Energy and the Simple Harmonic Oscillator

Learning Objectives

By the end of this section, you will be able to:

• Determine the maximum speed of an oscillating system.

To study the energy of a simple harmonic oscillator, we first consider all the forms of energy it can have We know from Hooke’s Law: Stress and Strain Revisited that the energy stored in the deformation of a simple harmonic oscillator is a form of potential energy given by:

PEel=12kx2.PEel=12kx2.

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Because a simple harmonic oscillator has no dissipative forces, the other important form of energy is kinetic energy KEKE. Conservation of energy for these two forms is:

KE+PEel=constantKE+PEel=constant

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or

12mv2+12kx2=constant.12mv2+12kx2=constant.

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This statement of conservation of energy is valid for all simple harmonic oscillators, including ones where the gravitational force plays a role

Namely, for a simple pendulum we replace the velocity with v=Lωv=Lω, the spring constant with k=mg/Lk=mg/L, and the displacement term with x=Lθx=Lθ. Thus

12mL2ω2+12mgLθ2=constant.12mL2ω2+12mgLθ2=constant.

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In the case of undamped simple harmonic motion, the energy oscillates back and forth between kinetic and potential, going completely from one to the other as the system oscillates. So for the simple example of an object on a frictionless surface attached to a spring, as shown again in Figure 16.14, the motion starts with all of the energy stored in the spring. As the object starts to move, the elastic potential energy is converted to kinetic energy, becoming entirely kinetic energy at the equilibrium position. It is then converted back into elastic potential energy by the spring, the velocity becomes zero when the kinetic energy is completely converted, and so on. This concept provides extra insight here and in later applications of simple harmonic motion, such as alternating current circuits.

Figure 16.14 The transformation of energy in simple harmonic motion is illustrated for an object attached to a spring on a frictionless surface.

The conservation of energy principle can be used to derive an expression for velocity vv. If we start our simple harmonic motion with zero velocity and maximum displacement (x=Xx=X), then the total energy is

12kX2.12kX2.

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This total energy is constant and is shifted back and forth between kinetic energy and potential energy, at most times being shared by each. The conservation of energy for this system in equation form is thus:

12mv2+12kx2=12kX2.12mv2+12kx2=12kX2.

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Solving this equation for vv yields:

v=±km(X2−x2)−−−−−−−−−−−√.v=±kmX2−x2.

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Manipulating this expression algebraically gives:

v=±km−−−√X1−x2X2−−−−−−√v=±kmX1−x2X2

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and so

v=±vmax1−x2X2−−−−−−√,v=±vmax1−x2X2,

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where

vmax=km−−−√X.vmax=kmX.

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From this expression, we see that the velocity is a maximum (vmaxvmax) at x=0x=0, as stated earlier in v(t)=−vmaxsin2πtTvt=−vmaxsin2πtT. Notice that the maximum velocity depends on three factors. Maximum velocity is directly proportional to amplitude. As you might guess, the greater the maximum displacement the greater the maximum velocity. Maximum velocity is also greater for stiffer systems, because they exert greater force for the same displacement. This observation is seen in the expression for vmax;vmax; it is proportional to the square root of the force constant kk. Finally, the maximum velocity is smaller for objects that have larger masses, because the maximum velocity is inversely proportional to the square root of mm. For a given force, objects that have large masses accelerate more slowly.

A similar calculation for the simple pendulum produces a similar result, namely:

ωmax=gL−−√θmax.ωmax=gLθmax.

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Example 16.6

Determine the Maximum Speed of an Oscillating System: A Bumpy Road

Suppose that a car is 900 kg and has a suspension system that has a force constant k=6.53×104N/mk=6.53×104N/m. The car hits a bump and bounces with an amplitude of 0.100 m. What is its maximum vertical velocity if you assume no damping occurs?

Strategy

We can use the expression for vmaxvmax given in vmax=km−−√Xvmax=kmX to determine the maximum vertical velocity. The variables mm and kk are given in the problem statement, and the maximum displacement XX is 0.100 m.

Solution

1. Identify known.
2. Substitute known values into vmax=km−−√Xvmax=kmX:vmax=6.53×104N/m900kg−−−−−−−−−−−−−√(0.100m).vmax=6.53×104N/m900kg(0.100m).16.44
3. Calculate to find vmax= 0.852 m/s.vmax= 0.852 m/s.

Discussion

This answer seems reasonable for a bouncing car. There are other ways to use conservation of energy to find vmaxvmax. We could use it directly, as was done in the example featured in Hooke’s Law: Stress and Strain Revisited.

The small vertical displacement yy of an oscillating simple pendulum, starting from its equilibrium position, is given as

y(t)=asinωt,y(t)=asinωt,

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where aa is the amplitude, ωω is the angular velocity and tt is the time taken. Substituting ω=2πTω=2πT, we have

y(t)=asin(2πtT).y(t)=asin2πtT.

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Thus, the displacement of pendulum is a function of time as shown above.

Also the velocity of the pendulum is given by

v(t)=2aπTcos(2πtT),v(t)=2aπTcos2πtT,

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so the motion of the pendulum is a function of time.