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What is conservation of energy?

Learn what conservation of energy means, and how it can make solving problems easier.

What is the principle of conservation of energy?

In physics, the term conservation refers to something which doesn't change. This means that the variable in an equation which represents a conserved quantity is constant over time. It has the same value both before and after an event.
There are many conserved quantities in physics. They are often remarkably useful for making predictions in what would otherwise be very complicated situations. In mechanics, there are three fundamental quantities which are conserved. These are energy, momentum and angular momentum.
If you have looked at examples in other articles—for example, the kinetic energy of charging elephants—then it may surprise you that energy is a conserved quantity. After all, energy often changes in collisions. It turns out that there are a couple of key qualifying statements we need to add:
  • Energy, as we'll be discussing it in this article, refers to the total energy of a system. As objects move around over time, the energy associated with them—e.g., kinetic, gravitational potential, heat—might change forms, but if energy is conserved, then the total will remain the same.
  • Conservation of energy applies only to isolated systems. A ball rolling across a rough floor will not obey the law of conservation of energy because it is not isolated from the floor. The floor is, in fact, doing work on the ball through friction. However, if we consider the ball and floor together, then conservation of energy will apply. We would normally call this combination the ball-floor system.
In mechanics problems, we are likely to encounter systems containing kinetic energy (E, start subscript, K, end subscript), gravitational potential energy (U, start subscript, g, end subscript), elastic—spring—potential energy (U, start subscript, s, end subscript), and heat (thermal energy) (E, start subscript, H, end subscript). Solving such problems often begins by establishing conservation of energy in a system between some initial time—subscript i—and at some later time—subscript f.
E, start subscript, K, i, end subscript, plus, U, start subscript, g, i, end subscript, plus, U, start subscript, s, i, end subscript, equals, E, start subscript, K, f, end subscript, plus, U, start subscript, g, f, end subscript, plus, U, start subscript, s, f, end subscript, plus, E, start subscript, H, f, end subscript
Which could be expanded out as:
start fraction, 1, divided by, 2, end fraction, m, v, start subscript, i, end subscript, squared, plus, m, g, h, start subscript, i, end subscript, plus, start fraction, 1, divided by, 2, end fraction, k, x, start subscript, i, end subscript, squared, equals, start fraction, 1, divided by, 2, end fraction, m, v, start subscript, f, end subscript, squared, plus, m, g, h, start subscript, f, end subscript, plus, start fraction, 1, divided by, 2, end fraction, k, x, start subscript, f, end subscript, squared, plus, E, start subscript, H, f, end subscript

What do we mean by system here?

In physics, system is the suffix we give to a collection of objects that we choose to model with our equations. If we are to describe the motion of an object using conservation of energy, then the system should include the object of interest and all other objects that it interacts with.
In practice, we always have to choose to ignore some interactions. When defining a system, we are drawing a line around things we care about and things we don't. The things we don't include are usually collectively termed the environment. Ignoring some of the environment will inevitably make our calculations less accurate. There is no indignity in doing this however. In fact, being a good physicist is often as much about understanding the effects you need to describe as it is about knowing which effects can be safely ignored.
Consider the problem of a person making a bungee jump from a bridge. At a minimum, the system should include the jumper, bungee, and the Earth. A more accurate calculation might include the air, which does work on the jumper via drag, or air resistance. We could go further and include the bridge and its foundation, but since we know that the bridge is much heavier than the jumper, we can safely ignore this. We wouldn't expect the force of a decelerating bungee jumper to have any significant effect on the bridge, especially if the bridge is designed to bear the load of heavy vehicles.
There is always some tenuous level of interaction between even distant objects, so we need to choose the boundary of our system intelligently.
There is always some tenuous level of interaction between even distant objects, so we need to choose the boundary of our system intelligently.

What is mechanical energy?

Mechanical energy, E, start subscript, M, end subscript, is the sum of the potential energy and kinetic energy in a system.
start box, E, start subscript, M, end subscript, equals, E, start subscript, P, end subscript, plus, E, start subscript, K, end subscript, end box
Only conservative forces like gravity and the spring force that have potential energy associated with them. Nonconservative forces like friction and drag do not. We can always get back the energy that we put into a system via a conservative force. Energy transferred by nonconservative forces however is difficult to recover. It often ends up as heat or some other form which is typically outside the system—in other words, lost to the environment.
What this means in practice is that the special case of conservation of mechanical energy is often more useful for making calculations than conservation of energy in general. Conservation of mechanical energy only applies when all forces are conservative. Luckily, there are many situations where nonconservative forces are negligible, or at least a good approximation can still be made when neglecting them.

