Phy 201: General Physics INewton’s 1st LawNewton’s 2nd LawNewton’s 3rd LawFree-Body DiagramsTypes of ForcesMass vs. WeightNormal ForceExamples of Normal ForceSurface Frictional ForcesStatic FrictionKinetic FrictionTension ForceTension ApplicationsSir Isaac Newton (1642-1727)Phy 201: General Physics IChapter 4: Forces & Newton’s Laws of MotionLecture NotesNewton’s 1st Law•Also known as the “Law of Inertia” •Key Points:–When an object is moving in uniform linear motion it has no net force acting on it–When there is no net force acting on an object, it will stay at rest or maintain its constant speed in a straight line•This property of matter to maintain uniform motion is called inertia•Stated a simpler way: Nature is lazy! {i.e. matter resists changes in motion}F=0 a = 0 {v = constant}S �rrrEvery object continues in its state of rest, or of motion unless compelled to change that state by forces impressed upon it.Newton’s 2nd Law•When a net force is exerted on an object its velocity will change:•The time rate of change of motion (acceleration) is related to:–Proportional to the size of the net force–Inversely proportional to the mass of the object (i.e. its inertia)•The relationship between them isor •The direction of will always correspond to the direction of( )ˆ ˆx yF1a= = F x + F ym m�� �rr rr( )ˆ ˆx yF = ma= m a x + a y�rr...1 2 3F=F F F+ + + ��rr r r rdvdtarF�rNewton’s 3rd LawWhen an object exerts a force on a second object, the second object exerts an equal but oppositely directed force on the first objectwhere:Consequences:•Forces always occur in action-reaction pairs (never by themselves)•Each force in an action-reaction pair acts on a different objectImportant: •Newton’s 3rd law identifies the forces produced by interactions between bodies•Newton’s 2nd law defines the accelerations that each object undergoes1 on 2FrBody 1Body 22 on 1Fr2 on 1 1 on 2F F=rrFree-Body Diagrams•Simplified drawing of a body with only the forces acting on it specified•The forces are drawn as vectors•Free-Body diagrams facilitate the application of Newton’s 2nd LawExamples:JoeyWrJoey in “Free Fall”JoeyWrfloorFrJoey standing on a floorJoeyWrfloorFrJoey getting slapped while standing on a floorslapFrTypes of ForcesNon-Contact:•Gravitational•Electric•MagneticContact:•Normal•Frictional•TensionIn our world, forces can be categorized as one of 2 types:•Non-Contact: force is exerted over a distance of space with out direct contact (a.k.a. “action-at-a-distance” forces)•Contact: forces is exerted due to direct contact (Note: at the microscopic level, ALL forces are non-contact)•In either case, Newton’s 3rd law still applies to the forces presentExamples of each type of force:Mass vs. Weight•The “weight” of an object is the gravitational force exerted on it by the gravitational attraction between the object and its environment:•On the surface of the Earth, the gravitational force is referred to as weight:•Mass is a measure of an object’s inertia (measured in kg)–Independent of object location•Weight is the effect of gravity on an object’s mass (measured in N)–Determined by the local gravitational acceleration surrounding the objectˆG G G G G G GF =ma =ma y F =m a F =ma� �r rr rˆG GF = W = (-mg)y F = W = mg�r rNotes:•Mass is a measure of an object’s inertia (measured in kg)–Independent of object location•Weight is the effect of gravity on an object’s mass (measured in N)–Determined by the local gravitational acceleration surrounding the objectNormal Force•The “support” force between 2 surfaces in contact•Direction is always perpendicular (or normal) to the plane of the area of contactExample: the force of floor that supports your weightConsider standing on a scale on the floor of an elevator. The reading of the scale is equal to the normal force it exerts on you:Task: Construct free body diagrams for the scale:1. At rest2. Constant velocity3. Accelerating upward4. Accelerating downwardExamples of Normal ForceSurface Frictional Forces•When an object moves or tends to move along a surface, there is an interaction between the microscopic contact points on the 2 surfaces. This interaction results in a frictional force, that is–parallel to the surface–opposite to the direction of the motion•There are 2 types of surface friction:–Static (sticking)–Kinetic (sliding)Static Friction•Static (or sticking) friction ( ) is the frictional force exerted when the object tends to move, but the external force is not yet strong enough to actually move the object. •Increasing the applied force, the static frictional force increases as well (so the net force is zero) . The force just before breakaway is the maximum static frictional force.•The direction of the static friction force is always in opposition to the external forces(s) acting on the body•The magnitude of the maximum static friction force is:Where:– is the coefficient (maximum) of static friction–FN is the normal forcemax maxs s Nf =μ FrrsfrmaxsμKinetic Friction•Kinetic frictional force ( ) is the frictional force exerted by the surface on an object that is moving along the surface•Kinetic frictional force:–always opposes the direction of the motion–the direction is along the surface (parallel to the surface)•The magnitude of the kinetic frictional force depends only on the normal force and the properties of the 2 surfaces in contactWhere:– k is the coefficient of kinetic friction–FN is the normal forceNotes:–Kinetic friction is independent to the rate of travel of the sliding body–Kinetic friction is independent to the surface area of contactk k Nf =μ FrrkfrTension Force•Force applied through a rope or cable•When the rope or cable is mass-less (negligible compared to the bodies it is attached to) it can be treated as a connection between 2 bodies–No mass means no force needed to accelerate rope–Force of pull transfers unchanged along the rope –Action force at one end is the same as the Reaction force at the other end•When attached to a pulley the tension force can be used to change the direction of force acting on a body•Calculation of a tension force is usually an intermediate step to connecting the free-body diagrams between 2 attached objectsTension Applications•With Pulley (flat surface):•Inclined Plane:•Atwood