# Week6b - Centre of Percussion Where would we apply a force...

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Physics Demonstrations in Mechanics II 11. Centre of Percussion Centre of Percussion Where would we apply a force so there is no reaction force at the pivot? We have pure rotation about the pivot point so α = a cm /(L/2) if the rod is of length L. Similar to above then F = ma cm and τ p = I p α. Except we have taken the moment of inertia (I) and the torque ( τ ) around the pivot. F p F 0 x F p F 0 x F p = 0 if x = (2/3)L See also Young & Freedman Problem 10.98

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http://www.egr.msu.edu/classes/me361/feeny/ChimneyGlasgow.gif Physics Demonstrations in Mechanics II 10. Free-Fall Paradox a can be > g !! Angular momentum Angular momentum is conserved in the same way as linear momentum. If the net external torque acting on a system is zero , the total angular momentum of the system is constant. Angular momentum L = r x p . For a system of particles that rotate about a symmetry axis L =I ω . How is angular momentum conserved when a ice skater pulls her arms in towards her body?
A spinning figure skater pulls his arms in as he rotates on the ice. As he pulls his arms in, (a) his angular momentum increases (b) his angular momentum remains the same (c) his angular momentum decreases (d) answer depends on how far he pulls his arms in (e) none of the above www.votapedia.com (a)02 81161820 (b)02 81161821 (c)02 81161822 (d)02 81161823 (e)02 81161824 Physics Demonstrations in Mechanics II 9. Conservation of Angular Momentum Under some circumstances a star can collapse into an extremely dense object made mostly of neutrons and called a neutron star . The density of a neutron star is roughly 10 14 times as great as that of ordinary solid matter. Suppose we represent the star as a uniform solid sphere, both before and after the collapse. The star’s initial radius was 7.0 10 5 km (comparable to our sun); its final radius is 16 km. If the original star rotated once in 30 days find the angular speed of the neutron star. (The moment of inertia of a sphere of mass m and radius r is 2 / 5 mr 2 .) Picture the problem The star’s angular momentum L must stay the same before and after the collapse. So, L star = L neutron star (1) However, L = I ω , where I is the rotational inertia and ω is the angular velocity. So, I star ω star = I neutron star ω neutron star (2) For a uniform sphere,

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## This note was uploaded on 08/20/2010 for the course PHYS 141 taught by Professor Xxx during the One '09 term at University of Wollongong, Australia.

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Week6b - Centre of Percussion Where would we apply a force...

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