# Lecture09 - Midterm 1 - Results Racquetball Striking a Wall...

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Midterm 1 - Results

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Potential General Physics I Mechanics Racquetball Striking a Wall Energy Physics 140 Mt. Etna Stewart Hall
Work-Energy Theorem • The work on the left-hand side of this equation is the total work done by all forces acting on a body KE W tot = • It is convenient to split this total work into two contributions: work of all conservative forces and work of all non-conservative forces acting on the body: • So far, this is just a purely formal “accounting” trick KE W W cons non cons = + -

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Conservative Forces • If work done by a force does not depend on the path taken between an initial and final points, this force is called conservative f • Alternatively, a force is conservative if its work over any closed path is equal to zero i W = W = W i W = 0
Conservative Forces • Examples of conservative forces: – Gravity – Elastic force – Electrostatic force • All other forces (forces that are not conservative) are called non-conservative • Examples of non-conservative forces: – Kinetic friction – Fluid resistance – Forces produced by a muscle

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Work Done by Gravity • Work of gravity: • But d sin θ = h o, rav gh d h ( 29 sin 90 cos mgd mgd d g m W grav - = + ° = = r r • So, W grav = - mgh • In the end, the total work is the same as for just going up vertically: - mgh • The same is true for any path: work of gravity depends only on height difference mg h mg
Work Done by Gravity • When the direction of motion changes, total work is summed over all little elements of the path: mg d s y • Calculate the dot product for each piece and sum: = s d g m W grav r r ( 29 mgdy mgdy j dy i dx j mg s d g m - = - = + - = 0 ˆ ˆ ˆ r r x

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## This note was uploaded on 02/15/2012 for the course PHYS 140 taught by Professor Uher during the Winter '10 term at University of Michigan.

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Lecture09 - Midterm 1 - Results Racquetball Striking a Wall...

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