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VICPhysicsQuick questions
Unit 2: How does physics help us to understand the world?
Quick questions on Friction and inclined planes (VCE Physics Unit 2)
8short 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 static friction?Show answer
Opposes motion that would otherwise begin. $f_s \le \mu_s N$. The inequality reflects that static friction takes whatever value is needed to keep the object stationary, up to the threshold.
What is kinetic friction?Show answer
Acts on a moving object opposing motion. $f_k = \mu_k N$. Constant magnitude (independent of speed, to a good approximation).
What is frictionless ramp?Show answer
Net force along slope = $mg \sin\theta$. Acceleration $a = g \sin\theta$.
What is with kinetic friction?Show answer
Net force along slope = $mg \sin\theta - \mu_k mg \cos\theta$. Acceleration $a = g(\sin\theta - \mu_k \cos\theta)$.
What is using $N = mg$ on an incline?Show answer
Wrong. On an incline, $N = mg \cos\theta$ (smaller than weight).
What is treating $f_s = \mu_s N$?Show answer
Static friction equals this only at the threshold. Below the threshold it equals whatever force is opposing motion.
What is mixing up which direction friction acts?Show answer
Friction always opposes relative motion (or the tendency of motion). Identify the direction of motion first.
What is forgetting friction's direction on an incline?Show answer
A block sliding down has friction pointing up the slope.