You are a passenger in a car, not wearing a seat belt. The
car makes a sharp left turn. From your perspective in
the car, what do you feel is happening to you?
(A) you are thrown to the right
(B) you feel no particular change
(C) you are thrown to the left
(D) you are thrown to the ceiling
(E) you are thrown to the floor
During that sharp left turn, you found yourself hitting
the passenger door. What is the correct description of
what is actually happening?
(A) centrifugal force is pushing you into the door
(B) the door is exerting a leftward force on you
(C) both of the above
(D) neither of the above
You drive your dad’s car too fast around a curve and the
car starts to skid. What is the correct description of this
situation?
(A) car’s engine is not strong enough to keep the car from being
pushed out
(B) friction between tires and road is not strong enough to keep
car in a circle
(C) car is too heavy to make the turn
(D) a deer caused you to skid
A rider in a “barrel of fun” finds
herself stuck with her back to the wall.
Which diagram correctly shows the
forces acting on her?
A
B
C
D
E
Playground Physics
Centripetal
forces keep
these children
moving in a
circular path.
Uniform Circular Motion
Uniform circular motion is motion along a
circular path in which there is no change in
speed, only a change in direction.
Fc
v
Constant velocity
tangent to path.
Constant force
toward center.
Question: Is there an outward force on the ball?
Uniform Circular Motion (Cont.)
The question of an outward force can be
resolved by asking what happens when the
string breaks!
Ball moves tangent to
v
path, NOT outward as
might be expected.
When central force is removed,
ball continues in straight line.
Centripetal force is needed to change direction.
Examples of Centripetal Force
You are sitting on the seat next to
the outside door. What is the
direction of the resultant force on
you as you turn? Is it away from
center or toward center of the turn?
• Car going around a
curve.
Fc
Force ON you is toward the center.
Car Example Continued
Reaction
Fc
F’
The centripetal
force is exerted
BY the door ON
you. (Centrally)
There is an outward force, but it does not act
ON you. It is the reaction force exerted BY you
ON the door. It affects only the door.
Another Example
Disappearing
platform at fair.
R
Fc
What exerts the centripetal force in this
example and on what does it act?
The centripetal force is exerted BY the wall
ON the man. A reaction force is exerted by
the man on the wall, but that does not
determine the motion of the man.
Spin Cycle on a Washer
How is the water removed
from clothes during the
spin cycle of a washer?
Think carefully before answering . . . Does the
centripetal force throw water off the clothes?
NO. Actually, it is the LACK of a force that
allows the water to leave the clothes
through holes in the circular wall of the
rotating washer.
Centripetal Acceleration
• An object traveling in a circle, even
though it moves with a constant speed,
will have an acceleration
• The centripetal acceleration is due to the
change in the direction of the velocity
Kinematics of Uniform Circular Motion
Uniform circular motion: motion in a circle of constant
radius at constant speed
Instantaneous velocity is always tangent to circle.
v2
v1
Radial Acceleration:
Similar Triangles
Δv v
vΔr

 Δv 
Δr r
r
v2
Dt
r2
Dq
-v1
Dq
Dr
r1
Dv
v1
Divide by time
Δv

Δt
Centripetal
Acceleration
v  Δr 
 
r  Δt 
2
v
ac 
r
v2
Kinematics of Uniform Circular Motion
In uniform circular motion the acceleration is called the
centripetal, or radial, acceleration. It is perpendicular to the
velocity and points towards the center of the circle.
v
a
c
r
v2
ac 
r
Dynamics of Uniform Circular Motion
For an object to be in uniform circular motion, there
must be a net force acting on it.
The radial force
on the ball is
provided by the string
v
a
 v2 
Fc  mac  m 
 r 
 v2 
Fc  m 
 r 
c
Fc
This radial force
is called a
centripetal force
There is no
centrifugal force
acting on the ball
Centripetal Acceleration, final
• The magnitude of the centripetal
acceleration is given by
2
v
ac 
r
– This direction is toward the center of the
circle
Forces Causing Centripetal
Acceleration
• Newton’s Second Law says that the
centripetal acceleration is accompanied
by a force
– FC = maC
– FC stands for any force that keeps an object
following a circular path
• Tension in a string
• Gravity
• Force of friction
Example 1: A 3-kg rock swings in a circle
of radius 5 m. If its constant speed is 8
m/s, what is the centripetal acceleration?
