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QLDPhysicsQuick questions
Unit 2: Linear motion and waves
Quick questions on Newton's laws and forces (QCE Physics Unit 2)
7short 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 first law?Show answer
An object continues in a state of rest or uniform motion in a straight line unless a net external force acts on it. A net force of zero means constant velocity (which includes being at rest).
What is second law?Show answer
The net force on an object equals its mass times acceleration:
What is third law?Show answer
For every action there is an equal and opposite reaction. If body A exerts a force on body B, then body B exerts a force of equal magnitude and opposite direction on body A. The forces act on different bodies and never on the same body, which is why a third-law pair does not "cancel out" the way two opposing forces on one body do.
What is confusing action-reaction with balanced forces?Show answer
A book on a table has weight (down) and normal force (up). These are not a third-law pair because both act on the book. The third-law pair to the book's weight is the book's gravitational pull on the Earth.
What is forgetting to resolve weight on an incline?Show answer
Weight does not act along the slope unless the slope is vertical.
What is treating static friction as $\mu_s N$ at all times?Show answer
Static friction is only equal to $\mu_s N$ at the threshold of slipping. Before slipping, it equals whatever force it needs to to keep the object stationary, up to $\mu_s N$.
What is using $F = ma$ without summing all forces?Show answer
Always tally the net force first.
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