A.3.8—Mechanical energy transformations
- Syllabus
- First assessment 2025
- Objective
- —
- Level
- SL
Transform Mechanical Energy Between Stores
Conservative transformations
When mechanical energy is conserved, energy can move between translational kinetic, gravitational potential and elastic potential stores without changing their sum.
Use the endpoints
For a car descending a frictionless track, gravitational potential energy decreases while kinetic energy increases. For a spring system, elastic potential energy can become kinetic energy and then return.
Add non-conservative transfers
If friction or drag acts, part of the mechanical energy transfers to internal energy. The endpoint equation must include that loss from the mechanical stores.
A.3.8 Exam Analysis
[ 2 ]
Show that the speed of the car at P is 1.7 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.