2
v
v
m
m = 3 kg
ac 
R
R
R = 5 m; v = 8 m/s
2
(8 m/s)
2
ac 
 12.8 m/s
5m
mv
Fc  mac 
R
2
F = (3 kg)(12.8 m/s2)
Fc = 38.4 N
Example 2: A skater moves with 15 m/s in a
circle of radius 30 m. The ice exerts a
central force of 450 N. What is the mass of
the skater?
Draw and label sketch
Fc R
mv 2
Fc 
; m 2
v = 15 m/s
R
v
Fc R
450 N
30 m
m=?
Speed skater
(450 N)(30 m)
m
2
(15 m/s)
m = 60.0 kg
Car Negotiating a Flat Turn
v
Fc
R
What is the direction of the
force ON the car?
Ans. Toward Center
This central force is exerted
BY the road ON the car.
Car Negotiating a Flat Turn
The centripetal force Fc is
that of static friction fs:
m
Fc
R
n
fs
Fc = fs
R
v
mg
The central force FC and the friction force fs
are not two different forces that are equal.
There is just one force on the car. The nature
of this central force is static friction.
Finding the maximum speed for
negotiating a turn without slipping.
n
fs
Fc = fs
m
v
R
Fc
R
mg
The car is on the verge of slipping when FC is
equal to the maximum force of static friction fs.
Fc = fs
Fc =
mv2
R
fs = msmg
Maximum speed without slipping (Cont.)
n
Fc = fs
fs
R
mv2
R
mg
v=
m
v
Fc
R
= msmg
msgR
Velocity v is maximum
speed for no slipping.
Example 4: A car negotiates a turn of
radius 70 m when the coefficient of static
friction is 0.7. What is the maximum
speed to avoid slipping?
m
v
Fc
Fc =
R
ms = 0.7
mv2
R
fs = msmg
From which: v =
msgR
g = 9.8 m/s2; R = 70 m
v  ms gR  (0.7)(9.8)(70m) v = 21.9 m/s
Optimum Banking Angle
By banking a curve at the
optimum angle, the normal
Fc
m
R
v
fs
w
force n can provide the
necessary centripetal force
without the need for a
friction force.
n
q
slow speed
n
w
fs
q
fast speed
fs = 0
w
n
q
optimum
Free-body Diagram
Acceleration a is toward the
center. Set x axis along the
direction of ac , i. e.,
horizontal (left to right).
n
x
mg
q
n
q
n cos q
q
n
n sin q
mg
q
mg
+ ac
Optimum Banking Angle (Cont.)
n
mg
n cos q
n
n sin q
q
Apply
Newton’s 2nd
Law to x and y
axes.
q
mg
mv2
SFx = mac
n sin q =
SFy = 0
n cos q = mg
R
Optimum Banking Angle (Cont.)
n
mg
q
n sin q =
n cos q
q
n
n sin q
n
sin q
tan q 
n cos q
mg
mv2
R
n cos q = mg
2
mv
2
v
R
tan q 

mg gR
1
Optimum Banking Angle (Cont.)
n
mg
q
Optimum Banking
Angle q
n cos q
n
q
n sin q
mg
2
v
tan q 
gR
Example 5: A car negotiates a turn of
radius 80 m. What is the optimum
banking angle for this curve if the speed
is to be equal to 12 m/s?
n
mg
n cos q
tan q =
q
q
q
n
n sin q
mg
v2
gR
=
(12 m/s)2
(9.8 m/s2)(80 m)
tan q = 0.184
q = 10.40
How might you 2find the
mv
centripetal
FC force on the
car, knowing R
its mass?
Motion in a Vertical Circle
v
Consider the forces on a
ball attached to a string as
it moves in a vertical loop.
