Lesson 3 of 6 · Conservation of energy
Account for every energy transfer
8 minute lesson · 3 checks along the way
Your goal for this lessonApply an energy balance to a chosen system and explain why decreasing mechanical energy does not mean total energy has been destroyed.
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Draw the boundary before balancing the account
Conservation of energy is a rule for careful accounting. In an , total energy remains constant. A selected subsystem can gain or lose energy when energy crosses its boundary. Radiation can enter a solar-powered spacecraft; electricity can leave a battery; hot exhaust can carry energy out of an engine. State what is inside your system before deciding whether its energy should stay constant.
The word is not always enough: a closed thermodynamic system exchanges no matter, but it may still exchange energy. An isolated system exchanges neither. For a chosen interval, write final energy as initial energy plus energy entering minus energy leaving. Include all relevant stores and transfers once. Changing the boundary changes the bookkeeping, not the physical event being described.
Mechanical energy is one part of the total
combines macroscopic and potential energy. For an ideal falling object with negligible drag, these can exchange while their sum stays constant. During a real landing or atmospheric entry, macroscopic motion also produces deformation, internal energy changes, sound, and radiation. Saying that mechanical energy decreased can be accurate. Saying that the missing amount ceased to exist is not.
Friction does not simply deposit all energy in whichever object you first named. The other contacting surface or surrounding fluid also matters. A spacecraft slowing in an atmosphere transfers energy to the gas and its own material, with some later escaping as radiation or carried-away material. A heat shield controls part of this transfer; it does not need to absorb the spacecraft’s entire initial kinetic energy.
Check 1 of 3
10 pointsUse the reading above. You can try again without losing points.
Work through a decision
Balance a fictional test enclosure
Consider an insulated enclosure containing a carriage and its braking mechanism. During a brief test, no matter or energy crosses the enclosure boundary. Initially the carriage has 120 of kinetic energy relative to the enclosure. It finishes with 30 joules. No potential-energy change occurs. Measurements find that 70 joules became in the brake and carriage, and 20 joules became internal energy elsewhere inside.
The final account is 30 + 70 + 20 = 120 joules. Mechanical energy decreased by 90 joules, but the complete account is unchanged. If 15 joules later radiate out through a window, the chosen enclosure system then has 15 joules less total energy. Including that outward transfer restores the balance. Conservation does not require every chosen subsystem to retain its initial energy.
Check 2 of 3
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Connect the accounting to a return from space
Hayabusa returned a sample capsule to Earth in June 2010. The accompanying photograph records luminous entry trails over Australia; it is an exposure of light received during the event, not a thermal diagram. Its streaks include activity associated with the spacecraft’s atmospheric return, and the photograph alone cannot assign every bright feature to a surviving capsule, a fragment, or a particular temperature.
A scientific energy account would combine trajectory and mass estimates with atmospheric and thermal measurements. Brightness depends on emission, distance, camera settings, and which wavelengths were recorded. The useful question is where energy was transferred, not whether a dramatic image looks powerful enough. Carefully chosen boundaries connect this return from space to the same conservation principle used for a small laboratory brake.
Hayabusa’s atmospheric return over Australia on June 13, 2010, photographed by Ed Schilling. The source is a photographic exposure containing luminous trails and clouds, preserved with its full foreground. It is not an instantaneous thermal map or an identification of every fragment.
Look closely
Locate the bright trails and the illuminated clouds. Which parts of a complete entry energy budget are not measured by the light in this photograph?
See what to notice
The photograph does not measure all changes in spacecraft and atmospheric internal energy, energy carried by material, or radiation outside the camera’s wavelength and viewing range. It also lacks the calibrated trajectory and mass data needed to estimate the mechanical-energy change.
Check 3 of 3
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Put it together
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