Chapter 5. Force and Motion - Physics & Astronomy

1 Chapter 5. Force and MotionChapter 5. Force and MotionIn this Chapter we study causes of Motion : Why does the windsurfer blast across the water in the way he does? The combined forces of the wind, water, and Copyright 2008 Pearson Education, Inc., publishing as Pearson the wind, water, and gravity accelerate him according to the principles of dynamics. Chapter Goal:To establish a connection between Force and : Force A Short Catalog of Forces Identifying Forces Chapter 5. Force and MotionChapter 5. Force and MotionCopyright 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. What Do Forces Do? Newton s Second Law Newton s First Law Free-Body DiagramsForce: Properties1. Push or Pull2. Acts on an object 3. Force is a vector4. Force is either a contact Force or long range forceCopyright 2008 Pearson Education, Inc.

2 , publishing as Pearson : PropertiesForce is a vector The net Force is the vector sum of the individual forcesCopyright 2008 Pearson Education, Inc., publishing as Pearson can we find the individual Force ? Major Forces:1. Weight gravitational Force pulls the objects down determines its directionmagnitude:wmg====rrrrrrrrm- Mass of the Copyright 2008 Pearson Education, Inc., publishing as Pearson acceleration Major Forces:1. Weight gravitational force2. Spring Force wmg====rrrrrrrrk- coefficient, which depends only on geometric parameters of the spring spFkx====Copyright 2008 Pearson Education, Inc., publishing as Pearson the spring -change in the length of the spring ||xl= = = = Major Forces:1. Weight gravitational force2. Spring Force 3. Tension Force direction is always in the direction of the ropemagnitude -usually found from the condition of equilibrium wmg====rrrrrrrrTrrrrspFkx====Copyright 2008 Pearson Education, Inc.

3 , publishing as Pearson -usually found from the condition of equilibrium Major Forces:1. Weight gravitational force2. Spring Force 3. Tension Force4. Normal Force direction is always perpendicular to the surfacemagnitude -usually found from the condition of equilibrium wmg====rrrrrrrrTrrrrspFkx====nrrrrCopyri ght 2008 Pearson Education, Inc., publishing as Pearson -usually found from the condition of equilibrium Major Forces:1. Weight gravitational force2. Spring Force 3. Tension Force4. Normal Force 5. Friction wmg====rrrrrrrrTrrrrspFkx====nrrrrkfrrrr Copyright 2008 Pearson Education, Inc., publishing as Pearson Kinetic friction opposes the Motion direction opposite the velocity vector- static friction prevent the Motion of the objectdirection opposite the direction in which the object would movesfrrrrMajor Forces:1.

4 Weight gravitational force2. Spring Force 3. Tension Forcewmg====rrrrrrrrTrrrrspFkx====Copyri ght 2008 Pearson Education, Inc., publishing as Pearson Normal Force 5. Friction - kinetic friction - static frictionnrrrrkfrrrrsfrrrrForce Identification wmg====rrrrrrrrsfrrrrnrrrrTrrrrnrrrrCopy right 2008 Pearson Education, Inc., publishing as Pearson Diagram 1) Object as a particle 2) Identify all the forces 3) Find the net Force (vector sum of all individual forces) 4) Find the acceleration of the object (second Newton s law)5)With the known acceleration find kinematics of the object Copyright 2008 Pearson Education, Inc., publishing as Pearson )With the known acceleration find kinematics of the object It was Isaac Newton (1642-1727) who realizedthe importance of Force and its connection with Three laws of Motion 1st Law: inertia 2nd Law: change in Motion 3rd Law: action and Newton s laws of MotionCopyright 2008 Pearson Education, Inc.

5 , publishing as Pearson 3rd Law: action and reaction pairsAbout NewtonIn 1665, shortly after getting a bachelor s degree at Cambridge, Newton was forced to return to his home because of the Great Plague. During the next 18 months he formulated most of his scientific discoveries: including, the development of his three laws of Motion , the Law of gravitation, the invention of the calculus, the dispersion of light, the building of a reflecting telescope, and so on. That short period was probably the most creative period in the history of man .. and it will never be repeated!Copyright 2008 Pearson Education, Inc., publishing as Pearson s book of 1687: the Principia MathematicaNewton s Second Law An object of mass subject to forces will undergo an acceleration given by netFam====rrrrrrrr12.