+
v
Note also that the positive
direction is always along
acceleration, i.e., toward
the center of the circle.
v
+
v
Bottom
Tmg
+
T
T
T
mg
+
+mg
mg
T
v
Top
ofSide
Path
Left
mg
Top Right
Right
Top
Tension is
Weight
hasas
no
Maximum
minimum
Weight
causes
Weight
has
no
effect
on
T
tension
T, W
Bottom
weight
helps
small
decrease
effect on T
opposes
Fc
Ftension
in
c force T
Note changes as you click
the mouse to show new
positions.
+
v
10 N
T
As an exercise, assume
that a central force of
Fc = 40 N is required to
maintain circular motion
of a ball and W = 10 N.
+
R
T
+
10 N
v
The tension T must
adjust so that central
resultant is 40 N.
At top: 10 N + T = 40 N
TT =
= _?_
30 N
Bottom: T – 10 N = 40 N
TT==__?___
50 N
Motion in a Vertical Circle
v
mg
T
mv2
Resultant force
Fc =
toward center
R
R
v
mg + T =
AT TOP:
+
mg
T
Consider TOP of circle:
T=
mv2
R
mv2
R
- mg
Vertical Circle; Mass at bottom
v
T
Resultant force
toward center
R
v
mg
T - mg =
mg
+
R
Consider bottom of circle:
AT Bottom:
T
Fc =
mv2
T=
mv2
R
mv2
R
+ mg
Visual Aid: Assume that the centripetal force
required to maintain circular motion is 20 N.
Further assume that the weight is 5 N.
v
mv 2
FC 
 20 N
R
Resultant central force FC
at every point in path!
R
v
FC = 20 N at top
AND at bottom.
FC = 20 N
Weight vector W is
downward at every point.
W = 5 N, down
Visual Aid: The resultant force (20 N) is the
vector sum of T and W at ANY point in path.
W
T
+
T
W
+
v
Top: T + W = FC
T + 5 N = 20 N
T = 20 N - 5 N = 15 N
R
v
FC = 20 N at top
AND at bottom.
Bottom:
T - W = FC
T - 5 N = 20 N
T = 20 N + 5 N = 25 N
For Motion in Circle
v
AT TOP:
R
+ T=
mg
mv2
R
- mg
T
v
AT BOTTOM:
T
mg
+
T=
mv2
R
+ mg
Example 10: A 2-kg rock swings in a vertical
circle of radius 8 m. The speed of the rock as it
passes its highest point is 10 m/s. What is
tension T in rope?
2
At Top:
v
mg
T
T=
R
v
mg + T =
mv2
mv
- mg
R
R
2
(2 kg)(10 m/s)
2
T
 2 kg(9.8 m/s )
8m
T = 25 N - 19.6 N
T = 5.40 N
Example 11: A 2-kg rock swings in a vertical
circle of radius 8 m. The speed of the rock as it
passes its lowest point is 10 m/s. What is
tension T in rope?
2
At Bottom:
v
R
T
mg
T=
v
T - mg =
mv2
mv
+ mg
R
R
2
(2 kg)(10 m/s)
2
T
 2 kg(9.8 m/s )
8m
T = 25 N + 19.6 N
T = 44.6 N
Example 12: What is the critical speed vc at
the top, if the 2-kg mass is to continue in a
circle of radius 8 m?
0
2
mv
v
At Top:
mg + T =
mg
R
vc occurs when T = 0
R
T
mv2
mg =
vc = gR
v
R
v=
gR =
(9.8 m/s2)(8 m)
vc = 8.85 m/s
The Loop-the-Loop
Same as cord, FN replaces T
v
AT TOP:
FW
R
v
AT BOTTOM:
FN
FW
+
+
FN =
mv2
R
- mg
FN
FN=
mv2
R
+ mg
The Ferris Wheel
v
AT TOP:
FN
R
+
v
mg
AT BOTTOM:
FN
mg
mg
+
FN
=
- FN =
FN = mg
mv2
R
+ mg
mv2
R
-
mv2
R
Example 13: What is the
n
apparent weight of a 60-kg
person as she moves through v
the highest point when R =
mg
45 m and the speed at that
point is 6 m/s?
Apparent weight will be the
normal force at the top:
mv2
mg - FN =
FN = mg
R
2
(60
kg)(6
m/s)
2
Fn
N  60 kg(9.8 m/s ) 
45 m
+
R
v
-
mv2
R
FN = 540 N