Springs Hookes Law The force exerted by a spring is proportional to the distance the spring is stretched or compressed from its relaxed position F X k x Where x is the displacement from the relaxed position and ID: 163363
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Slide1
SHM -1Slide2
Springs
Hooke’s Law:
The force exerted by a spring is proportional to the distance the spring is stretched or compressed from its relaxed position.
FX = -k x Where x is the displacement from the relaxed position and k is the constant of proportionality.
relaxed position
F
X
= -kx > 0
x
x
0
x=0
18Slide3
Springs ACT
Hooke’s Law:
The force exerted by a spring is proportional to the distance the spring is stretched or compressed from its relaxed position.
FX = -k x Where
x is the displacement from the relaxed position and k is the constant of proportionality.
What is force of spring when it is stretched as shown below. A) F > 0 B) F = 0 C) F < 0
x
F
X
= - kx < 0
x > 0
relaxed position
x=0
14Slide4
Spring
ACT
A mass on a spring oscillates back & forth with simple harmonic motion of amplitude A. A plot of displacement (x) versus time (t) is shown below.
At what points during its oscillation is the magnitude of the acceleration of the block biggest?
1. When x = +A or -A (i.e. maximum displacement) 2. When x = 0 (i.e. zero displacement) 3. The acceleration of the mass is constant
+A
t
-A
x
CORRECT
F=ma
17Slide5
Potential Energy in Spring
Force of spring is Conservative
F = -k x
W = -1/2 k x
2Work done only depends on initial and final positionDefine Potential Energy Uspring = ½ k x2
Force
x
work
20Slide6
Oscillations
Oscillations (whether sinusoidal or otherwise) have some common characteristics:
They take place around an equilibrium position;
The motion is periodic and repeats with each cycle.Slide7
Periodic Motion
Period:
time required for one cycle of periodic motionFrequency: number of oscillations per unit time
The frequency unit is called a hertz (Hz):Slide8
Frequency and PeriodSlide9
Example:
Radio Station Frequency and Period
What is the oscillation period of an FM radio station that broadcasts at 100 MHz?
Note that 1/Hz = sSlide10
Simple Harmonic Motion
A
spring exerts a restoring force that is proportional to the displacement from equilibrium:Slide11
Simple Harmonic Motion
A
mass on a spring has a displacement as a function of time that is a sine or cosine curve:
A
is called the amplitude of the motion.Slide12
Simple Harmonic Motion
If
we call the period of the motion T (this is the time to complete one full cycle) we can write the position as a function of time as:
It
is then straightforward to show that the position at time
t + T is the same as the position at time t (one period earlier), as we would expect.Slide13
Connections between Uniform Circular Motion and Simple Harmonic Motion
An object in simple harmonic motion has the same motion as one component of an object in uniform circular motion:Slide14
Uniform circular motion projected into one dimension is simple harmonic motion (SHM).
Consider a particle rotating ccw, with the angle
f
increasing linearly with time:
Connections between Uniform Circular Motion and Simple Harmonic MotionSlide15
Connections between Uniform Circular Motion and Simple Harmonic Motion
Here, the object in circular motion has an angular speed of
where
T
is the period of motion of the object in simple harmonic motion.Slide16
Connections between Uniform Circular Motion and Simple Harmonic Motion
The position as a function of time:
The angular frequency:Slide17
Connections between Uniform Circular Motion and Simple Harmonic Motion
The velocity as a function of time:
And the acceleration:
Both of these are found by taking components of the circular motion quantities.Slide18
The Period of a Mass on a Spring
Since the force on a mass on a spring is proportional to the displacement, and also to the acceleration, we find that . Substituting the time dependencies of a and
x gives:Slide19
The Period of a Mass on a Spring
Therefore, the period is:Slide20
Mass+Spring
Simple Harmonic Motion
In simple harmonic motion (SHM), the acceleration, and thus the net force, are both proportional to and oppositely directed from the displacement from the equilibrium position.
A =
amplitude
w
= angular frequencyd = phaseSlide21
SHM Prototype Experiment
Consider Fig. (a). An air-track glider attached to a spring. The glider is pulled a distance
A
from its rest position and released.
Fig. (b) shows a graph of the motion of the glider, as measured each 1/20 of a second.
The graphs on the right show the position and velocity of the glider from the same measurements. We see that A
=0.17 m and T=1.60 s. Therefore the oscillation frequency of the system is f = 0.625 HzSlide22
Two Oscillating Systems
The diagram shows two identical masses attached to two identical springs and resting on a horizontal frictionless surface. Spring 1 is stretched to 5 cm, spring 2 is stretched to 10 cm, and the masses are released at the same time.
Which mass reaches the equilibrium position first?
Because
k and m are the same, the systems have the same period, so they must return to equilibrium
at the same time. The frequency and period of SHM are independent of amplitude.Slide23
Clicker Question 1
Shown are two mass + spring systems. The blocks have the same mass.
When set into oscillation, what is the relation between the oscillation periods T1,2 of the two systems?
