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**Unformatted text preview: **Lecture 15- Static Equilibrium- Elasticity, Stress and Strain Conditions for equilibrium: ⃗ F net = ⃗ τ net = Equilibrium Equilibrium A system is in equilibrium if no part of it is moving (or moving at constant rate.). and CG is at rest Parts are not moving about the CG. A very useful feature: The system should not rotate about ANY axis. We can take the torques about any point we like . Example: Board on scales A 1.00 m long board with a mass of 5.00 kg is supported at each end by a scale. A 10.0-kg mass is placed 25.0 cm from the left side. What value does each scale read? L = 1.00 m M = 10.0 kg m = 5.00 kg d = 25.0 cm A. 15.0 kg (L), 0 kg (R) B. 12.5 kg (L), 2.50 kg (R) C. 10.0 kg (L), 5.00 kg (R) D. 7.50 kg (L), 7.50 kg (R) E. 5.00 kg (L), 10.0 kg (R) d = 25.0 cm L = 1.00 m M = 10.0 kg m = 5.00 kg N 1 N 2 mg Mg L /2 2 equations, 2 unknowns N 1 N 2 − Mg − mg = F net = Let us take the torque about the left end: L N 2 − dMg − L 2 mg = net = N 1 g = M m − N 2 g On the left On the right DEMO: Plank and scales Answer C d = 25.0 cm L = 1.00 m M = 10.0 kg m = 5.00 kg N 1 N 2 mg Mg L /2 N 1 N 2 − Mg − mg = L N 2 − dMg − L 2 mg = N 2 g = M d L m 2 = 10.0kg 25.0cm 100cm 5.00kg 2 = 5.00kg = 5.00kg 10.0kg − 5.00kg = 10.0kg ACT: How to weight a turkey on a tiny kitchen scale You bought a frozen turkey and forgot how many pounds it was. All you have is a tiny kitchen scale that can weigh a maximum of 2 lb. But here’s a good trick you can use to get an estimation… turkey Light rod scale d L When d = 20 cm , the scale reads 2 lb exactly. If L = 1 m , how big is your turkey?...

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