A.2.20—Elastic and inelastic collisions

Momentum first

In an isolated collision, total linear momentum is conserved for both elastic and inelastic collisions.

Kinetic energy distinguishes them

In an elastic collision, total kinetic energy is also conserved. In an inelastic collision, some kinetic energy is transferred to internal energy, sound or deformation; in a perfectly inelastic collision the bodies move together afterward.

Use the right conservation law

Apply momentum conservation to find final velocities, then compare initial and final kinetic energy if the collision type is required.

Common trap

“Inelastic” does not mean momentum is lost. It means kinetic energy is not conserved.

How it is assessed

The evidence asks for the speed of a ship after an object joins it, requiring a shared final velocity and momentum conservation.

Command terms

Calculate / Show

What earns marks

Use momentum conservation for the final speed, especially when bodies stick together. To show that a collision is inelastic, compare initial and final total kinetic energy and identify the energy transferred to other forms.

Watch for

Using kinetic-energy conservation for a sticking collision or assigning separate final velocities after the bodies have joined.

Representative question

[Maximum number: 2]

Calculate the speed of the ship after the collision.

Ice in a still lake will usually form in a single layer on the surface.

$\begin{aligned} \frac{1}{40} m 12=\left(\frac{1}{40} m+m\right) v v=0.29\left\langle\mathrm{~m} \mathrm{~s}^{-1}\right\rangle \end{aligned}$

A.2.20—Elastic and inelastic collisions

Distinguish Elastic and Inelastic Collisions

A.2.20 Exam Analysis

Assessment in practice

Question 1