(a)
T1>T
2 (b) T
1=T2 (c) T1<T
2(d) Need to know m and k to answerSlide24
Example
: A Block on a Spring
A 2.00 kg block is attached to a spring as shown.The force constant of the spring is k = 196 N/m.The block is held a distance of 5.00 cm fromequilibrium and released at
t = 0.(a) Find the angular frequency w
, the frequency f, and the period T.
(b) Write an equation for x vs. time.Slide25
Example:
A System in SHM
An air-track glider is attached to a spring,pulled 20 cm to the right, and releasedat t-=0. It makes 15 completeoscillations in 10 s.What is the period of oscillation?What is the object’s maximum speed?
What is its position and velocity at t=0.80 s?Slide26
Example:
Finding the Time
A mass, oscillating in simple harmonic motion, starts at x = A and has period T. At what time, as a fraction of T, does the mass first pass through x = ½
A?Slide27
The Phase Constant
But what if
f is not zero at t=0?
A
phase constant
f¹
0 means that the rotation starts at a different point on the circle, implying different initial conditions.Slide28
SHM Initial ConditionsSlide29
***Energy ***
A mass is attached to a spring and set to motion. The maximum displacement is
x=
A
SWnc = DK + DU 0 = DK + DU or Energy U+K is constant! Energy = ½ k x2
+ ½ m v2At maximum displacement x=
A, v = 0 Energy = ½ k A2
+ 0 At zero displacement x = 0 Energy = 0 + ½ mv
m2 Since Total Energy is same
½ k A2 = ½ m vm2 vm = sqrt(k/m) A
m
x
x=0
0
x
PE
S
25Slide30
Preflight 3+4
A mass on a spring oscillates back & forth with simple harmonic motion of amplitude
A
. A plot of displacement (x) versus time (t) is shown below.
At what points during its oscillation is the total energy (K+U) of the mass and spring a maximum? (Ignore gravity). 1. When x = +A or -A (i.e. maximum displacement) 2. When x = 0 (i.e. zero displacement) 3. The energy of the system is constant.
+A
t
-A
x
CORRECT
27Slide31
Preflight 1+2
A mass on a spring oscillates back & forth with simple harmonic motion of amplitude A. A plot of displacement (x) versus time (t) is shown below.
At what points during its oscillation is the speed of the block biggest?
1. When x = +A or -A (i.e. maximum displacement)
2. When x = 0 (i.e. zero displacement) 3. The speed of the mass is constant
+A
t
-A
x
CORRECT
29
“There is no potential energy at x=0 since U=1/2kx^2=0, therefore allowing all the energy of the spring to be allocated toward KE . Slide32
Simple Harmonic Motion:
x(t) = [A]cos(
t)
v(t) = -[A]sin(t)
a(t) = -[A2]cos(t)x(t) = [A]sin(
t)v(t) = [A]cos(t)
a(t) = -[A2]sin(t)
x
max = Avmax
= Aamax = A2
Period = T (seconds per cycle)Frequency = f = 1/T (cycles per second)
Angular frequency = = 2f = 2/T
For spring: 2
= k/m
OR
36Slide33
Example
A
3 kg mass is attached to a spring (k=24 N/m). It is stretched 5 cm. At time t=0 it is released and oscillates.
Which
equation describes the position as a function of time x(t) =A) 5 sin(wt) B) 5 cos(wt) C) 24sin(wt)
D) 24 cos(wt) E) -24 cos
(wt)
We are told at t=0, x = +5 cm. x(t) = 5 cos
(wt) only one that works.
39Slide34
Example
A
3 kg mass is attached to a spring (k=24 N/m). It is stretched 5 cm. At time t=0 it is released and oscillates.
What
is the total energy of the block spring system? A) 0.03 J B) .05 J C) .08 J E = U + KAt t=0, x = 5 cm and v=0:
E = ½ k x2 + 0
= ½ (24 N/m) (5 cm)2
= 0.03 J
43Slide35
Example
A
3 kg mass is attached to a spring (k=24 N/m). It is stretched 5 cm. At time t=0 it is released and oscillates.
What is the maximum speed of the block?
A) .45 m/s B) .23 m/s C) .14 m/s E = U + KWhen x = 0, maximum speed: E = ½ m v
2 + 0 .03 = ½ 3 kg v
2
v = .14 m/s
46Slide36
Example
A 3 kg mass is attached to a spring (k=24 N/m). It is stretched 5 cm. At time t=0 it is released and oscillates.
How long does it take for the block to return to x=+5cm?
A) 1.4 s B) 2.2 s C) 3.5 s
w = sqrt(k/m)
= sqrt
(24/3) = 2.83 radians/secReturns to original position after 2
p radiansT = 2
p / w = 6.28 / 2.83 = 2.2 secondsSlide37
Summary
Springs
F = -
kx
U = ½ k x2w = sqrt(k/m)Simple Harmonic MotionOccurs when have linear restoring force F= -kx x(t) = [A] cos(wt) or [A] sin(wt)
v(t) = -[Aw] sin(
wt) or [Aw]
cos(wt) a(t) = -[A
w2] cos(wt) or -[Aw
2] sin(wt)