F cos f) has no tendency to produce a
rotation Torque will have direction If the turning tendency of the
force is counterclockwise, the torque will be positive If the
turning tendency is clockwise, the torque will be negative Net
Torque The force F1 will tend to cause a counterclockwise rotation
about O The force F2 will tend to cause a clockwise rotation about
O St = t1 + t2 = F1d1 F2d2 Torque vs. Force Forces can cause a
change in linear motion
Described by Newtons Second Law Forces can cause a change in
rotational motion The effectiveness of this change depends on the
force and the moment arm The change in rotational motion depends on
the torque Torque Units The SI units of torque are N.m
Although torque is a force multiplied by a distance, it is very
different from work and energy The units for torque are reported in
N.m and not changed to Joules Torque and Angular Acceleration
Consider a particle of mass m rotating in a circle of radius r
under the influence of tangential force Ft The tangential force
provides a tangential acceleration: Ft = mat Torque and Angular
Acceleration, Particle cont.
The magnitude of the torque produced by Ft around the center of the
circle is t = Ft r = (mat) r The tangential acceleration is related
to the angular acceleration t = (mat) r = (mra) r = (mr 2) a Since
mr 2 is the moment of inertia of the particle, t = Ia The torque is
directly proportional to the angular acceleration and the constant
of proportionality is the moment of inertia Motion in Accelerated
Frames
A fictitious force results from an accelerated frame of reference A
fictitious force appears to act on an object in the same way as a
real force, but you cannot identify a second object for the
fictitious force Centrifugal Force From the frame of the passenger
(b), a force appears to push her toward the door From the frame of
the Earth, the car applies a leftward force on the passenger The
outward force is often called a centrifugal force It is a
fictitious force due to the acceleration associated with the cars
change in direction Coriolis Force This is an apparent force caused
by changing the radial position of an object in a rotating
coordinate system The result of the rotation is the curved path of
the ball Fictitious Forces, examples
Although fictitious forces are not real forces, they can have real
effects Examples: Objects in the car do slide You feel pushed to
the outside of a rotating platform The Coriolis force is
responsible for the rotation of weather systems and ocean currents
Fictitious Forces in Linear Systems
The inertial observer (a) sees The noninertial observer (b) sees
Fictitious Forces in a Rotating System
According to the inertial observer (a), the tension is the
centripetal force The noninertial observer (b) sees