Lect08 - Physics 211 Lecture 8 Todays Agenda Friction Recap...

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Physics 211: Lecture 8, Pg 1 Physics 211: Lecture 8 Physics 211: Lecture 8 Today Today ’s Agenda s Agenda z Friction Recap z Drag Forces ¾ Terminal speed ¾ A special very cool demo…. z Dynamics of many-body systems ¾ Atwood’s machine ¾ General case of two attached blocks on inclined planes ¾ Some interesting problems
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Physics 211: Lecture 8, Pg 2 Model for Surface Friction Model for Surface Friction z The direction of the frictional force vector f F is perpendicular to the normal force vector N , in the direction opposing relative motion of the two surfaces. z Kinetic (sliding): Kinetic (sliding): The magnitude of the frictional force vector is proportional to the magnitude of the normal force N . f F = µ K N It moves, but it heats up the surface it moves on! z Static: Static: The frictional force balances the net applied forces such that the object doesn’t move. The maximum possible static frictional force is proportional to N . f F µ S N and as long as this is true, then f F = f A in opposite direction It doesn’t move!
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Physics 211: Lecture 8, Pg 3 Lecture 8, Lecture 8, Act 1 Act 1 friction dynamics friction dynamics z A block of mass m , when placed on a rough inclined plane ( µ > 0 ) and given a brief push, keeps moving down the plane with constant speed . ¾ If a similar block (same µ ) of mass 2m were placed on the same incline and given a brief push, it would: (a) (a) stop (b) (b) accelerate (c) (c) move with constant speed m
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Physics 211: Lecture 8, Pg 4 Lecture 8, Lecture 8, Act 1 Act 1 Solution Solution z Since the velocity is constant, it’s just broken free from S. F. ¾ Net force down ramp is essentially zero z Draw FBD and find the total force in the x-direction F NET,X = mg sin θ − µ K mg cos θ i j mg θ N θ µ K N = ma = 0 (first case) Doubling the mass will simply double both terms…net force will still be zero! Speed will still be constant! Speed will still be constant! Increase the friction and the Increase the friction and the downhill force by the same downhill force by the same factor factor Æ nothing changes! nothing changes! mg sin θ
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Physics 211: Lecture 8, Pg 5 Friction in Fluids: Drag Forces Friction in Fluids: Drag Forces z When an object moves through a viscous medium, like air or water, the medium exerts a “ drag ”or“ retarding ”forcethat opposes the motion of the object relative to the medium.
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This note was uploaded on 02/10/2009 for the course PHYS 211 taught by Professor Tonyliss during the Spring '09 term at University of Illinois, Urbana Champaign.

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Lect08 - Physics 211 Lecture 8 Todays Agenda Friction Recap...

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