Physics 2A Lecture 8: Feb 2, 2017 Kaixuan Ni UCSD Physics
•Kinematics: – Language for describing motion •displacement, velocity, acceleration •Dynamics: –Cause of change in motion •New Concepts : Force and Mass •Newton ± s Three Laws of Motion –Obtained from first principles ! Fundamental – Based on experimental observations of motion, NOT product of some math derivation
The Force Vector • Force is a push or pull on some object – Contact or no contact (gravity, EM) • Gives quantitative description of interaction between an object(s) & its environment – i.e: pulling a box • Force is a vector: – has direction and magnitude – SI unit of magnitude is the Newton (N) – adding and decomposing force vectors
Newton ± s First Law Of Motion • Actually first noticed by Galileo in his experiments with smooth surfaces (technology!) • Later generalized by Newton: When there is no net force acting on an object, the object maintains its motion with a constant velocity ( zero acceleration ) Conversely: When an object moves with constant velocity, the net force acting on it must be zero Tendency of an object to maintain its state of rest or uniform motion is called inertia
Inertial Frame Of Reference • A frame of ref. where Newton ± s 1 st law is valid is called an inertial frame of reference • Any reference frame that moves with constant velocity w.r.t an inertial frame is also an inertial frame (Newton ± s 1 st law is valid in this ref. frame too) • Reference frame where Newton ± s 1 st law does not hold " non-inertial frames – e.g: An accelerating frame of reference
Relative Perspectives ! Consider 3 inertial frames of reference & their experiences 1 Observer A claims he is at rest B & C are moving towards him with v =v ! ! 1 Observer B claims she & C are at rest, A is moving towards him with v = -v ! ! Who is right ? !! Everyone !!
Perspective In Non-Inertial F.O.R Girl on frictionless skates initially at rest in a boxcar with frictionless floor Consider perspective of a observer stuck to boxcar as it accelerates from v=0 What does he think of girl±s motion compared to the ground? What does the girl think?
Newton ± s 2 nd Law Of Motion F ma or m inertial mass= measure of an object's resistence to F a cha in its mo tion (The "I F a m ner nge ; tia" of 1st law ) = ∝ = = ! ! ! ! ! !
At all points in circular motion, a & F are in same direction: towards the c enter ∑ ! ! Newton ± s 2 nd Law Of Motion F a ∝ ! !
Interpreting Newton ± s 2 nd Law • Vector equation: • refers to external forces exerted on the object by the environment – impossible for object to affect its own motion by applying force on itself ! • Can’t lift yourself to ceiling by pulling up you belt • Valid only in inertial frames of reference • Only true when mass m = constant – think of acceleration of a fuel tank due to a constant force when the tank is leaking fuel ?? x x y y z z F ma F ma , F ma , F ma = ⇒ = = = ∑ ∑ ∑ ∑ ! ! ! ! ! ! ! !
Mass Of An Object F Ratio is & is constant a regardless of the magnitu property o de of the f the obje applied f
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- Fall '09