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Jetzt kostenlos anmeldenHave you ever been to an amusement park and tried the pendulum ride?
The pendulum ride applies a harmonic motion going back and forth. When it is at the highest level, it stops for some time and then starts moving downward by increasing speed. Then, it reaches its peak again, stops again, and starts moving faster again, repeating the cycle. Why is that? This article will answer that question by explaining how to use the Energy-Time Graph. Throughout the next few paragraphs, we will explore the definition of an Energy Time Graph, compare a Displacement-Time graph to a Potential and Kinetic Energy-Time graph, and do some calculations and examples.
We'll kick it off with a definition.
An Energy-Time Graph is a model used to show how an object's energy changes over time.
Let's say we have a block \(A\) attached to a spring. What happens if we compress it to position \(+x\) and then let it go? When we compress the block to position \(+x\), it will gain elastic potential energy. When the block returns to equilibrium, it converts its potential energy to kinetic energy. The same case will happen when it stretches to position \(-x\); it converts its kinetic energy to elastic potential energy. While going back to the original state, the potential energy is again converted to kinetic energy. The mechanical energy is conserved if the block is in a closed system, meaning that there is no dissipation or addition of energy. That's why the kinetic and potential energies will convert nonstop, but their total combined value will be constant.
Now, let's do an example of Simple Harmonic Motion (SHM). Below are two graphs that describe what is happening to the block \(A\) (from the image above) as it oscillates back and forth.
This graph shows the position of block \(A\) as a function of time.
The motion plots a sine curve. Notice how it has an amplitude of \(x\) and a period of \(4\,\mathrm{s}\). Remember that the amplitude is the height of the curve from \(0\) and the period is the amount of time it takes the curve to go from one peak to another peak.
This graph is the Energy-Time Graph for block \(A\).
With these two graphs, we can make sense of how the block moves, as well as how the block transfers its energy. Let's take it step by step.
This cycle continues forever, since in SHM the effect of friction is ignored and the mechanical energy is conserved.
When the object is at the equilibrium position, all its energy is kinetic energy; when it is at the maximum displacements, it's all potential energy.
During oscillation, while the mechanical energy is constant, the kinetic and potential energies may vary with time. For example, the variation of a linear oscillator's potential energy depends on how compressed or stretched the spring is, which is \(x(t)\mathrm{.}\)
From principles of simple harmonic motion we can express the displacement at any point in time as \(x(t)=x_\text{m} \cos{(\omega t + \phi )}\), where \(x(t)\) is the function of the displacement with respect to time, \(x_\text{m} \) is the maximum displacement, \(\omega \) is the angular velocity, \(t\) is the time, and \(\phi \) is the phase shift of the cosine function.
The elastic potential energy is calculated as \(U=\frac{1}{2}\\ kx^2 \), so we can insert \(x(t)\) into the potential energy formula like so:
$$U(t)=\frac{1}{2}\\ kx^2 = \frac{1}{2}\\k(x_\text{m} \cos{(\omega t + \phi )})^2$$
$$U(t) = \frac{1}{2}\\ k x_\text{m} ^2 \cos^2{(\omega t + \phi)}\mathrm{.}$$
On the other hand, the kinetic energy variation depends on the block's velocity. We know that kinetic energy can be found from \(K=\frac{1}{2}\\ mv^2 \). If \(x(t) = x_\text{m} \cos{(\omega t + \phi )}\), we can derive velocity as \(V(t) = - \omega x_\text{m} \sin{(\omega t + \phi )}\).
