Lect19

# Lect19 - PHYS 172 Modern Mechanics Fall 2009 Lecture 19...

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Fall 2009 PHYS 172: Modern Mechanics Lecture 19 – Angular Momentum Read 10.1–10.4

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Consider the falling block of mass m as shown in the figure. The force of gravity is mg ; there are no other forces acting on it. With respect to the point A, CLICKER QUESTION #1 Reading Question (Sections 10.1 – 10.4) A A L r p = ! r r r r ! A = r r A " r F A. There is angular momentum and a torque. B. There is no angular momentum or torque, because nothing is rotating. C. There can be angular momentum, but no torque because that would require a rigid connection to A.
Spinning Objects How can we easily relate the child’s motion with the subsequent rotation of the merry-go-round? Most convenient method is to consider angular momentum. Child runs towards and jumps onto a merry-go-round.

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The “Rotation” of a Point Particle? v r Consider a particle fixed to the edge of a rotating disk. Particle is rotating: it’s moving through some angle. Consider another particle moving with speed v to the right relative to the axis shown. This particle also moves through some angle. Δθ Δθ This particle is also rotating (about the specific point A). A v r
Angular Momentum of Point Particle L A angular momentum of point particle relative to point A. This is a vector: it has magnitude and direction. sin A A A L L r p r p ! " # = = \$ r Magnitude: This definition seems sensible, since larger values of both r and p result in faster spinning of the merry-go-round.

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