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QLDPhysicsQuick questions
Unit 2: Linear motion and waves
Quick questions on Momentum, impulse and conservation in collisions (QCE Physics Unit 2)
4short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is treating kinetic energy as conserved in every collision?Show answer
$KE$ is only conserved in elastic collisions. Always check.
What is dropping the sign on velocity?Show answer
In a head-on collision the bodies have opposite signs of velocity. Mixing them up gives a momentum total that is too large by a factor of two or three.
What is applying momentum conservation to a system with an external force?Show answer
If a car is being pushed by a person during the collision, the system is not isolated. For typical QCAA problems, the collision time is short enough that external forces (friction, weight) deliver negligible impulse.
What is confusing impulse with force?Show answer
Impulse has units of N s (or kg m s$^{-1}$). Force has units of N. They are equal only when multiplied or divided by $\Delta t$.
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