We can insert this into the kinetic energy formula,
$$K(t)=\frac{1}{2}\\ m(v(t))^2 = \frac{1}{2}\\ m(-\omega x_\text{m} \sin{(\omega t + \phi )})^2$$
$$K(t) = \frac{1}{2}\\ m \omega ^2 x_\text{m} ^2 \sin^2{(\omega t +\phi )}\mathrm{,}$$
and since \(\omega ^2 = \frac{k}{m}\), we can transform the kinetic energy formula into
$$K(t) = \frac{1}{2}\\ k x_\text{m} ^2 \sin^2{(\omega t + \phi )}\mathrm{.}$$
During the oscillation, the total mechanical energy \(E\) is conserved. So, the sum of kinetic energy and potential energy is equal to mechanical energy:
$$E=U+K$$
$$E=\frac{1}{2}\\ k x_\text{m} ^2 \cos^2{(\omega t + \phi )} + \frac{1}{2}\\ k x_\text{m} ^2 \sin^2{(\omega t +\phi )}\mathrm{.}$$
Since \(\frac{1}{2}\\ k x_\text{m} ^2 \) is the common part of the equation, we can rewrite the sum as
$$E=\frac{1}{2}\\ k x_\text{m} ^2 (\cos^2{(\omega t + \phi )} + \sin^2{(\omega t + \phi )})\mathrm{.}$$
It is important to know that \(\sin^2{a} + \cos^2{a} = 1\). So, the sum will be equal to \(E=\frac{1}{2}\\k x_\text{m} ^2 \).
In SHM, the total mechanical energy is independent of time! It only depends on the spring constant and the amplitude.
So far, we've really only shown how Energy vs. Time Graphs can help us to understand oscillators. But that is not their only use! Energy-Time Graphs can also aid us in modeling the energy in capacitors.
Although AP Physics 2 focuses mostly on electricity and magnetism, it is still useful to know that Energy Time Graphs have other uses than just to model mechanics problems. An Energy-Time Graph for a capacitor is usually logarithmic or exponential. Below is an example of a possible Energy \(U\) vs. Time \(t\) for a capacitor as it charges.
From this graph, we can see that a capacitor gains energy at a decreasing rate as it approaches its maximum charge.
There is no real application for the area under an Energy-Time Graph unless we're talking quantum physics and phase shifts. My guess is that the only knowledge you have about quantum physics derives from Avengers: Endgame (which is all wrong: shocker!), so we won't go down that rabbit hole.
The slope of an Energy-Time Graph does tell us something useful, however. But, before we get into that, we need a little background.
Power equals the rate of energy over time. Therefore, power gives us a quantity for how much punch our energy packs. A little bit of energy over a large amount of time will give us little punch. Whereas a large amount of energy over a little amount of time gives us massive punch. A relevant equation for this principle is
$$P=\frac{\Delta E}{\Delta t}\\\mathrm{.} $$
From this knowledge, we glean that the slope of an Energy-Time Graph equals the power! Remember that slope is the rate of change of \(y\) over the rate of change of \(x\). Since the energy is the \(y\) and the time is the \(x\), plugging these into the slope equation yields
$$m=\frac{\Delta E}{\Delta t}\\$$
where \(m\) is the slope. That is the exact equation for power. To illustrate this principle, we'll go over the Energy-Time Graph below.
Fig. 6 shows that the slope of an Energy vs. Time Graph equals power. To find the power of the above function of energy with respect to time, we need only find the slope:
$$\begin{align*} m&=\frac{\Delta y}{\Delta x} \\ m&=\frac{100\,\mathrm{J}-0\,\mathrm{J}}{1\,\mathrm{s}-0\,\mathrm{s}} = 100\,\mathrm{\frac{J}{s}} \\ m&=\text{Power}=100\,\mathrm{\frac{J}{s}.} \\ \end{align*}$$
Energy equals the area under a Power vs. Time Graph because power equals energy over time, therefore, energy equals power times time:
$$\Delta E = P\Delta t.$$
Now, it's time for some examples.
Suppose the block in the diagram below has a mass of \(m=2.00\,\mathrm{kg} \) and is designed to oscillate at the end of a spring at frequency \(f=20.0\,\mathrm{Hz}\) and with amplitude \(x_m = 50.0\,\mathrm{cm}\).
a) What is the total mechanical energy \(E\) of the spring-block system?
b) What is the block's speed as it passes through the equilibrium position?
a) The mechanical energy is dependent on the spring constant and amplitude and can be visualized with the graph below.
