# Lecture02 - PHYSICS 220 Lecture 02 Forces: Equilibrium...

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Lecture 2 Purdue University, Physics 220 1 Lecture 02 Forces: Equilibrium Examples Textbook Sections 2.5 - 2.8 PHYSICS 220

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Lecture 2 Purdue University, Physics 220 2 Overview • Last Lecture – Newton’s Laws of Motion • 1st: inertia • 2nd: F=ma – Forces: contact and non-contact forces • Today – Newton’s Third Law – Free Body Diagram – More on Normal and Frictional Forces – Spring and Tension
Lecture 2 Purdue University, Physics 220 3 Question • Compare the net force on the two books A) F physics > F biology B) F physics = F biology C) F physics < F biology Physics Biology Net force is zero on both books! The normal force from table exactly cancels downward forces.

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Lecture 2 Purdue University, Physics 220 4 Mass and Weight • Gravitational force G = 6.7 × 10 -11 Nm 2 /kg 2 • The mass of an object quantifies the amount of inertia that an object possesses – an intrinsic property of the object • Weight W = mg g = GM earth /(r earth ) 2 = 9.8 m/s 2
Lecture 2 Purdue University, Physics 220 5 For every action there is an equal an opposite reaction Forces in nature come in pairs T F mg N The object accelerates IF F > F f F F F f F f Newton’s Third Law

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Lecture 2 Purdue University, Physics 220 6 Equilibrium • Net Force: If are all the forces acting on an object • When an object is in equilibrium, the net force acting on it is zero
Lecture 2 Purdue University, Physics 220 7 Contact Forces • Normal force: a contact force acting at an interface – perpendicular to the surface of contact • Frictional force: another contact force acting at an interface – parallel to the surface of contact – resist motion of the two surfaces sliding past each other – always act opposite the direction of motion mg N F F f

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Lecture 2 Purdue University, Physics 220 8 Friction • Magnitude of frictional force is proportional to the normal force. – f
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## This note was uploaded on 09/17/2010 for the course PHYS 220 taught by Professor Chang during the Spring '09 term at Purdue University-West Lafayette.

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Lecture02 - PHYSICS 220 Lecture 02 Forces: Equilibrium...

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