Three Laws of Newton

  ’ “l” “il” i iii ial ii illAlillili itiiiiiiill iii iiLast Lecture More 2-D examples Circular motion Today Newtons Three Laws Important Concepts Remember how vectors change Remember how vectors add Separate externaand nternaforces Think carefully about what object each force acts on Important Remnders Exam #1 s ths Frday at 10am Matercovered s through 2-D moton Informaton and sampe probems are posted on the web page. so copes of PRS, InCass probems and presentaton sdes. In addon to tutorng sessons, rectatons, and offce hours, there wbe a Questons and Answer sesson tonght from 7-9pm. Next Masterng Physcs due next Monday ’i jli(ii) il ’l ΣΣ’  ilill iiliil m“”  !F=m!a!Newtons Frst Law Obects move at constant veocty and reman at rest f they start at restf they are acted on by no net externaforce. Constant velocity means both magnitude and direction No net force doesnt mean no force at all, just that whatever forces are present add up to zero Only forces between objects, not internaforces count This works both ways, if v is constant, then F must be zero and if F=0, then v must be constant. Newtons Second Law Everythng ese we wdo n 8.01L s an exampe or a consequence of ths equaton. Forces add as vectors The totaforce points in the direction of the acceleration The force and acceleration are related by , a property of the object itself. Basically, how much stuffis there. Note distinction between mass and weight 12 Units of Force Acceleration is Use caution not to confuse m the mass, a property of an object, and m the meter, a unit of length Mass is in kg So, Force is N is the Newton, the short unit for force. ms2 kg m s2 = N Newton’s Third Law The most confusing of them all! Force due to object A on object B is always exactly equal in magnitude and always exactly opposite in direction to the force due to object B on object A. The two forces are called an “action-reaction” pair The two forces are equal and opposite in direction but do not “add to zero and go away” in solving problems because they act of different objects It only makes sense to add forces on a single object If you consider A+B as a single object, then these forces become “internal” and do drop out Inertial Reference Frames These principles only apply for observers who are not accelerating, a so-called “inertial” frame There are philosophical objections to this concept since it’s hard to precisely define an inertial frame except as one where Newton’s Laws are valid Alternative formulations are beyond the scope of this course, you don’t need to worry about it Summary Force is related to the change in the velocity vector The unit of force is the Newton which equals  Action-reaction pairs are an important concept in solving problems but need to be considered very carefully, especially the fact that the two forces in the pair act on different objects These concepts only apply as phrased here in inertial reference frames ! F = m!a ! kg m s2