Physics 111: Mechanics Lecture 3

Physics 111: MechanicsLecture 3Bin ChenNJIT Physics Department

Chapter 4 Newton’s Laws of Motionq4.1 Force and Interactionsq4.2 Newton’s First Lawq4.3 Mass and Weightq4.4 Newton’s Second Lawq4.5 Newton’s Third Lawq4.6 Free-Body DiagramsIsaac Newton’s work represents one of the greatestcontributions to science ever made by an individual.

Kinematics and DynamicsqKinematics: Describing object’s motion byanswering: When? Where? How fast? How far?How long? without asking: Why is object movingin a certain way?

Kinematics and DynamicsDynamics: Describing object’s motion byanswering: Why is the object moving in acertain way? What causes the object to changeits velocity?q Dynamics studies motion on a deeper level thankinematics: it studies the causes of changes inobjects’ motion!q

DynamicsDescribes the relationship between the motionof objects in our everyday world and the forcesacting on themq Language of DynamicsqnForce: The measure of interaction between twoobjects (pull or push). It is a vector quantity – it has amagnitude and directionnMass: The measure of how difficult it is to changeobject’s velocity (sluggishness or inertia of the object)

ForcesqqqThe measure of interactionbetween two objects (pull orpush)Vector quantity: hasmagnitude and directionMay be a contact force or afield forcennContact forces result fromphysical contact between twoobjectsField forces act betweendisconnected objectsnAlso called action at a distance

ForcesGravitational Forceq Tension Forceq Normal Forceq Friction Forceq Spring Forceq

Vector Nature of ForceVector force: has magnitude and directionq Net Force: a resultant force acting on object!! ! ! !Fnet å F F1 F2 F3 .qqYou must use the rules of vector addition toobtain the net force on an object

Newton s First LawqqqqAn object at rest tends to stay at rest and anobject in motion tends to stay in motion withthe same speed and in the same directionunless acted upon by an unbalanced forceAn object at rest remains at rest as long as no net force acts on itAn object moving with constant velocity continues to move withthe same speed and in the same direction (the same velocity) aslong as no net force acts on it“Keep on doing what it is doing”

Newton s First LawqqWhen forces are balanced, the acceleration of the objection is zeronnqAn object at rest tends to stay at rest and anobject in motion tends to stay in motion withthe same speed and in the same directionunless acted upon by an unbalanced forceObject at rest: v 0 and a 0Object in motion: v ¹ 0 and a 0The net force is defined as the vector sum of all the external forcesexerted on the object. If the net force is zero, forces are balanced. Fnet F F1 F2 F3 . 0

Mass and InertiaqqqqqqEvery object continues in its state of rest, or uniformmotion in a straight line, unless it is compelled to changethat state by unbalanced forces impressed upon itInertia is a property of objectsto resist changes in motion!Mass is a measure of theamount of inertia.Mass is a measure of the resistance of an object tochanges in its velocityMass is an inherent property of an objectScalar quantity and SI unit: kg

Newton s Second LawqThe acceleration of an object is directlyproportional to the net force acting onit and inversely proportional to its mass!"# &!( ''!"# &! '(

Units of Forceq Newton’ssecond law:!"# &! '(q SIunit of force is a Newton (N)kg - m1N 1 0s

More about Newton’s 2nd LawqYou must be certain about which body we areapplying it toq!"# must be the vector sum of all the forces that acton that bodyq Only forces that act on that body are to be includedin the vector sumq Acceleration along an axis isdetermined by the net forcecomponent along the same axis

Net Force and AccelerationqThe figure shows overhead views of four situations inwhich two forces accelerate the same block across africtionless surface. Rank the situations below accordingto the magnitude of the horizontal acceleration of theblock, greatest first.3NA.B.C.D.E.I, IV, III, IIII, I, IV, IIIIII, II, IV, II, IV, II, IIIIII, IV, I, II5N5N3NIII3N5N5N3NIIIIV

F ma F ma F ma Newton s Third LawxqxyyzIf object 1 and object 2 interact, the forceexerted by object 1 on object 2 is equal inmagnitude but opposite in direction to the(magnitudeofw mgforce exerted by object2 on object1 FA on B FB on AqEquivalent to saying a single isolated force cannot existzth(New

Newton s Third Law cont.qF12 may be called theaction force and F21 thereaction forcenqActually, either force canbe the action or thereaction forceThe action and reactionforces act on differentobjects

Action and Reaction ForceqIf a bird collides with the windshield of a fastmoving plane, which experiences an impactforce with a larger magnitude?A) The bird.B) The plane.C) The same force is experienced by both.D) Not enough information is given

Action and Reaction ForceqWhich experiences greater acceleration?A) The bird.B) The plane.C) The same acceleration is experienced byboth the bird and plane.D) Not enough information is given

Applying Newton’s Third Law I An apple rests on a table. Identify the forces thatact on it and the action-reaction pairs.[Conceptual Example 4.9 in the textbook]

Applying Newton’s Third Law II A person pulls on a block across the floor. Identifythe action-reaction pairs.[Conceptual Example 4.10 in textbook]

A paradox?nIf an object pulls back on you just as hard as you pull onit, how can it ever accelerate?

Chapter 4 Newton’s Laws of Motion q4.1 Force and Interactions q4.2 Newton’s First Law q4.3 Mass and Weight q4.4 Newton’s Second Law q4.5 Newton’s Third Law q4.6 Free-Body Diagrams Isaac Newton’s work represents one of the greatest contributions to science ever made by an individual.