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17-1 Oscillating Systems

Each day we encounter many kinds of oscillatory motion,such as swinging pendulum of a clock, a person bouncing on a trampoline,a vibrating guitar string, and a mass on a spring.

They have common properties:

- The particle oscillates back and forth about a equilibrium position. The time necessary for one complete cycle (a complete repetition of the motion) is called the period T.

2. No matter what the direction of the displacement,

the force always acts in a direction to restore the

system to its equilibrium position. Such a force is

called a “restoring force(恢复力)”.

3. The number of cycles per unit time is called the

“frequency” f.

(17-1)

Unit: period (s)

frequency(Hz, SI unit), 1 Hz = 1 cycle/s

4. The magnitude of the maximum displacement

from equilibrium is called the amplitude of the motion.

17-2/3 The simple harmonic oscillator and its motion

1. Simple harmonic motion

An oscillating system which can be described in

terms of sine and cosine functions is called a

“simple harmonic oscillator” and its motion is called

“simple harmonic motion”.

2. Equation of motion of the simple harmonic oscillator

Fig 17-5 shows a simple harmonic oscillator,

consisting of a spring of force constant K acting on

origin is chosen at here

a body of mass m that slides on a frictionless horizontal surface. The body moves in x direction.

m

Relaxed state

x

o

F

m

x

o

(17-4)

Eq(17-4) is called the “equation of motion of the simple harmonic oscillator”. It is the basis of many complex oscillator problems.

Rewrite Eq(17-4) as

(17-5)

We write a tentative solution to Eq(17-5) as

(17-6)

3. Find the solution of Eq. (17-4)

We differentiate Eq(17-6) twice with respect to the

Time.

Putting this into Eq(17-5) we obtain

Therefore, if we choose the constant such that

(17-7)

Eq(17-6) is in fact a solution of the equation of

motion of a simple harmonic oscillator.

a) :

If we increase the time by in Eq(17-6), then

Therefore is the period of the motion T.

(17-8)

(17-9)

The quantity is called the angular frequency.

is the maximum value of displacement. We call it

the amplitude of the motion.

- and :

The quantity is called phase of the motion.

is called “phase constant （常相位)”.

and are determined by the initial position and

velocity of the particle. is determined by the system.

How to understand ?

Fig 17-6 shows several simple harmonic motions.

Fig 17-6

(a)

(b)

(c)

(a) same: ,

different:

(b) same: ,

different:

(c) same: ,

different:

d).Displacement, velocity, and acceleration

Displacement

Velocity

Acceleration

When the displacement is a maximum in either

direction, the speed is zero, because the velocity

must now change its direction.

(17-11)

17-4 Energy in simple harmonic motion

1.The potential energy

(17-12)

2.The kinetic energy

U(t)

K(t)

T/2

T

(17-13)

Fig 17-8(a)

Fig17-8(a), both potential and kinetic energies oscillate with time tand vary between zero and maximum value of .

- BothU and K vary with twice the frequency of the displacement and velocity.

3. The total mechanical energy E is

(17-14)

E

U(x)

K(x)

x

Fig 17-8 (b)

At the maximum displacement , .

At the equilibrium position , .

Eq(17-14) can be written quite generally as

(17-15)

then

(17-16)

or

Sample problem 17-2

Fig 17-5

x

m

o

In Fig 17-5, m=2.43kg, k=221N/m, the block is

stretched in the positive x direction a distance of

11.6 cm from equilibrium and released. Take time

t=0 when the block is released, the horizontal

surface is frictionless.

(a) What is the total energy?

(b) What is the maximum speed of the block?

(c) What is the maximum acceleration?

(d) What is the position, velocity, and acceleration

at t=0.215s?

Solution:

(a)

(b)

(c) The maximum acceleration occurs just at the

instant of release, when the force is greatest

(d)

Since at t=0, then

So at t=0.215s

Sample problem 17-3

Fig 17-5

v

m

x

0

In Fig17-5, m=2.43kg, k=221N/m, when the block

m is pushed from equilibrium to x=0.0624m, and

its velocity , the external force is

removed and the block begins to oscillate on the

horizontal frictionless surface. Write an equation

for x (t) during the oscillation.

:

Setting this equal to , we have

To find the phase constant , we still need to use the

information give for t=0:

At t=0

17-5 Applications of simple harmonic motion

Fixed clamp

O’

1. The torsional oscillator(扭转振子)

Fig 17-9 shows a torsional oscillator.

If the disk is rotated in the horizontal (xy)

plane, the reference line op will move

to the OQ, and the wire oo’ will be

twisted. The twisted wire will exert a

restoring torque on the disk, tending to

return the system to its equilibrium.

o

Q

p

R

Fig 17-9

For small twist, the restoring torque is

(17-17)

Here is constant ( the Greek letter Kappa ), and

is called torsional constant.

The equation of motion for such a system is

(17-18)

where is the rotational inertia of the disk about z

axis. Using Eq(17-17) we have

or (17-19)

Eq(17-19) and (17-5) are mathematically identical.

The solution should be a simple harmonic oscillation in the angle coordinate ,

(17-20)

or (17-21)

A torsional oscillator is also called torsional pendulum(扭摆). The Cavendish balance, used to measure the gravitational force constant G, is a torsional pendulum.

2. The simple pendulum(单摆)

Fig(17-10) shows a simple pendulum of length L

and particle mass m.

The restoring force is:

(17-22)

If the is small,

T

L

k

m

(17-23)

x

mg

(17-24)

k

Fig(17-10)

- Any rigid body mounted so that it can swing in
- vertical plane about some axis passing through it
- is called “physical pendulum”.