6 F Fr rr rr rr rmCopyright 2008 Pearson Education, Inc., publishing as Pearson n Tfam+ +++ +++ +++ ++====rrrrrrrrr rr rr rr rrrrrNewton s Second Law TrrrrnrrrrnetFam====rrrrrrrrkw n Tfam+ +++ +++ +++ ++====rrrrrrrrr rr rr rr rrrrrIt is convenient to introduce coordinate system and write the Newton s second law in terms of vector components yCopyright 2008 Pearson Education, Inc., publishing as Pearson components xNo Motion in y-direction:0ya====0ywn+=+=+=+=For Motion in x-direction:xxkmawTf=+ =+ =+ =+ Free-fall motionwmg====rrrrrrrrThen from the second Newton s law: wmgagmm============rrrrrrrrrrrrrrrrCopyr ight 2008 Pearson Education, Inc., publishing as Pearson acceleration is the same for all objects (does not depend on the mass of the object) agmm============Newton s First Law An object that is at rest will remain at rest, or an object that is moving will continue to move in a straight line with constant velocity, if and only if the net Force acting on the object is zero.

7 0a====rrrr0F====rrrrthenvelocity is Copyright 2008 Pearson Education, Inc., publishing as Pearson is constant Static equilibrium Dynamic equilibrium Inertial reference frames Inertial reference frame is the coordinate system in which Newton s laws are valid. The earth is an inertial reference frame Any other coordinate systems, which are traveling with constant Copyright 2008 Pearson Education, Inc., publishing as Pearson other coordinate systems, which are traveling with constant velocity with respect to the earth is an inertial reference frame Car traveling with constant velocity is an inertial reference frameCar traveling with acceleration is NOTan inertial reference frame (violation of Newton s law) Copyright 2008 Pearson Education, Inc., publishing as Pearson 5.

8 Summary Chapter 5. Summary Copyright 2008 Pearson Education, Inc., publishing as Pearson 5. Summary Chapter 5. Summary Copyright 2008 Pearson Education, Inc., publishing as Pearson 5. QuestionsChapter 5. QuestionsCopyright 2008 Pearson Education, Inc., publishing as Pearson 5. QuestionsChapter 5. QuestionsTwo of three forces exerted on an object are shown. The net Force points to the left. What is the missing third Force ?Copyright 2008 Pearson Education, Inc., publishing as Pearson of three forces exerted on an object are shown. The net Force points to the left. What is the missing third Force ?Copyright 2008 Pearson Education, Inc., publishing as Pearson ve just kicked a rock, and it is now sliding across the ground about 2 meters in front of you. Which of these forces act on the ball?

9 A. Gravity, acting normal Force , acting upwardCopyright 2008 Pearson Education, Inc., publishing as Pearson normal Force , acting upwardC. The Force of the kick, acting in the direction of motionD. Friction, acting opposite the direction of motionE. A, B, and D but not ve just kicked a rock, and it is now sliding across the ground about 2 meters in front of you. Which of these forces act on the ball?A. Gravity, acting normal Force , acting upwardCopyright 2008 Pearson Education, Inc., publishing as Pearson normal Force , acting upwardC. The Force of the kick, acting in the direction of motionD. Friction, acting opposite the direction of motionE. A, B, and D but not rubber bands stretched the standard distance cause an object to accelerate at2 m/s2. Suppose another object with twice the mass is pulled by four rubber bands stretched the standard length.

10 The acceleration of this second object isCopyright 2008 Pearson Education, Inc., publishing as Pearson 16 m/s2. B. 8 4 2 1 rubber bands stretched the standard distance cause an object to accelerate at 2 m/s2. Suppose another object with twice the mass is pulled by four rubber bands stretched the standard length. The acceleration of this second object isCopyright 2008 Pearson Education, Inc., publishing as Pearson 16 m/s2. B. 8 4 2 1 forces act on an object. In which direction does the object accelerate?Copyright 2008 Pearson Education, Inc., publishing as Pearson forces act on an object. In which direction does the object accelerate?Copyright 2008 Pearson Education, Inc., publishing as Pearson elevator suspended by a cable is moving upward and slowing to a stop.