’ “ l” “i l” i i i i ial i i i l lAl i l li li iti i i i ii ill ii i i iLast Lecture More 2-D examples Circular motion Today Newton s Three Laws Important Concepts Remember how vectors change Remember how vectors add Separate externa and nterna forces Think carefully about what object each force acts on Important Rem nders Exam #1 s th s Fr day at 10am Mater covered s through 2-D mot on Informat on and samp e prob ems are posted on the web page. so cop es of PRS, InC ass prob ems and presentat on s des. In add on to tutor ng sess ons, rec tat ons, and off ce hours, there w be a Quest ons and Answer sess on ton ght from 7-9pm. Next Master ng Phys cs due next Monday ’ i j l i ( ii ) il ’ l ΣΣ’ i l ill i i li i l m“ ” !F = m!a!Newton s F rst Law Ob ects move at constant ve oc ty and rema n at rest f they start at rest f they are acted on by no net externa force. Constant velocity means both magnitude and direction No net force doesn t mean no force at all, just that whatever forces are present add up to zero Only forces between objects, not interna forces count This works both ways, if v is constant, then F must be zero and if F=0, then v must be constant. Newton s Second Law Everyth ng e se we w do n 8.01L s an examp e or a consequence of th s equat on. Forces add as vectors The tota force points in the direction of the acceleration The force and acceleration are related by , a property of the object itself. Basically, how much stuff is there. Note distinction between mass and weight 12Units of ForceAcceleration isUse caution not to confuse m the mass, a property of anobject, and m the meter, a unit of lengthMass is in kgSo, Force isN is the Newton, the short unit for force.ms2kg ms2= NNewton’s Third LawThe most confusing of them all!Force due to object A on object B is always exactlyequal in magnitude and always exactly opposite indirection to the force due to object B on object A.The two forces are called an “action-reaction” pairThe two forces are equal and opposite in direction but donot “add to zero and go away” in solving problemsbecause they act of different objectsIt only makes sense to add forces on a single objectIf you consider A+B as a single object, then theseforces become “internal” and do drop outInertial Reference FramesThese principles only apply for observers who arenot accelerating, a so-called “inertial” frameThere are philosophical objections to this conceptsince it’s hard to precisely define an inertial frameexcept as one where Newton’s Laws are validAlternative formulations are beyond the scope ofthis course, you don’t need to worry about itSummaryForce is related to the change in the velocity vectorThe unit of force is the Newton which equals Action-reaction pairs are an important concept insolving problems but need to be considered verycarefully, especially the fact that the two forces in thepair act on different objectsThese concepts only apply as phrased here in inertialreference frames !F = m!a!kg
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