A.3.6—Mechanical energy
- Syllabus
- First assessment 2025
- Objective
- —
- Level
- SL
Identify Mechanical Energy
Mechanical energy stores
Mechanical energy is the sum of translational kinetic energy, gravitational potential energy and elastic potential energy:
Emech=Ek+Ep,g+Ep,elastic
Use the chosen system
Mechanical energy describes these stores within the system. Internal energy, chemical energy and sound may also be present in the full energy account but are not mechanical energy.
Common trap
Do not call all conserved energy mechanical energy; classify the store before applying a mechanical-energy equation.
A.3.6 Exam Analysis
[ 1 ]
show that the speed of the ball is about 4.3 ms−1.
Retrieve the A.3 Work, Energy and Power Model
Account for energy
Define the system, identify energy stores and describe transfers. Work done by a force transfers energy; total energy is conserved even when mechanical energy is not.
Use the mechanical model
E_k= rac12mv^2,\quad \Delta E_{p,g}=mg\Delta h,\quad E_{p,elastic}= rac12k(\Delta x)^2
Conserve their sum only when resistive transfers are absent or included explicitly.
Use rates and ratios
P= rac{\Delta E}{\Delta t}=Fv,\qquad \eta= rac{E_{useful}}{E_{input}}= rac{P_{useful}}{P_{input}}
Fuel energy density connects available input energy to a chosen volume.
Final checks
Check the system boundary, signs of work and potential-energy changes, the reference height, extension from natural length, and whether the quantity is energy, power, efficiency or energy density.