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2007-10-25 Lab 07 - Passive Forces and Newton's Laws

# 2007-10-25 Lab 07 - Passive Forces and Newton's Laws -...

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/ Physics 295 Introductory Laboratory I Partners Section Date LAB 7: PASSIVE FORCES AND NEWTON'S LAWS There ISno way of avoiding the tension of tightrope walking. Anonymous Circus Performer OBJECTIVES · To explore how the presence of friction forces is incorporated into Newton'sFirst and Second Laws of Motion. · To explore the interaction forces between objects and Newton's Third Law of Motion. · To explore tension forces and their mechanism. · To apply Newton's Laws of Motion to a situation with tension included. Physics Material: Newton's Laws of Motion, Special Forces. OVERVIEW In Lab 6, you "invented" an invisible force--the gravitational force. By noting that the motion of falling objects near the surface of the earth is with a constant acceleration, you concluded according to Newton's Second Law, that there must be a constant (gravitational) force acting on the object. Finding invisible forces is often hard because some of them are not active forces. Rather, they are passive forces which only arise in response to active ones. For example, forces normal (perpendicular) to a surface are elestic forces between the particles in the surface which arise in response to the active forces such as the push you exert on a wall, or the gravitational pull on a book sitting on a table.) Friction and tension forces are other examples of passive forces. The passive nature of friction is obvious when you think of an object like a block being pulled along a rough surface. There is an applied force (active) in one direction and a friction force in the @1993-94 P.Laws,D.Sokoloff,R. Thornton Supported by National ScienceFoundation and the U.S.Dept. of Education (FIPSE) Note: These materials have been modified locally for use in the U of L laboratories. Page7-1

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Page 7-2 Real Time Physics: Active Learning Laboratory V1.40--8/94 other direction which opposes the motion. If the applied force is discontinued the block will slow down to rest but it will not start moving in the opposite direction due to friction. This is because the friction force is passive and stops acting as soon as the block comes to rest. Likewise, tension forces, such as those exerted by a rope pulling on an object can only exist when there is an active force pulling on the other end of the rope. In this lab you will use Newton's Laws of Motion to "invent" friction and tension forces. Along the way you will examine the Third of Newton's Laws of Motion. INVESTIGATION 1: NEWTON'S LAWS WHEN FRICTION IS PRESENT In previous labs we have been trying hard to ignore the effects of friction and we have been concentrating on applied forces involving pushes and pulls that we can see and measure directly. The time has come to take friction into account. You can make observations by applying a force directly to your force probe mounted on a cart and comparing its acceleration when no friction is present to that when the friction pad under your cart is allowed to drag on the track. In order to make observations on the effects of friction you will need the following
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2007-10-25 Lab 07 - Passive Forces and Newton's Laws -...

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