How can conservation of energy describe how objects move?

When energy is conserved, we can set up equations which equate the sum of the different forms of energy in a system. We then may be able to solve the equations for velocity, distance, or some other parameter on which the energy depends. If we don't know enough of the variables to find a unique solution, then it may still be useful to plot related variables to see where solutions lie.
Consider a golfer on the moon—gravitational acceleration 1.625 m/ssquared—striking a golf ball. By the way, Astronaut Alan Shepard actually did this. The ball leaves the club at an angle of 45degrees to the lunar surface traveling at 20 m/s both horizontally and vertically—total velocity 28.28 m/s. How high would the golf ball go?
We begin by writing down the mechanical energy:
E, start subscript, M, end subscript, equals, start fraction, 1, divided by, 2, end fraction, m, v, squared, plus, m, g, h
Applying the principle of conservation of mechanical energy, we can solve for the height h—note that the mass cancels out.
start fraction, 1, divided by, 2, end fraction, m, v, start subscript, i, end subscript, squared, equals, m, g, h, start subscript, f, end subscript, plus, start fraction, 1, divided by, 2, end fraction, m, v, start subscript, f, end subscript, squared
h=12vi212vf2g=12(28.28 m/s)212(20 m/s)21.625 m/s2=123 m\begin{aligned} h &= \frac{\frac{1}{2}v_i^2-\frac{1}{2}v_f^2}{g} \\ &=\frac{\frac{1}{2}(28.28~\mathrm{m/s})^2-\frac{1}{2}(20~\mathrm{m/s})^2}{1.625~\mathrm{m/s^2}} \\ &= 123~\mathrm{m}\end{aligned}
As you can see, applying the principle of conservation of energy allows us to quickly solve problems like this which would be more difficult if done only with the kinematic equations.
Exercise 1: Suppose the ball had an unexpected collision with a nearby american flag hoisted to a height of 2 m. How fast would it be traveling at the time of collision?
Exercise 2: The image below shows a plot of the kinetic, gravitational potential and mechanical energy over time during the flight of a small model rocket. Points of interest such as maximum height, apogee, and the time of motor stop, burnout, are noted on the plot. The rocket is subject to several conservative and nonconservative forces over the course of the flight. Is there a time during the flight when the rocket is subject to only conservative forces? Why?
Energy transfer during the flight of a small model rocket [1].
Energy transfer during the flight of a small model rocket [1].

Why can perpetual motion machines never work?

The perpetual motion machine is a concept for a machine which continues its motion forever, without any reduction in speed. An endless variety of weird and wonderful machines have been described over the years. They include pumps said to run themselves via their own head of falling water, wheels which are said to push themselves around by means of unbalanced masses, and many variations of self-repelling magnets.
Though often interesting curiosities, such a machine has never been shown to be perpetual, nor could it ever be. In fact, even if such a machine were to exist, it wouldn't be very useful. It would have no ability to do work. Note that this differs from the concept of the over-unity machine, which is said to output more than 100% of the energy put into it, in clear violation of the principle of conservation of energy.
From the most basic principles of mechanics, there is nothing that strictly makes the perpetual motion machine impossible. If a system could be fully isolated from the environment and subject to only conservative forces, then energy would be conserved and it would run forever. The problem is that in reality, there is no way to completely isolate a system and energy is never completely conserved within the machine.
It is possible today to make extremely low friction flywheels which rotate in a vacuum for storing energy. Yet, they still lose energy and eventually spin down when unloaded, some over a period of years [2]. The earth itself, rotating on its axis in space is perhaps an extreme example of such a machine. Yet, because of interactions with the moon, tidal friction, and other celestial bodies, it too is gradually slowing. In fact, every couple of years, scientists have to add a leap second to our record of time to account for variation in the length of day.

Attributions

[1] Figure made using OpenRocket 15.03. Custom expressions for calculating energy detailed in openrocket documentation.
[2] Abbasi, Tasneem. Renewable Energy Sources: Their Impact On Global Warming And Pollution. A.S.A., 2010. ISBN: 9788120339941

Want to join the conversation?

