Mechanics & Simple Machines · AP Physics 1 Unit 2 (Dynamics)
Free Body Diagram & Static vs Kinetic Friction Simulator
Key exam takeaway: Static friction matches the driving force exactly until it reaches maximum static friction, dropping to kinetic friction once sliding starts.
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3D Simulation Stage● 60 FPS
Parameter Controls
N
kg
°
Live Derivations & Metrics
Actual Friction f
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Max Static Friction
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Normal Force N
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Acceleration a
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Real-Time Formula Derivation
Governing Formula:
--Substitution:
--Regime: --
Worked Examples & Exam Question Deep Dive
Worked Problem
Problem: A 100N box on a horizontal floor has μs=0.4 and μk=0.3. What is the friction when pushed with 30N? With 50N?
Step-by-Step Derivation:
- 1) Max static friction fs,max = 0.4*100 = 40N. Since 30N < 40N, the box remains static, so f = 30N. 2) 50N > 40N triggers sliding, so kinetic friction f = 0.3*100 = 30N.
Worked Problem
Problem: A 5kg block rests on a 37° incline. Find the friction and normal force. (g=10m/s², sin37°=0.6, cos37°=0.8)
Step-by-Step Derivation:
- Normal force N = mg*cos37° = 5*10*0.8 = 40N; gravity downhill component Gx = mg*sin37° = 30N. Since block is at rest, static friction f = Gx = 30N uphill.
Student Inquiry Worksheet & Practice Problems
Q1. Variant 1: A 2kg block moves uniformly under a 6N horizontal pull. Find μ. If pull increases to 10N, what is friction?
Answer Key: Uniform motion means f = 6N, μ = 6 / (2*9.8) ≈ 0.306. When pull increases to 10N, kinetic friction remains 6N.
Q2. Variant 2: Tilting an incline, the block just begins to slip at θ = 30°. Determine μs.
Answer Key: At critical slip, mg*sinθ = μs*mg*cosθ, thus μs = tan(30°) ≈ 0.577.
Q3. Variant 3: A 20N block is pressed against a vertical wall with 50N horizontal force and stays still. Find friction.
Answer Key: Vertical equilibrium requires f = G = 20N upward. The 50N push provides the normal force.
Frequently Asked Questions & Misconceptions
Does static friction always increase as push increases?
Yes, as long as the object remains static, static friction exactly equals the net applied parallel force. Once the force exceeds maximum static friction, the object begins to slip and friction drops to kinetic friction.
Why is kinetic friction coefficient typically lower than static?
Microscopically, surfaces interlock and form micro-welds at rest. Once relative motion begins, peaks shear past each other before full interlocking can occur, reducing resistance.
At what incline angle does an object slide naturally?
When tanθ > μs, the component of gravity down the slope exceeds maximum static friction. This critical angle is known as the angle of repose.
Does gripping a cup tighter increase friction?
No. In vertical equilibrium, static friction equals the weight of the cup and stays constant. Gripping tighter increases maximum static capacity, not actual friction.