PH125 Ch. 5 Notes
PH125 Ch. 5 Notes Physics 125
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This 2 page Class Notes was uploaded by Nora Salmon on Thursday January 29, 2015. The Class Notes belongs to Physics 125 at University of Alabama - Tuscaloosa taught by Prof. Andreas Piepke in Fall2015. Since its upload, it has received 186 views. For similar materials see Physics 1 w/Calculus in Physics 2 at University of Alabama - Tuscaloosa.
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Date Created: 01/29/15
Chapter 5 Force and Motion I 51 What is physics 0 Physics is a study of motion but also what causes motion I What causes acceleration A force a push or a pull quotactsquot on an object to change its velocity 52 Newtonian Mechanics o Newtonian mechanics does not apply to all situations I Very large bodies use Einstein s theory of relativity I Very small bodies quantum mechanics 53 Newton s First Law 0 A body will keep moving with constant velocity if no force acts on it I Newton s First Law states that if no force acts on a body the body s velocity cannot change that is the body cannot accelerate I The body moves with the same magnitude AND direction 54 Force 0 A force is measured by the acceleration it produces forces like acceleration are vector quantities I When two or more forces act on a body we can find their net or resultant force by adding the forces vectorially o The principle of superposition for forces states that a single force that has the magnitude and direction of the net force has the same effect on the body as all the individual forces together I This is why we can add the forces to get an accurate result 0 Inertial reference frames I Newton s first law is not true in all reference frames however it works in inertial frames or reference frames in which Newton s laws hold 55 Mass 0 The ratio of the masses of two bodies is equal to the inverse of the ratio of their accelerations when the same force is applied to both I For unknown body X we have mX 0 quot1 0 E 0 Mass is a scalar quantity it has magnitude but no direction 0 What is mass exactly I We can only describe it indirectly the mass of the body is the characteristic that relates a force on the body to the resulting acceleration 56 Newton s Second Law 0 Newton s Second Law states that the net force on a body is equal to the product of the body s mass and its acceleration gt Fnet The net force must be the vector sum of ALL the forces acting on that body Chapter 5 Force and Motion I 0 Solving 2nd law problems We draw a freebody diagram in which the only body shown is the one for which we are summing forces A system consists of one or more bodies Any force on the bodies inside the system from bodies outside the system is called an external force 57 Some Particular Forces 0 The gravitational force A gravitational force on a body is a certain type of pull that is directed toward a second body When we speak of the gravitational force of the Earth on a body we mean a force that pulls on the body directly toward the center of the Earth For this we assume the ground is an inertial frame a o 0 Weight The weight of a body is the magnitude of the net force required to prevent the body from falling freely This means that the weight W of a body is equal to the magnitude Fg of the gravitational force on the body W mg 0 Normal Force The normal force is a force that acts perpendicular to the surface that a body is pressing on For example when you stand on the oor the normal force from the oor is perpendicular to the oor If a body is pressing down on a surface and not accelerating downward we say that the normal force FN W mg 0 Friction A force that opposes motion by a bonding between a body and a surface is called friction o Tension The pull of a cord on a body pointing away from the body and along the cord is called tension Cords are usually said to be massless and unstretchable This means that the cord exists only as a connection between two bodies 58 Newton s Third Law 0 Two bodies are said the interact when they push or pull on each other Newton s Third Law states that when two bodies interact the forces on the bodies from each other are always equal in magnitude and opposite in direction The forces between two interacting bodies comprise a third law force pair