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# 1111111111 - PY211 Lecture 18(08 Nov 2016 Equilibrium...

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PY211 Lecture 18 (08 Nov 2016) Equilibrium Statics (cont.) [Ch. 11 Young and Freedman] Gravitation [Ch. 13 Young and Freedman]

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Class info Midterm 2: being graded now No pre-lecture today. Next one is on Fluids (Ch. 12) – Due this Thursday and next Tuesday. Discussion this week: on topics from Chs. 12, 13. • Lab: next week in SCI-B9 – Lab6: Fluids Problem Set 8: due next Friday, Nov. 20 by 5pm. – To be posted. Problems from Chapters 12, 13. – Ten problems online. Half also submitted on paper Help: SCI-121 More help: on Piazza.com
Worksheet: Leaning Ladder A 5.0 m long ladder with mass m = 12.0 kg leans against a smooth wall at a point 4.0 m above a rough floor. Quiz – How many forces are acting on the ladder? Step 1 – Sketch a free body diagram of just the ladder. – What are all the forces acting on the rod? – At which point on the rod does each force act? Step 2 – Choose a coordinate system and apply Newton's 2 nd law (twice).

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Worksheet: Leaning Ladder Step 3 – Choose an appropriate axis of rotation and apply Newton's 2 nd law for rotation. – Should we choose an axis at the top or the bottom? Step 4 – Solve for the forces shown: – The horizontal force from the wall – The horizontal force from the ground – The vertical force from the ground Follow-up – Will the ladder stay in place? F N F fr
Worksheet: Leaning Ladder Step 4 – Combine equations for sum of forces and sum of torques. F N F fr τ = F N 0 ( ) + F fr 0 ( ) F x = F fr F W = 0 F y = F N mg = 0 + mg 3 2 use lever arm! F W 4 ( ) = 0 F fr = F W F N = mg = 120 N F W = 3 8 mg = 45 N Follow-up – Will the ladder stay in place?

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