Static vs. Kinetic Friction: The Dresser That Won't Budge (Then Slides) | Physics

A 25.0 kg dresser sits on a wood floor (μs = 0.450, μk = 0.320). Push it with 80.0 N: does it move, and what is the friction force? How hard do you have to push to get it going? Once it's sliding, push with 120. N: what's the acceleration? Then let go at 1.20 m/s: how far does it coast before it stops? One problem, start to finish, from static friction to kinetic friction to kinematics. In this video: • Why the coefficients of friction have no units (they're dimensionless) • The free-body diagram: F_N = mg = 245 N • The maximum static friction: μs F_N = 110. N • The common mistake: static friction is NOT always μs F_N. It's ≤ μs F_N, and it matches the push (80.0 N) • Breaking free: friction drops to kinetic, μk F_N = 78.4 N, and that's why things lurch • Newton's second law while it slides: ΣF = ma gives a = 1.66 m/s² • Letting go: a = −μk g = −3.14 m/s² (the mass cancels) • Picking the kinematics equation with no time in it: v_f² = v₀² + 2ad gives d = 0.230 m Chapters 00:00 Static vs. kinetic friction 00:20 The problem 00:34 Side note: why μ has no units 01:01 Free-body diagram and maximum static friction 01:47 Pushing with 80.0 N: does it move? 02:04 The common mistake: friction isn't 110. N 02:49 How hard to push to get it going 03:04 It breaks free: kinetic friction 03:32 Pushing while it slides 03:58 Letting go 04:28 Which kinematics equation? 04:57 How far it slides 05:17 The takeaway Answers: F_N = 245 N · F_f,s,max = μs F_N = 110. N · push 80.0 N: it doesn't move, F_f,s = 80.0 N · it starts once the push passes 110. N · F_f,k = μk F_N = 78.4 N · push 120. N while sliding: a = 1.66 m/s² · let go at 1.20 m/s: a = −3.14 m/s², d = 0.230 m (23.0 cm), stops in 0.383 s New to free-body diagrams? Start here:    • How to Draw Free-Body Diagrams: 6 Examples...   Kinetic friction at constant velocity (angled pull):    • Kinetic Friction from an Angled Pull: Cons...   Narrated by Mr. Johnson · Units Matter · Animated with Manim (Community Edition) #physics #friction #staticfriction