Lecture06

Lecture06 - PHYSICS 149: Lecture 6 Chapter 2 2.7 Contact...

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PHYSICS 149: Lecture 6 • Chapter 2 – 2.7 Contact Forces: Normal Force and Friction – 2.8 Tension Lecture 6 Purdue University, Physics 149 1
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ILQ 1 Lecture 6 Purdue University, Physics 149 2 If the distance to the moon were halved, then the force of attraction between the earth and moon would be: A) quartered (.25 x) B) halved (.5 x) C) doubled (2 x) D) quadrupled (4 x)
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Normal (= Perpendicular) Force • The normal force is a contact force perpendicular to the contact surfaces that prevents two objects from passing through each other. • Normal force is a vector. – Direction: always perpendicular to the “contact surface” (rather than the horizon) – Magnitude: depends on the weight of the object (see different cases on next pages) • Type: contact force (not a long-range force) • Normal force is usually denoted by N . Lecture 6 3 Purdue University, Physics 149
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Normal Force • Symbol for FBD: N • Type: Contact force • Direction is normal (perpendicular) to surface • This is the normal component for the contact force between two (planar) surfaces Lecture 6 4 Purdue University, Physics 149
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What Causes Normal Force? • Atoms inside solid objects are inter-connected by molecular bonds which act like springs. • When you place an object on top of a table, the table deforms slightly. This bend is usually not visible to the eye. • The "springs" holding the atoms in the table compress or stretch exerting a force on the object on the table. Lecture 6 5 Purdue University, Physics 149
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Normal Force: Case 1 • If the table’s surface (contact surface) is horizontal, – Direction of the normal force is perpendicular to the “contact surface.” In this case, vertically upward. – Magnitude of the normal force is the book’s weight, according to Newton’s First Law of Motion. N = W (= mg) according to Σ F y = 0 for an object in equilibrium Lecture 6 6 Purdue University, Physics 149
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Normal Force: Case 2 • If the contact surface is horizontal and there is another vertical force acting on the book, – Direction of the normal force is perpendicular to the “contact surface.” In this case, vertically upward. – Magnitude of the normal force is the book’s weight plus the magnitude of the additional force, according to Newton’s First Law of Motion. N = W (= mg) + F according to Σ F y = 0 for an object in equilibrium Lecture 6 7 Purdue University, Physics 149
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Normal Force: Case 3 • If the contact surface is not horizontal (with an inclination angle θ ), – Direction of the normal force is perpendicular to the “contact surface.” In this case, it is not vertical.
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Lecture06 - PHYSICS 149: Lecture 6 Chapter 2 2.7 Contact...

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