p140w07_ct_17

# p140w07_ct_17 - Racquetball Striking a Wall Copyright:...

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Physics 140: Winter 2007 Lecture #17 March 13, 2007 Dave Winn Racquetball Striking a Wall Copyright: Loren M. Winters Mt. Etna Andrew Davidhazy

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Exam #2 • More difficult than Exam #1 • This is common for the second 140 exam • Course is graded on a straight scale with a 28/40 the cutoff between B- and C+ • If the average on midterm 3 and the final is below 70% I will consider lowering the scale. • Average of 63%
Recapping and looking forward • Newtonian mechanics – Kinematics – Newton’s laws • Momentum and Energy – Work and impulse – Conservation laws • Rotational motion – Parallels • Remainder of 140: some applications – Solids and elasticity – Gravity and Orbital Dynamics – Fluids and their dynamics – Oscillations – Coupled oscillators and waves – Sound waves and some of their phenomenology These are the real underpinnings

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Equilibrium and elasticity • Conditions for equilibrium • Applying these equations gives information about the forces acting on an object • Equilibrium doesn’t require no motion, just no changes in motion 0 0 center any = Σ = Σ τ r r F F SL F SR m b g m t g x + rotation L
Weight and center of gravity • Where does the weight, the tug of the Earth on an object, act? • Pulls on each little part straight down • We can treat it as all acting at the center of mass (= center of gravity) • If gravitational pull varies over the object CG CM • For homogeneous, symmetric objects, it is just the middle i i i cm m r m r Σ Σ = r r Bigger mass here pulls CM to this side mg T

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from one end of a 1 m measuring stick. What is the weight of the measuring stick if it is balanced by a support force at the 0.25 m mark? 1 kg
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## p140w07_ct_17 - Racquetball Striking a Wall Copyright:...

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