  • blobby green style avatar for user S26Patel
    In the golfer problem why is Em (Mechanical Energy) = 0 in the equation Em = Ep + Ek? In general What does Em = 0 mean in relation to PE and KE? I understand how the algebra works but I don't understand the general relationship of Em to the other terms in the equation. Does Em = 0 generally mean that Em is constant? And, when Em is constant, does that always mean Ek = 0 and Ep will be at its maximum positive value?
    (14 votes)
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    • leaf yellow style avatar for user joshua wallace
      The mechanical energy does not equal zero. Think of it this way. The conservation of energy formula goes Ki+Ui=Kf+Uf. U is potential energy and K is kinetic energy. In this case the golf ball at the start has zero potential energy. We are considering the surface of the moon to be the height h. The height is zero therefore, we have no initial potential energy(mgh). We are left with Ki=Kf+Uf=>1/2m(vi)^2=1/2m(vf)^2+mgh.

      Also Em is a measure of energy at a certain time. We can rewrite the conservation of energy formula as Em(i)=Em(f). If Em equals zero that means U+K=0 . I hope this helps.
      (24 votes)
  • purple pi teal style avatar for user Yasin Aksit
    If an object hits another but if friction force doesn't allow the second object to move then where is the energy of the first object gone ? Heat ? How ?
    (6 votes)
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    • female robot ada style avatar for user Rodrigo Campos
      The collision's kinetic energy vibrates both body and nearby air's molecules, increasing their quadratic average speed, which is directly related to temperature. Small but quick compression/expansion processes from the impact also dissipate energy as sound waves. The latter is usually a neglectible amount, though. Like Andrew said, missing energy usually turns out to be heat.
      (11 votes)
  • blobby green style avatar for user Aliya Dunbar
    How did you get 28.16m/s as your answer for Problem 1? I am very confused and I need help
    (2 votes)
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    • starky tree style avatar for user Jonathan Morales
      I can help you with this.
      For get 28.16m/s you have to do the next equation:
      Vf = sqrt[(28.28^2) - (2·2·1.625)]*

      Take care about:
      - "g" is 1.625 (*one coma six hundred and tweenty-five
      ) & not 1625 (one thousand six hundred and tweenty-five)
      - "Vi" is 28.28 squared.
      - Remeber to do the square root to the result.

      I hope I help you & sorry if my english isn't the best.
      (6 votes)
  • piceratops ultimate style avatar for user David Ninan Kurien
    isnt mass also conserved? howcome it isnt a fundamental ?
    (3 votes)
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    • piceratops ultimate style avatar for user Anujith Krishnan
      mass can be destroyed when an antimatter and matter collide with each other and also annihilate energy in the form light energy.Antimatter posses the properties just opposite to matter , like antimatter have positively charged electrons called positron . Though mass can be destroyed energy remains constant. This process involves the conversion of mass into energy. The law of conservation of mass-energy states the same
      (2 votes)
  • male robot hal style avatar for user jjain654
    Would friction be an example of a non-conservative force because it releases thermal energy?
    (1 vote)
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  • blobby green style avatar for user Anwesha Donna
    In the model rocket question, I have understood the problem in terms of the graph (mechanical energy is flat, so energy is conserved). But can someone explain what actually happens in reality? Is the energy conserved because the rocket went past the atmosphere and gravitational force? Could someone explain how is the energy conserved practically at this stage - "During this time, the rocket is coasting upwards—motor has stopped burning—but going slow enough that the work being done by drag on the rocket is mostly negligible."
    (3 votes)
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  • blobby green style avatar for user weyapaca
    In Ex. 1 above, I used the conservation of energy equation in the vertical direction. I got a final velocity of 19.84 m/s then divided by sin 45 and got close to the same answer (28.05 m/s instead of 28.16 m/s). Why is the direction not considered?
    (2 votes)
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    • blobby green style avatar for user Pete Sillifant
      Because the calculation is for energy, which is not a vector quantity.
      I also tried to calculate in the vertical direction after reading this post. I was curious as to why the difference between the two answers. It's because the angle also shifts slightly from 45 degrees as the vertical velocity drops slightly. Using the pythagorean theorem on the vertical and horizontal directions gives the correct diagonal velocity, and taking the inverse tan of the horizontal and vertical velocities gives the new angle of 44.77 degrees. :)
      (2 votes)
  • aqualine seed style avatar for user TamaRanda
    In Exercise 1 of How can conservation of energy describe how objects move, how did you get 28.16m/s for the final velocity? I practiced the problem before I saw the answer, and I used the same formula, but I got 6.2m/s for the final velocity.
    (3 votes)
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  • leaf green style avatar for user Annling Wang
    In the golfer question, why is Mechanical Energy = m*vi^2/2?
    (0 votes)
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  • aqualine ultimate style avatar for user jeffrey.deleon113
    So is conservation of energy the same as conservatives forces?
    (2 votes)
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