In Fig. 17-11 a body of irregular shape is pivoted about a horizontal frictionless axis through P

P

d

C

y

x

Mg

Fig(17-11)

The restoring torque for an angular displacement is

(17-26)

For small angular displacement .

(17-27)

then

(17-28)

(a)The rotational inertia can be found from Eq(17-28).

(17-29)

Center of oscillation (振动中心 )”

Suppose the mass of the physical pendulum were concentrated at one point with distance L from the pivot, it will form a simple pendulum.

P

d

L

(17-30)

C

The resulting simple pendulum would have same period as the original physical pendulum.

O

Fig(17-11)

P

d

L

C

C

O

O

Fig(17-11)

The point O is called the “center of oscillation” of the physical pendulum.

- If we pivot the original physical pendulum from point O, it will have the same period as it does when pivoted from point P.

- The center of oscillation is often also called the “center of percussion（撞击中心）”.

If an impulsive force in the plane of oscillation acts at the center of oscillation, no effect of this force is felt at the pivot point P.

Sample problem 17-4

A thin uniform rod of mass M=0.112kg and

length L=0.096m is suspended by a wire that

passes through its center and is perpendicular

to its length. The wire is twisted and the rod

set oscillating. The period is found to be 2.14s.

When a flat body in the shape of an equilateral

triangle is suspended similarly through its

center of mass, the period is found to be 5.83m.

Find the rotational inertia of the triangle about this axis.

A uniform disk is pivoted at

its rim ( Fig17-12). Find its

period for small oscillations

and the length of the

equivalent simple pendulum.

Solution:

The rotational inertia about

pivot at the rim is

Sample problem 17-5P

R

C

O

Fig 17-12

From Eq(17-28) with d=R, then

The simple pendulum having the same period has

a length (Eq(17-30))

The center of oscillation of the disk pivoted at P is

therefore at o, a distance below the point of support.

You may check that the period of the pendulum pivoted at O is the same as that pivoted at P.

17-6 Simple harmonic motion and uniform circular motion

Simple harmonic motion can be described as a

projection of uniform circular motion along a

diameter of the circle.

1. Fig17-14 shows a particle

P in uniform circular motion.

y

P

Q

r

0

x

Fig 17-14

At a time t, the vector ( ) makes an angle

with x axis, and the x component of is

(17-31)

This is of course identical to Eq(17-6) for the

displacement of the simple harmonic oscillator.

- The radius r corresponding to ;
- Point , which is the projection of P on the x axis, executes simple harmonic motion along the x axis.

2. The magnitude of the tangent velocity of the

point P is , the x component of is

(17-32)

3. The centripetal acceleration is , and its x

component is (17-33)

Eqs(17-32) and (17-33) are identical with Eqs(17-

11) for simple harmonic motion, again with

replace by r.

See动画库\波动与光学夹\1-07辅助圆

4. In Fig17-14，the y projection OQ of at time t, is

(17-34)

So the projection of uniform circular motion along the y direction also gives simple harmonic motion.

On the contrary, the combination of two simple harmonic motions at right angles, with identical amplitude and

frequencies, can form a uniform circular motion.

See动画库\波动与光学夹\1-10垂直振动的合成

17-7 Damped (阻尼) harmonic motion

v

Up to this point we have assumed that no frictional

force act on the system.

For real oscillator, there may be friction, air resistance act on the system, the amplitude will decrease.

1. This loss in amplitude is called “damping” and

the motion is called “damped harmonic motion”.

m

x

f

0

When we add a small damping force, the amplitude gradually decreases to zero but the frequency changes by a negligible amount. In this case Eq(17-6) becomes

(17-36)

where is called the “damping time constant” or

the “mean lifetime” of the oscillator.

is the time necessary for the amplitude to drop to 1/e of its initial value.

(b) When the damping force is not large, the mechanism energy is

(17-37)

Eq(17-37) shows that the mechanical energy of

the oscillator decreases exponentially with time.

The energy decreases twice as rapidly as the

amplitude.

2*. Mathematical analysis

Assume the damping force is , where b is a

positive constant called the “damping constant”.

With , Newton’s second law gives

or

(17-38)

The solution is

(17-39)

where

(17-40)

- If b is negligible, . It is ideal simple harmonic oscillation.

If , that is , damping slows down

the motion. This case is called underdamping (欠阻尼)

Comparing Eqs(17-39) and (17-36) we have

(c)

(a)

(b) When , , the motion decays

exponentially to zero with no oscillation at all.

This condition is called “critical damping (临界阻尼)”. The lifetime has its smallest possible value, .

(c) When , the motion also decays

exponentially to zero with no oscillation, called overdamping （过阻尼）.

17-8 Forced oscillations and resonance

Forced oscillations:

Oscillations of a system carried out under the action of an external periodical force, such as

or a successive action of an external non-periodical force.

and ,

,Which frequency will the forced oscillation system take?

Forced oscillation system takes the frequency

of the external force, namely .

The amplitude of the forced oscillation can increase much as approaches .

This condition is known as “resonance” and is called “resonant angular frequency”.

In the case with damping, the rate at which energy is provided by the driving force exactly matches the rate at which energy is dissipated by the damping force.

Small damping

Amplitude

Fig 17-19

Large damping

String theory: Theory of everything (万物之理)

Theory of anything (任意之理)

Where string theory stands today

弦理的根本问题是

它很难或不可能得到实验证明

Peter Woit (Princeton)

弦论根本不是理论

Not even wrong!

简直比错还差！

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