P161Sp08Chapter_05 - Chapter 5 Forces & Motion...

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Unformatted text preview: Chapter 5 Forces & Motion Quiz2:An object falls a distance of 5.0 m from rest. How long does it take? Hint approximate g by 10.m/s2 1. 2. 3. 4. 5. 0.20s O.50s 0.75s 1.0s 1.2s 0% 0. 20 s 91% 3% O .5 0s 1% 0. 75 s 1. 0s 6% 1. 2s Force n Forces are what cause any change in the velocity of an object The net force is the vector sum of all the forces acting on an object n n Also called total force, resultant force, or unbalanced force Forces n n A force is a push or pull A force acts on an object n forces do not exist unless acting on something n n n n Something has to exert a force Forces are vectors Contact forces: e.g. billiard ball collisions long range forces: e.g. gravity, electric & magnetic fields Major Types of Forces n n Contact forces involve physical contact between two objects Field forces act through empty space n No physical contact is required Fundamental Field Forces n Gravitational force n Between two objects Between two charges Between subatomic particles Arise in certain radioactive decay processes n Electromagnetic forces n n Nuclear force n n Weak forces n Zero Net Force n When the net force is equal to zero: n n The acceleration is equal to zero The velocity is constant, not necessarily = 0 n Equilibrium occurs when the net force is equal to zero n n The object, if at rest, will remain at rest If the object is moving, it will continue to move at a constant velocity Forces - Weight n n n The gravitational pull of the earth on an object is called weight The weight vector always points down toward the center of the earth Mass and weight are two different quantities - more later on mass Forces - Spring Forces n Spring Forces Forces - Tension n n Pulling forces in strings or cables are called tension Tension forces do not push on an object Normal Forces n n Normal forces are forces that are exerted by a surface due to compressed atomic "springs" in the surface. They are reactions to forces exerted on the surface. Normal forces are always perpendicular to the surface Normal Forces -2 Static Friction Forces: fs n n Static friction is a force that keeps an object from moving on a surface The static friction force points opposite to the direction an object would move if there were no friction. Kinetic Friction Forces - fk n n The kinetic friction force opposes the motion of an object moving on a surface. The kinetic friction forces points in the direction opposite the velocity vector Drag Forces n n Drag forces are a kind of friction force of an object moving through a fluid media (gas or liquid) The Drag force is opposite to the velocity vector Thrust n Thrust is the force that propels jet planes and rockets. Electric & Magnetic Forces n n Electric and magnetic forces act over long distances like gravitational forces. These force act on charged particles like electrons or protons Rubber Band Forces Proportionality Proportionality Looking at the graph we can say a is proportional to F a F, where ,means proportional to n Mathematically we can write n n We can also write F = ma, where m is a constant. a = cF, where c is a constant In this example m = 1 c Newton's First Law n n An object at rest will stay at rest, or an object that is moving will continue to move at constant velocity if and only if the net (i.e. total) force acting on it is zero. Objects with no net force acting on them are said to be in equilibrium Newton's Second Law n The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass a = SF/ m Algebraically we can also write, SF = m a More About Newton's Second Law n SF is the net force n This is the vector sum of all the forces acting on the object n Newton's Second Law can be expressed in terms of components: n n n SFx = m ax SFy = m ay SFz = m az Units of Force Inertia and Mass n n The tendency of an object to resist any attempt to change its velocity is called inertia Mass is that property of an object that specifies how much resistance an object exhibits to changes in its velocity Mass vs. Weight n n Mass and weight are two different quantities Weight is equal to the magnitude of the gravitational force exerted on the object n Weight will vary with location Gravitational Force n n n n The gravitational force, Fg, is the force that the earth exerts on an object This force is directed toward the center of the earth Its magnitude is called the weight of the object Weight = |Fg|= mg More About Weight n Because it is dependent on g, the weight varies with location n g, and therefore the weight, is less at higher altitudes (@10km g = 9.77m/s2) n Weight is not an inherent property of the object, e.g. the weight of a given mass would be different on the moon. More About Mass n n n n n Mass is an inherent property of an object dependent on the number & type of atoms Mass is independent of the object's surroundings Mass is independent of the method used to measure it Mass is a scalar quantity The SI unit of mass is kg Inertial Frames Any reference frame (i.e. coordinate system) in which Newton's Laws are valid is an inertial frame. If a = 0 in some reference system only Generally non accelerating reference frames are inertial frames when Fnet = 0 that reference frame is an inertial frame Examples of Inertial & Non-Inertial Frames n n The a= 0 plane is an inertial system and the ball stays in place wrt the plane In the accelerating plane the ball accelerates backward with no visible force acting on it. This is a non-inertial frame. Free Body Diagrams n A free body diagram shows all the forces acting on a body n The body is represented as a point particle n Free body diagrams facilitate calculations by showing the fordes present and their directions Free Body Diagrams 1) Identify all forces acting on the object. 2) Draw a coordinate system. Use the axes defined in your pictorial representation. If those axes are tilted, for motion along an incline, then the axes of the free-body diagram should be similarly tilted. 3) Represent the object as a dot at the origin of the coordinate axes. This is the particle model. 4) Draw vectors representing each of the identified forces. Be sure to label each force vector. 5) Draw and label the net force vector. Draw this vector beside the diagram, not on the particle. Or, if appropriate, write . Then check that points in the same direction as the acceleration vector on your motion diagram. Start 2/22 An Elevator Accelerates Upward Skier Pulled up a Hill Skier Pulled up a Hill n A tow rope pulls a skier up a hill at constant velocity. Applications of Newton's Law n Assumptions n n Objects can be modeled as particles Masses of strings or ropes are negligible n When a rope attached to an object is pulling it, the magnitude of that force, T, is the tension in the rope n Interested only in the external forces acting on the object n can neglect internal reaction forces n Initially dealing with frictionless surfaces ...
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This note was uploaded on 05/19/2008 for the course PHYS 161 taught by Professor Hammer during the Spring '07 term at Maryland.

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