Spring Forces and Simple
Harmonic Motion
Chapter 10
Expectations
At the end of this chapter, students will be able to:
 Apply Hooke’s Law to the calculation of spring
forces.
 Conceptually understand how simple harmonic
motion is caused by forces and torques obeying
Hooke’s Law.
 Calculate displacements, velocities, accelerations,
and frequencies for objects undergoing simple
harmonic motion.
Expectations
At the end of this chapter, students will be able to:
 Calculate the elastic potential energy resulting
from work done by spring forces.
 Understand how pendulums approximate simple
harmonic motion.
 Calculate the natural frequencies of both simple
and physical pendulums.
 Conceptually understand the ideas of damped,
driven, and resonant simple harmonic motion.
Expectations
At the end of this chapter, students will be able to:
 Analyze the elastic deformation of objects in
terms of stress, strain, and the elastic moduli of
materials.
 Express Hooke’s Law in terms of stress and
strain.
Spring Forces
A spring resists
being stretched
or compressed.
Spring Forces
The force with
which the spring
resists is
proportional to
the distance
through which it
is compressed or
stretched:
F  kx
“spring constant”
SI units: N/m
Spring Forces – Hooke’s Law
F  kx
This relationship is
called Hooke’s
Law.
Hooke’s Law
Robert Hooke
1635 – 1703
English mathematician and
natural philosopher;
contemporary of Isaac
Newton
Early builder of microscopes
and telescopes
Hooke’s Law
The direction of
the spring force
is always
opposite the
direction of the
stretching or
compression of
the spring –
hence, the minus
sign.
F  kx
A Consequence of Hooke’s Law
Consider Newton’s second law and Hooke’s Law
simultaneously:
F  ma  kx
k
a x
m
Now, a small and sneaky bit of calculus:
d 2x
k
a 2  x
dt
m
differential equation

x  A cos t
its solution
Simple Harmonic Motion
Motion described by this equation:
x  Acost
displacement (m)
angular frequency (rad/s)
time (s)
amplitude (m)
is called simple harmonic motion (SHM). It is:
 periodic (repeats itself in time)
 oscillatory (takes place over a limited spatial
range)
Simple Harmonic Motion x  Acost
displacement vs.  t
A = 1.00 m
1.00
0.80
0.60
displacement, m
0.40
0.20
0.00
-0.20
-0.40
-0.60
-0.80
-1.00
0.00
2.00
4.00
6.00
8.00
 t, rad
10.00
12.00
14.00
SHM: Reference Circle Representation
Y
A vector of magnitude
A rotates about the
origin with an angular
velocity .
A cos  t
A
t
X
The x component of
the vector represents
the displacement.
SHM: Frequency
Since there are 2p radians in each trip (“cycle”)
around the reference circle, the “cycle” frequency
is related to the angular frequency by
  2pf
or

f 
2p
SI units of “cycle” frequency, f:
cycles / s = Hertz (Hz)
SHM: Velocity
Y
We can calculate the
velocity from the
reference circle
representation:
vT  r  A
 vT sin t   A sin t
v   A sin t
- vT sin  t
vT
t
A
t
X
SHM: Acceleration
Y
aC  r  A
2
2
a  aC cos t
-aC cos  t
t
aC
t
X
a   A cos t
2
SHM: Velocity and Acceleration






 must be expressed in rad/s.
Like displacement, velocity and acceleration are
periodic in time.
Maximum velocity: vmax  A
2
a

A

Maximum acceleration: max
Acceleration has maximum magnitude at extremes
of displacement.
Velocity has maximum magnitude when
displacement is zero.
Mass on a Spring System
Natural frequency for a mass m on a spring with
spring constant k:
d 2x
k
a 2  x
dt
m

k
  2pf 
m
x  A cos t
Work Done in Straining a Spring
Stretch or compress a spring by a displacement x
from its unstrained length.
Initial force: F0 = 0
Average force:
Final force: Fmax = kx
1
F  kx
2
1 2
Work over a displacement x: W  F x  W  kx
2
Elastic Potential Energy
The spring force is a conservative force:


