Back to the full dot-point answer
VICPhysicsQuick questions
Unit 3: How do fields explain motion and electricity?
Quick questions on Newton's laws, momentum and impulse: VCE Physics Unit 3
12short 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 newton's three laws?Show answer
First law (inertia). An object continues at rest or in uniform straight-line motion unless acted on by a net external force.
What is momentum and impulse?Show answer
Momentum is a vector: $p = mv$, units kg m/s.
What is conservation of momentum?Show answer
In an isolated system (no net external force), total momentum is conserved:
What is elastic vs inelastic collisions?Show answer
In VCE, almost all real collisions are inelastic. Steel ball bearings come close to elastic; cars colliding, clay landing on a board, and bullets embedding in blocks are all inelastic.
What is first law?Show answer
An object continues at rest or in uniform straight-line motion unless acted on by a net external force.
What is second law?Show answer
The net force on an object equals its mass times its acceleration: $F_{net} = ma$. In two dimensions, resolve forces along perpendicular axes and apply $F_{net} = ma$ on each axis independently.
What is third law?Show answer
When object A exerts a force on object B, object B exerts an equal and opposite force on A. The two forces act on different objects, so they never cancel on a single free-body diagram.
What is treating Newton's third law as cancelling forces?Show answer
The action-reaction pair acts on two different objects. In a free-body diagram of one object, only one of the pair appears.
What is forgetting that momentum is a vector?Show answer
When a ball rebounds, $\Delta p$ uses signed velocities. A ball striking a wall at $8$ m/s and rebounding at $6$ m/s changes momentum by $m(8 - (-6)) = 14m$, not $2m$.
What is assuming kinetic energy is conserved?Show answer
Only momentum is automatically conserved in an isolated collision. KE is conserved only if the collision is explicitly elastic.
What is confusing impulse with force?Show answer
Impulse has units N s, force has units N. Always include $\Delta t$.
What is using speeds instead of velocity components in 2D?Show answer
Conservation of momentum is applied separately to the x and y axes.