Worked example
A moving sled has 32 J of mechanical energy before it crosses a rough patch and 21 J after. If the sled, ground and nearby air exchange negligible energy with their surroundings, the remaining 11 J goes to other forms within this system, mainly thermal energy in this simplified model.
Contrast and common error
In an ideal frictionless track with negligible air resistance, a cart at the same height and mass has the same speed if the total mechanical energy is unchanged. A different comparison concerns time: two devices may do the same work, but the one that does it sooner has more average useful power.
Transfer check
A toy lift transfers 60 J of useful work in 3 seconds. Another delivers the same work in 5 seconds. Compare average useful power. Check: 20 W versus 12 W. This is a rate difference, without information about input energy or efficiency.
Try it, then explain it.
Complete a short set without looking at the example. After each answer, explain which evidence or rule supports your decision and why the alternatives do not fit.
Sources & authorship
Original OpenExam explanations and examples. References support the concepts; they are not the source of copied questions. Independent educator review is pending.
OpenStax Physics 9.2 Mechanical Energy and Conservation of EnergyOpenStax Physics 9.1 Work Power and the Work–Energy Theorem