Mechanical energy — Science definition

Defined in 2 courses:

Where it is taught

In use

Their mechanical energy — the combination of kinetic energy (motion) and potential energy (position) — clearly shrinks over time.

Conservation of Energy

Mechanical energy includes kinetic and appropriate potential-energy stores for a selected system.

Conservation of Energy

Questions using mechanical energy

Problem 1

Mechanical energy can decrease while total energy stays accounted forMechanical energy can decrease while total energy stays accounted forFull system includes object, Earth and relevant surroundings.InitialGPE 100 JLaterGPE 30 JKE 50 JThermal 20 JMechanical energy: 100 → 80 J. Total represented energy: 100 → 100 J.
The supplied initial account has 100 J gravitational energy. The later account has 30 J gravitational, 50 J kinetic and 20 J thermal increase, giving the same 100 J total. Mechanical energy, the sum of kinetic and gravitational contributions here, has fallen to 80 J. The bar widths use one common energy scale and the particular values are illustrative.
For this unit’s illustrated case, how much mechanical energy has changed into the represented thermal contribution? Why is conservation of mechanical energy the wrong assumption for this case?
Answer and explanation

20 J. The account explicitly includes a thermal increase from interactions, so mechanical energy alone does not stay constant even though the full stated total does.

Mechanical energy is a subset of an energy account, not another name for every store. Check whether friction and other interactions transfer energy out of that subset before applying a mechanical-conservation shortcut.

20 J. The account explicitly includes a thermal increase from interactions, so mechanical energy alone does not stay constant even though the full stated total does.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 2

An illustrative rolling system starts with 80 J mechanical energy. Later it has 55 J mechanical energy, 20 J additional thermal energy in the system and 5 J transferred to surroundings. Write a complete energy account and explain the system boundary. Distinguish reduced mechanical energy from reduced total accounted energy.
Answer and explanation

55+20+5=80 J accounts for the initial amount. Mechanical energy fell by 25 J, but energy is redistributed within and across the boundary. A closed boundary for matter can still exchange energy; an isolated ideal system exchanges neither. State the boundary and include transfers before claiming energy disappeared.

55+20+5=80 J accounts for the initial amount. Mechanical energy fell by 25 J, but energy is redistributed within and across the boundary. A closed boundary for matter can still exchange energy; an isolated ideal system exchanges neither. State the boundary and include transfers before claiming energy disappeared.

Response criteria

  • Use the supplied conditions and relevant evidence.
  • Explain the scientific mechanism or calculation.
  • State a meaningful limitation or testable next step.

Problem 3

An object starts with 80 J mechanical energy and loses 15 J to thermal transfer. Find final mechanical energy. Show the reasoning, not just the conclusion.
Hints

Mass cancels in this ideal gravitational-energy calculation.

Answer and explanation

65 J under the stated accounting.

Problem 4

A system begins with 200 J mechanical energy. It ends with 120 J mechanical,50 J thermal increase and 20 J transferred outward as sound/heat. Find the unaccounted energy and a useful next check.
Answer and explanation

The accounted final/output total is 190 J, leaving 10 J. Check boundary heat transfer, deformation/storage and instrument uncertainty rather than claiming energy was destroyed.

The accounted final/output total is 190 J, leaving 10 J.

Check boundary heat transfer, deformation/storage and instrument uncertainty rather than claiming energy was destroyed.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical or environmental mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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