Download Phy 211: General Physics I

Phy 201: General Physics I
Chapter 5: Uniform Circular Motion
Lecture Notes
Uniform Circular Motion
r
UCM, a special case of 2-D motion where:
• The radius of motion (r) is constant
• The speed (v) is constant
• The velocity changes as object continually changes
direction
• Since v is changing (v is not but direction is), the object
vx ˆ vy ˆ
must be accelerated:
ac=ax  ay 
•
•
t
x+
t
y
The direction of the acceleration vector is always toward the
center of the circular motion and is referred to as the
centripetal acceleration
v2
The magnitude of centripetal acceleration: ac =ac=
r
v
Derivation of Centrifugal Acceleration
y
Consider an object in uniform circular motion, where:
v
r = constant {radius of travel}
v = constant {speed of object}
r
q
x
The magnitude of the centripetal acceleration can be
obtained by comparing the similar triangles for position
and velocity:
time, t1
time, t2
v1
v2
r
r1
q
r1
r
r2
Position only
v
v2
q
v1
Velocity only
2
The direction of position and velocity vectors both shift by the
same angle, resulting in the following relation:
r v
vr
v v  r 
v2
=
 v=

=    ac=
r
v
r
t
r  t 
r
y
x
Centripetal Force
• Centripetal force is the center-seeking force that pulls an
object into circular motion (object is not in equilibrium)
Fc= mac
• The direction of the centripetal force is the same as the
centripetal acceleration
• The magnitude of centripetal force is given by:
mv2
Fc = Fc = mac =
r
• For uniform circular motion, the centripetal force is the net
mv2
force:
F = Fc= mac  F = Fc= mac=

r
Notes:
– the faster the speed the greater the centripetal force
– the greater the radius of motion the smaller the force
Centripetal Force & Gravitation
Assume the Earth travels about the Sun in UCM:
Sun
Earth
Questions:
1.
2.
3.
4.
5.
What force is responsible for maintain the Earth’s circular orbit?
Draw a Free-Body Diagram for the Earth in orbit.
Calculate gravitational force exerted on the Earth due to the Sun.
What is the centripetal acceleration of the Earth?
What is the Earth’s tangential velocity in its orbit around the Sun?
Centripetal Force vs. Centrifugal Force
• An object moving in uniform circular motion experiences an
outward “force” called centrifugal force (due to the object’s
inertia)
• However, to refer to this as a force is technically incorrect.
More accurately, centrifugal force _______.
– is a consequence of the observer located in an accelerated (revolving)
reference frame and is therefore a fictitious force (i.e. it is not really a
force it is only perceived as one)
– is the perceived response of the object’s inertia resisting the circular
motion (& its rotating environment)
– has a magnitude equal to the centripetal force acting on the body
Centripetal force & centrifugal force are
NOT action-reaction pairs…
Artificial Gravity
• Objects in a rotating environment “feel” an
outward centripetal force (similar to gravity)
• What the object really experiences is the
normal force of the support surface that
provides centripetal force
• The magnitude of “effective” gravitational
acceleration is given by:
2
ac = gartificial =
v
r
• If a complete rotation occurs in a time, T,
then the rotational (or angular) speed is
w =
2p
q
=
T
t
{where T is the period of the motion}
Note: there are 2p radians (360o) in a complete rotation/revolution
2
4
p
r
2
• Since v = wr (rotational speed x radius), the artificial gravity is: gartificial = ω r =
2
T
• Thus, the (artificial) gravitational strength is related to size of the structure (r),
rotational rate (w) and square of the period of rotation (T2)
Banked Curves
• Engineers design banked curves to increase
the speed certain corners can be navigated
• Banking corners decreases the dependence
of static friction (between tires & road)
necessary for turning the vehicle into a
corner.
• Banked corners can improve safety
particularly in inclement weather (icy or wet
roads) where static friction can be
compromised
• How does banking work?
• The normal force (FN) of the bank on the car
assists it around the corner!
Vertical Circular Motion
• Consider the forces acting on a motorcycle performing a
loop-to-loop:
• Can you think of other objects that undergo similar
motions?