Transcript KD-4 power point review

Questions for Thought
Which moves faster on a merry-go-round, a
horse on the outside or inside?
What happens if you are the passenger in a car
and the driver takes a right-hand turn?
Left-hand turn?
What happens during the spin cycle in a washing
machine?
Rotation vs. Revolution
Rotation (spin) – Motion of an object spinning
around an internal axis
Ex) Merry-go-round spins
around its own axis
Revolution- Motion of an object
turning around an external axis
Person on merry-go-round
revolves around the same axis
Hmmm?
Which moves faster a person near the outside of
the merry-go-round or one near the inside of the
same merry-go-round?
Rotational vs. Tangential
Speed
Rotational speed (angular)-# of rotations/time
Both have the same rotational speed
Tangential speed (linear)-Distance moved/time
The person on the outside has a greater tangential
speed because he/she is moving a greater
distance
Part 1: Circular Motion
Uniform Circular Motion-The motion of an object
traveling in a circular path at a constant speed
Ex) A tetherball moving in a circular path
Is the tetherball accelerating?
Circular Motion
Centripetal acceleration- Acceleration due to
changing direction
“Center seeking”
All objects moving
in a circle are
accelerating
(towards the
center)
Centripetal Acceleration
a = v²/R
a = 2лv/T
a = 4л²R/T²
R=Radius
T=period (time for 1 revolution)
Centripetal Acceleration
An object which experiences an acceleration
must also experience a force
F=ma
Centripetal Force
Force acting towards the
center which causes the
object to seek the center
Works against inertia
What is the Centripetal
Force?
A car moving around a track?
Frictional force between tires and Earth forcing
car inward keeping it in its path
Planets orbiting the sun?
Gravitational force pulling on the
object keeps it in its path
Centripetal Force
Formulas:
Fc =(m) (ac)
Fc = (m) (V2/R)
Fc = (m) (2πv/T)
Fc = (m) (4π2R/T2)
Centrifugal Force
“Center fleeing”
This is a FAKE force
Outward force is a
misconception due
to inertia
If swing a can on the end of a string over your
head and the string breaks, what happens to the
can?
The can moves in a straight line tangent to its
circular path
Example:
Calculate the maximum speed a 1200 kg car can
travel around a curve of 35 m radius if the
frictional force between the tires and the road
surface is 2.4 X 103 N.
Fc = m v2 / r
2.4 x 103 = 1200 v2 / 35
V = 8.4 m/s
Centripetal acceleration is always toward the center
Centrifugal acceleration does NOT exist since centrifugal
force does NOT exist
What happens to a person
in the backseat if driver
makes a right turn? Left?
The person goes STRAIGHT (due to inertia), but you
get the misconception you are going outward
Part 2: Projectile Motion
Motion resulting from the sum of 2 independent
velocities
Horizontal constant velocity (neglecting air
resistance)
Vertical increasing velocity (due to gravity)
Projectile Motion
Makes a parabolic path
Projectile Motion:
Equations
Range (horizontal distance)
Constant
Dx = vx t
Altitude (height)
Changes due to gravity
Dy = ½ g t2
vx
t
Example
If a bullet is dropped at the same moment a
bullet is shot out of a gun (and they both start at
the same height), which bullet hits the ground
1st?
They both hit at the same time!
Example:
If a bullet is fired 20 m above the ground, how
long does it take to hit the ground?
Dy = ½ g t2
20 = ½ (9.8) t2
t=2s
What if the object is fired
up at an angle?
Horizontal vs. Vertical
Vertical
velocity changes due to gravity
Time calculated would be the same up OR down
Horizontal
velocity is constant (neglecting air)
Time would be the same as down vertically is the object is
projected horizontally (only going down)
Time would be doubled if the object is projected at an angle (due
to the object traveling horizontally for the time up AND the time
down