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All 11 Flashcards — Friction (static & dynamic)
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Question
Define friction.
Answer
The force that **resists sliding** between two surfaces in contact; it always **opposes the motion** (or attempted motion).
Question
Static vs dynamic friction?
Answer
**Static** acts while the object is **still** (grows to match the push, up to μ_s R). **Dynamic** acts while it is **sliding** (a fixed μ_d R).
Question
Rule for static friction?
Answer
$F_f \le \mu_s R$ — friction can be anything up to a maximum of μ_s R.
Question
Rule for dynamic (sliding) friction?
Answer
$F_f = \mu_d R$ — a fixed value while the object moves.
Question
What is R (the normal force)?
Answer
The **support force** from the surface, perpendicular to it. On flat ground **R = mg**. Also written F_N.
Question
Minimum force to start an object moving?
Answer
**μ_s R** — you must beat the **maximum static** friction (use μ_s, not μ_d).
Question
Which is usually bigger, static or dynamic friction?
Answer
The **maximum static** friction — that's why it's harder to start something moving than to keep it moving.
Question
Why is μ dimensionless?
Answer
μ = F_f ÷ R is a **force ÷ a force**, so the newtons cancel — it has **no unit** (a pure number).
Question
Find the friction on a 10 kg box sliding on flat ground, μ_d = 0.20 (g = 9.8).
Answer
R = mg = 98 N, so F_f = μ_d R = 0.20 × 98 = 19.6 ≈ 20 N.
Question
Does friction depend on the contact area?
Answer
**No** (in this model) — it depends on μ and the normal force R, not on how big the contact patch is.
Question
Typical range of μ values?
Answer
Usually between **0 and 1** (e.g. ~0.3 for many everyday surfaces); it can exceed 1 for very grippy surfaces.
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Forces and momentum
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