Account for energy transfers, transformations and dissipation

Learning goal: Choose a system boundary, distinguish energy transfer from a change in energy form, and explain why slowing motion does not mean energy has ceased to exist.

A system is the part of the world selected for study. Its boundary can be a real container or an imagined line around several interacting objects. A skateboard alone, a skateboard plus rider, and the rider-board-Earth-track-air combination are different systems. Energy can be transferred between objects or transformed from one form into another, but it is never lost from the complete account. For example, a slowing board transfers energy to its surroundings as motion energy becomes thermal energy; the energy still exists even when the board stops. Energy can cross one boundary while remaining inside another. Write the system definition before calculating or drawing an energy account.

An open system can exchange matter and energy. A closed system prevents matter transfer but can exchange energy; an isolated system exchanges neither. Real apparatus only approximates perfect isolation. Most systems on Earth, such as an ecosystem, the atmosphere or the human body, are open systems that exchange matter and energy with their surroundings. The total energy of an isolated model system remains constant while energy changes form and location within it. If a smaller system gains or loses energy, identify a transfer across its boundary rather than assuming conservation has failed.

Gravitational potential energy is associated with interacting masses and their relative position. A raised rider-Earth system has more of this energy than the same system at a lower reference height. As a rider moves down, some can become kinetic energy of motion. On a climb, kinetic energy can become gravitational potential energy again. The reference height must remain consistent within a comparison; changing the zero mark is not a physical energy transfer.

Friction and air resistance transfer organized mechanical energy into thermal energy of the rider, board, track and air, with some energy also transmitted through sound. This spreading makes energy less available for the original useful motion. “Dissipated” does not mean destroyed. A rider can stop while the larger environment retains energy that is spread among many particles and difficult to recover for another climb.

A small energy transfer into a large surrounding system can produce a temperature change too small for a classroom thermometer to resolve. Failure to detect a change is not proof of zero transfer. If a small amount of warm water joins a much larger room-temperature sample, the final temperature may change only slightly. Keep the amount of material and the instrument's resolution in the explanation instead of equating temperature with total energy.

The diagram gives eight invented snapshots for a model with 100 total energy units. Each row has gravitational, kinetic and thermal portions: (100,0,0), (50,40,10), (0,80,20), (60,10,30), (20,40,40), (0,50,50), (40,0,60), and (0,0,100). Every row sums to 100. Thermal energy increases as mechanical energy decreases, though gravitational and kinetic shares can rise or fall as the model moves over hills.

These are explicit teaching values, not a recorded simulation run. At snapshot three, zero gravitational energy means the object is at the chosen reference height, not that gravity has disappeared. At snapshot seven, zero kinetic energy can be a momentary turning point; motion may resume downhill. At snapshot eight, all modeled energy has entered the thermal account at the reference level. The diagram does not show exactly how warm each real component becomes or account separately for sound; that transfer is included within the model's eventual thermal destination.

An independent simulation investigation should generate its own records. In an energy simulation, first examine the meaning of the displayed energy categories and total. Choose a fixed starting position and mass, then compare a negligible-friction run with one including friction while holding other settings consistent. Record several positions and times. Do not copy the illustrative eight rows as if they were observations from the software.

A graph or bar display helps test the model. At equal heights in a no-friction idealization, the total mechanical energy is unchanged and the motion pattern can repeat. With friction, later returns generally reach smaller heights unless energy is added. If a real rider pumps, pushes or is towed, energy enters from another store or system. That input must appear in an expanded account rather than being attributed to the track creating energy.

Worked snapshot: At snapshot four, 60 + 10 + 30 = 100 units. Only 70 remain in the modeled gravitational-plus-kinetic account, while 30 are thermal. Calling the missing 30 “destroyed” ignores a listed part of the system.

Worked boundary comparison: For a moving board alone, friction can transfer energy out to a track. For board plus track, the same interaction changes energy distribution internally. The physical event is unchanged; the chosen boundary determines whether the arrow is drawn crossing the system boundary.

Supported account: Construct eight energy bars from the supplied triples, verify every total and describe a possible sequence of descents, climbs and turning points. State what the model lumps together and what remains unmeasured.

Independent investigation: Use the linked simulation or a teacher-provided recorded simulation with documented settings. Compare two friction conditions, retaining mass, initial height and reference level. Record at least eight snapshots per condition and explain three transfers or transformations. Include a conclusion about dissipation and a limitation about extending a simplified simulation to a real rider and track.