Physics 121

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Physics 121
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9. Equilibrium and Elasticity
9.1 Statics Forces in Equilibrium 9.2
Conditions for Equilibrium 9.3 Solving Statics
Problems 9.4 Measurement and Uncertainty 9.5
Stability and Balance 9.6 Elasticity Stress and
Strain
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Example 9.1 . . . The String

• What is the tension in the string?

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The Conditions for Equilibrium

• ? Fx 0
• ? Fy 0
• ? ? 0

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Solution 9.1 . . . The String
? ? 0 (2)(10)(L) (1)(10)(L/2) (T sin
30)(L) T 50 N
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Example 9.2 . . . The Hinge

• Are there any (reaction) forces at the hinge?

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Solution 9.2 . . . The Hinge

• All the forces on the beam are shown
• Solution continues on the next slide

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Solution 9.2 . . . The Hinge
? Fx 0 So H T cos 300 or H 50 0.87 or
H 44 N ? Fy 0 So V 50 sin 300 10 20
or V 5 N
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Elasticity

• Stress Force / Area
• Strain ? L / L
• Youngs Modulus Stress / Strain

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Example 9.3 . . . The Piano
A 1.6 m long steel piano wire has a diameter of
0.20 cm. How great is the tension in the wire if
it stretches 0.30 cm when tightened. Youngs
Modulus for steel is E 2.0 x 10 11 N / m2
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Solution 9.3 . . . The Piano
strain ? L / L 0.003/1.6 1.875 x 10 - 3
m stress F / A F/ ? r2 F /3 .1 x 10 - 6
m2 E stress / strain 2.0 x 10 11 F /(3 .1 x
10 - 6 )(1.875 x 10 - 3 ) F 1200 N
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Thats all folks!