A.3.8—Mechanical energy transformations

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.

How it is assessed

The evidence asks for speeds of a car at different points on a track using gravitational potential to kinetic-energy conversion.

Command terms

Show / Calculate

What earns marks

Choose the initial and final mechanical stores, then equate their sum when no dissipative transfer is present. Use the same mass and potential-energy reference, and state any frictionless assumption.

Watch for

Using a height change with the wrong sign or applying the conservative equation after an unmentioned frictional section.

Representative question

[Maximum number: 2]

Show that the speed of the car at P is $1.7 \mathrm{~ms}^{-1}$ .

$\begin{aligned} m g \times 0.15=\frac{1}{2} m v^{2} v=\sqrt{2 \times 9.81 \times 0.15} \text { OR } \quad 1.72\left\langle<m s^{-1} \gg \right. \end{aligned}$

Award MP1 for recognition that KE of car at P is GPE lost.

Do not award MP1 for answers based on suvat equations. MP2 can still be awarded for a correct answer.

A.3.8—Mechanical energy transformations

Transform Mechanical Energy Between Stores

A.3.8 Exam Analysis

Assessment in practice

Question 1