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# lec9b - Lecture 9 P112 Feb 8 2008 Agenda for today Newtons...

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Lecture 9 P112 Feb 8, 2008

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Agenda for today Newton’s 3rd law Springs
Newton’s 3rd law If body A exerts a force on body B, then body B exerts a force on body A. The two forces have the same magnitude and are opposite in direction. **The two forces are sometimes called “3rd law partners”

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Tell whether or not each of the following is a pair of 3rd Law interaction force partners : (a) A mule pulls on a plow, moving it forward; the ground pulls back on the plow. (b) A car pulls forward on a trailer; the trailer tugs back on the car. (c) You push forward on a box without moving it; the floor (friction) pushes backward on the box. (d) A motorboat's propeller pushes backward on the water; the water pushes forward on the propeller. (e) Gravity pulls down on a person sitting in a chair; the chair pushes back up on the person.
Recap: Newton’s 3 laws of motion N1: if no net force is acting, a body moves with constant velocity (may be zero) N2: if a net force acts on a body, it accelerates: N3: if body A exerts a force on B, B will exert a force on A:

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