notes7aa - Chapter 7: Energy and Work Alternative method...

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Chapter 7: Energy and Work Alternative method for the study of motion In many ways easier, gives additional information Kinetic energy : consider an object of mass m and speed v, we define the kinetic energy as - a scalar, not a vector - units kg m 2 /s 2 = N m = Joule (J) in S.I. (ft lb in British units and erg in CGS) - like speed, gives a measure of an object’s motion (a car and tractor-trailer may have the same v, but different K) K m E K = = 2 2 1 v
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Work : the work done on an object by an applied constant net force F which results in the object undergoing a displacement of d (or r ) - a scalar, units of N m = J - if F and d are perpendicular, W=0 - work can be negative ( φ >90°) Work-Energy Theorem : when a net external force does work on an object, there is a change in the object’s KE W = F d d = ( F cos φ ) d = F ( d cos ) F d F d i f total K K K W W = Δ = =
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2 2 1 2 2 1 v v i f total m m W = Example A crate on a incline is held in place by a rope. The
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notes7aa - Chapter 7: Energy and Work Alternative method...

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