16.5 Energy and the Simple Harmonic Oscillator

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:

Because a simple harmonic oscillator has no dissipative forces, the other important form of energy is kinetic energy KEKE size 12{ ital "KE"} {}. Conservation of energy for these two forms is:

or

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 size 12{v=Lω} {}, the spring constant with k=mg/Lk=mg/L size 12{k= ital "mg"/L} {}, and the displacement term with x=Lθx=Lθ size 12{x=Lθ} {}. Thus

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 16.16, 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.

The conservation of energy principle can be used to derive an expression for velocity vv size 12{v} {}. If we start our simple harmonic motion with zero velocity and maximum displacement (x=Xx=X size 12{x=X} {}), then the total energy is

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:

Solving this equation for vv size 12{v} {} yields:

Manipulating this expression algebraically gives:

and so

where

From this expression, we see that the velocity is a maximum (vmaxvmax) at x=0x=0 size 12{x=0} {}, as stated earlier in vt=−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:

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 size 12{k=6 "." "53" times "10" rSup { size 8{4} } `"N/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?

We can use the expression for vmaxvmax size 12{v rSub { size 8{"max"} } } {} given in vmax=kmXvmax=kmX size 12{v size 8{"max"}= sqrt { { {k} over {m} } } X} {} to determine the maximum vertical velocity. The variables mm size 12{m} {} and kk size 12{k} {} are given in the problem statement, and the maximum displacement XX size 12{X} {} is 0.100 m.

This answer seems reasonable for a bouncing car. There are other ways to use conservation of energy to find vmaxvmax size 12{v rSub { size 8{"max"} } } {}. 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 size 12{v rSub { size 8{"max"} } } {} of an oscillating simple pendulum, starting from its equilibrium position, is given as

where aa size 12{a} {} is the amplitude, ωω size 12{ω} {} is the angular velocity and tt size 12{t} {} is the time taken. Substituting ω=2πTω=2πT size 12{ω= { {2π} over {T} } } {}, we have

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

Also the velocity of the pendulum is given by

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

Why does it hurt more if your hand is snapped with a ruler than with a loose spring, even if the displacement of each system is equal?

The ruler is a stiffer system, which carries greater force for the same amount of displacement. The ruler snaps your hand with greater force, which hurts more.

You are observing a simple harmonic oscillator. Identify one way you could decrease the maximum velocity of the system.

You could increase the mass of the object that is oscillating.