Like all conservative forces, its work is pathindependent.
Like all conservative forces, it is associated with a
form of stored or potential energy.
Elastic potential energy:
1 2
EPE  kx
2
Total Mechanical Energy
We add another term:
E  KE  KER  GPE  EPE
1 2 1 2
1 2
E  mv  I  mgh  kx
2
2
2
The Simple Pendulum
A simple pendulum is a particle
attached to one end of a
massless cord of length L. It is
able to swing freely and
without friction from the other
end of the cord.
Its frequency:
  2pf 
g
L
L
The Physical Pendulum
A physical pendulum is any real
object (mass m) suspended a
distance L from its center of
gravity, able to swing freely
and without friction from the
suspension point.
mgL
Its frequency:   2pf 
I
L
Physical-Simple Correspondence
Notice that a simple pendulum would have a
moment of inertia: I  mL2
Substitute:
mgL
mgL



2
I
mL
g
L
As a physical pendulum becomes a simple one, its
frequency “collapses” to that of a simple
pendulum.
Small-Angle Approximation

L
 R  TL sin  cos

T sin 
Result: the restoring torque
increases with angle, but at less
than a linear rate.
T cos 
The restoring torque on a
pendulum does not actually
have the Hooke’s Law form:
mg
Small-Angle Approximation
How much less?
, °
% under linear
0.10
0.0002%
1.0
0.02%
2.0
0.08%
5.0
0.51%
10
2.0%
Damped Oscillations
If the only force doing work on an object is the
spring force (conservative), its mechanical energy
is conserved. If frictional forces also do work, the
object’s mechanical energy decreases, and the
SHM is called damped.
If the frictional force is just large enough to prevent
oscillation as the object reaches its equilibrium
position, it is called critically damped.
Driven Oscillations
If a driving force acts on an object in addition to a
Hooke’s Law restoring force, the harmonic
motion of the object is called driven.
Example: a tree in a gusty wind.
Driven Oscillations: Resonance
If the driving force is periodic, and is applied at the
natural frequency of the oscillating object, the
work done on the object adds up over multiple
cycles of motion, and large-amplitude motion
results. This is called resonance. The natural
frequency is sometimes called the resonant
frequency.
Example: a person on a swing, being pushed by
another person.
Material Deformation: Everything is a
Spring
Solid materials are interconnected, microscopically,
by powerful intermolecular bonding forces.
These forces behave like springs … with really large
spring constants.
Because of them, material objects resist
deformations, such as compression, elongation, or
shearing.
Tension and Compression
A force acts to increase the length of an object:
fractional change in
length
applied force
L
F Y
A
L0
cross-
Young’s modulus
sectional
SI units = N/m2
area
Thomas Young
1773 - 1829
English physicist,
physician, and
Egyptologist
Famous mostly for his
work in optics
Shear
A pair of forces act to shear an object (deform it slantwise):
applied force
X
FS
A
L0
Shear modulus
SI units = N/m2
cross-sectional
area
Shear
1945 - ?
Has only one name
English physicist and pop
musician
Inventor of the shear
modulus
Rumored to have appeared in the 1983 version of Dune
Volume Deformation
In order to discuss volume deformation, it is necessary
to define a new force-related quantity: pressure.
Pressure is the ratio of the magnitude of a force
applied perpendicular to a surface to the area of that
surface:
F
P
A
SI units: N/m2 = Pascals (Pa)
Blaise Pascal
1623 – 1662
French mathematician
Invented the first digital
calculator (the “Pascaline”)
Volume Deformation
A change in pressure changes
the volume of an object:
pressure
change
V
P   B
V0
fractional
bulk modulus
change in
SI units: N/m2
volume
Stress and Strain
Stress is the deforming force applied to an object,
divided by its cross-sectional area:
F
stress 
A
Stress has SI units of N/m2 (just as pressure and the
elastic moduli have).
Stress and Strain
Strain is the change in a dimensional quantity
expressed as a fraction of its un-deformed value:
L
X
V
strain 
or strain 
or strain 
L0
L0
V0
Strain is a dimensionless, unitless ratio.
Strain is the result of stress on a material object.
Stress and Strain
Consider the defining equation for Young’s modulus:
L
F Y
A
L0
Rearrange:
F
L
Y
A
L0
To restate: stress  elastic modulus strain 
This is the stress-strain formulation of Hooke’s Law.
Summary: Elastic Moduli
deformation
elastic modulus
length
Young’s modulus (Y)
shear
shear modulus (S)
volume
bulk modulus (B)
all moduli have the same SI units: N/m2 = Pa
equation
L
F Y
A
L0
X
FS
A
L0
V
P   B
V0