In the example, the frequency and mass of the block are given so that we can calculate the spring constant using \(\omega ^2 = \frac{k}{m}\\\). Also, \(\omega \) is dependent on the frequency and equal to \(\omega = 2\pi f\). Therefore, we can calculate:
$$(2\pi f)^2 =\frac{k}{m}\\$$
$$4\pi ^2 f^2 = \frac{k}{2}\\$$
to give us an answer of
$$k=8\pi ^2(20.0\,\mathrm{Hz})^2=3.16 \times 10^4 \,\mathrm{\frac{N}{m}\\}\mathrm{.}$$
Now that we found the spring constant \(k\), we can calculate the mechanical energy (without forgetting to convert amplitude into units of meters!):
$$E=\frac{1}{2}\\ k x_m ^2 $$
$$E=\frac{1}{2}\\(3.16\times 10^4 \,\mathrm{\frac{N}{m}\\})(0.500\,\mathrm{m})^2$$
$$E=3.95\times 10^3 \,\mathrm{J}\mathrm{.}$$
b) When the block passes the equilibrium position, its potential energy is converted into kinetic energy. So, the mechanical energy will be equal to kinetic energy:
$$\frac{1}{2}\\k x_m ^2 = \frac{1}{2}\\ mv^2\mathrm{.}$$
From that equation, we can find the speed.:
$$3.95\times 10^3 \,\mathrm{J}=\frac{1}{2}\\(2.00\,\mathrm{kg})v^2$$
$$3.95\times 10^3 \,\mathrm{J}=v^2$$
$$\sqrt{3.95\times 10^3 \,\mathrm{J}}=\sqrt{v^2}$$
$$v=62.8\,\mathrm{\frac{m}{s}\\}\mathrm{.}$$
That was quite the rollercoaster ride! There was a lot of back and forth, some ups and downs, and even more stops and drops. Hopefully, by now, you aren't nauseous from all the physics frenzy, and you're ready to absorb the essential key takeaways from our article on Energy-Time Graphs.
Systems with internal structure have potential energy. A system can have potential energy if the objects that make up the system interact with conservative forces.
If the composition of the system changes, the potential energy can change. One example of this is mass-spring oscillators.
In a closed system, mechanical energy is constant during oscillation.
Mechanical energy is independent of time.
Mechanical energy is dependent on the amplitude and the spring constant.
Kinetic and potential energies depend on time and convert to each other during oscillation.
The kinetic and potential energies of a system constitute the internal energy of the system.
Since energy is constant in a closed system, changes in a system's potential energy can result in changes to the system's kinetic energy.
There is no real application for the area under an Energy-Time Graph.
Power is the slope of an Energy-Time Graph.
We can use Energy vs. Time Graphs to model the energy in a capacitor.
An energy time graph tracks the energy of an object over a period of time. It plots an object's energy as a function of time.
An energy time graph shows how the energy of an object or system changes over a period of time.
One example of an energy time graph is the energy of an object oscillating due to the force of a spring being plotted as a function of time.
You plot an energy time graph by plotting an object's energy at each second on a graph.
When the object is at the equilibrium position, all of its energy is kinetic.
True.
When the object is at the amplitude, all of its energy is potential.
True.
Suppose a block is designed to oscillate at the end of spring with a spring constant \(k = 0.50\) and with amplitude \(x_m=4.0 \,\text m\).
What is the total mechanical energy of the spring-block system?
\(4.0 \,\text J\).
Suppose a block is designed to oscillate at the end of spring with a spring constant \(k=0.2\) and with amplitude \(x_m=10 \,\text m\).
What is the total mechanical energy of the spring-block system?
\(1 \,\text J\).
Suppose the block has a mass of \(1.0 \,\text{kg}\) and is designed to oscillate at the end of spring at a frequency of \(10. \,\text{Hz}\) and with an amplitude of \(1.0 \,\text m\).
What is the total mechanical energy of the spring-block system?
\(1800 \,\text J\).
Suppose the block has a mass of \(5.00 \,\text{kg}\) and is designed to oscillate at the end of spring at a frequency of \(4.00 \,\text{Hz}\) and with an amplitude of \(0.250 \,\text m\).
What is the total mechanical energy of the spring-block system?
\(90.0 \,\text J\).
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