Physics 121, Sections 9, 10, 11, and 12 Lecture 3 Announcements Lectures available on the web(short version) For over-enrollment please go to the physics office P107 aboratory sessions start next week Gotomywebsitewww.phys.uconn.edu/-rcote Syllabus t homeworks lectures, etc WebAssign gotowww.webassign.netandlogin username: first letter of first name plus last name e.g. John Fernando Lachance: lachance 》 institution: uconn > password: People Soft ID#(without the initial") Physics 121: Lecture 3, Pg 1
Physics 121: Lecture 3, Pg 1 Physics 121, Sections 9, 10, 11, and 12 Lecture 3 Announcements Lectures available on the web (short version) For over-enrollment please go to the Physics office P107 Laboratory sessions start next week Go to my web site www.phys.uconn.edu/~rcote Syllabus + homeworks + lectures, etc. WebAssign: go to www.webassign.net and log in » username: first letter of first name plus last name » e.g. John Fernando Lachance: jlachance » institution: uconn » password: PeopleSoft ID # (without the initial “0”)
Physics 121, Sections 9, 10, 11, and 12 Lecture 2 Today's Topics Homework 1: Due Friday Sept 6 @6: 00PM Ch.1:#4,10,14,19,31,and33 Ch.2:#1.23,37,and63 Chapter 2 Forces and vectors Types of forces Newton's Laws of motion Net force and vector addition Contact force and tension Chapter 3: Forces and motion along a line Position, velocity, acceleration Physics 121: Lecture 3, Pg 2
Physics 121: Lecture 3, Pg 2 Physics 121, Sections 9, 10, 11, and 12 Lecture 2 Today’s Topics: Homework 1: Due Friday Sept. 6 @ 6:00PM Ch.1: # 4, 10, 14, 19, 31, and 33. Ch.2: # 1, 23, 37, and 63. Chapter 2: Forces and vectors Types of forces Newton’s Laws of motion Net force and vector addition Contact force and tension Chapter 3: Forces and motion along a line Position, velocity, acceleration
Problem solution method Five steps: 1) Focus the Problem draw a picture- what are we asking for? 2)Describe the physics what physics ideas are applicable what are the relevant variables known and unknown 3) Plan the solution what are the relevant physics equations 4)Execute the plan solve in terms of variables solve in terms of numbers 5) Evaluate the answer are the dimensions and units correct? do the num bers make sense? Physics 121: Lecture 3, Pg 3
Physics 121: Lecture 3, Pg 3 Problem Solution Method: Five Steps: 1) Focus the Problem - draw a picture – what are we asking for? 2) Describe the physics - what physics ideas are applicable - what are the relevant variables known and unknown 3) Plan the solution - what are the relevant physics equations 4) Execute the plan - solve in terms of variables - solve in terms of numbers 5) Evaluate the answer - are the dimensions and units correct? - do the numbers make sense?
Chap 2: Forces and vectors In classical mechanics Need to study interactions between objects Described by forces We have an idea of what a force is from everyday life Physicist must be precise A force is that which causes a body to accelerate (See Newton's Second Law) A Force is a push or a pull A Force has magnitude& direction(vector) F Physics 121: Lecture 3, Pg 4
Physics 121: Lecture 3, Pg 4 Chap.2: Forces and vectors In classical mechanics Need to study interactions between objects Described by forces We have an idea of what a force is from everyday life. Physicist must be precise. A force is that which causes a body to accelerate. (See Newton’s Second Law) A Force is a push or a pull. A Force has magnitude & direction (vector). F
Fundamental Forces Example of Forces Hooke' s law for ideal spring: F=-kX Units of a force are 1 N 1 kg m/s 2 Fundamental Forces Gravity(more later) >) For motion of planets, etc Strong and weak nuclear forces(not here !) >)Explains behavior of nucleus in atoms Electromagnetic force(next semester in PHY122) >)Relevant for electric systems, chemical properties, etc Physics 121: Lecture 3, Pg 5
Physics 121: Lecture 3, Pg 5 Fundamental Forces Example of Forces Hooke’s law for ideal spring: F = -k x Units of a force are 1 N= 1 kg m/s2 Fundamental Forces Gravity (more later) »For motion of planets, etc. Strong and weak nuclear forces (not here !) »Explains behavior of nucleus in atoms Electromagnetic force (next semester in PHY122) »Relevant for electric systems, chemical properties, etc
The laws of motion Isaac Newton (1642-1727) published Principia Mathematica in 1687. In this work, he proposed three " laws"of motion Law 1: An object subject to no external forces is at rest or moves with a constant velocity if viewed from an inertial reference frame Law 2: For any object, FNET =2 F=ma Law 3: Forces occur in pairs: FAB =-FBA (For every action there is an equal and opposite reaction More in following chapters Physics 121: Lecture 3, Pg 6
Physics 121: Lecture 3, Pg 6 The Laws of Motion Isaac Newton (1642 - 1727) published Principia Mathematica in 1687. In this work, he proposed three “laws” of motion: Law 1: An object subject to no external forces is at rest or moves with a constant velocity if viewed from an inertial reference frame. Law 2: For any object, FNET = F = ma Law 3: Forces occur in pairs: FA ,B = - FB ,A (For every action there is an equal and opposite reaction.) More in following chapters
Net Force: adding vectors A Force has magnitude direction(vector) Adding forces is like adding vectors(more next chapter) The net force is obtained by adding all forces Adding collinear vectors 500N 2 vectors in the same direction 200N300N >) Magnitude is the sum of both magnitudes 100N Direction remains the same 300N 2 vectors in opposite direction 200N >)Magnitude is the absolute value of the difference of both magnitudes > Direction is the same as the longest vector Sum of 2 vectors is zero if they have opposite directions and same magnitude 500N 500N Physics 121: Lecture 3, Pg 7
Physics 121: Lecture 3, Pg 7 Net Force: adding vectors A Force has magnitude & direction (vector). Adding forces is like adding vectors (more next chapter) The net force is obtained by adding all forces Adding collinear vectors 2 vectors in the same direction »Magnitude is the sum of both magnitudes »Direction remains the same 2 vectors in opposite direction »Magnitude is the absolute value of the difference of both magnitudes »Direction is the same as the longest vector Sum of 2 vectors is zero if they have opposite directions and same magnitude 500 N 200 N 300 N 100 N 200 N 300 N 500 N 500 N
The Free Body Diagram Newton's 2nd Law says that for an object F= ma Key phrase here is for an object. So before we can apply F= ma to any given object we isolate the forces acting on this object We obtain the fbd Physics 121: Lecture 3, Pg 8
Physics 121: Lecture 3, Pg 8 The Free Body Diagram Newton’s 2nd Law says that for an object F = ma. Key phrase here is for an object. So before we can apply F = ma to any given object we isolate the forces acting on this object: We obtain the FBD
FBD: an example A mass is suspended to the ceiling with a rope The fbd of the mass is simply given by all forces on acting on it mg mg Physics 121: Lecture 3, Pg 9
Physics 121: Lecture 3, Pg 9 FBD: an example A mass is suspended to the ceiling with a rope The FBD of the mass is simply given by all forces on acting on it mg T m mg T m
Internal and External Forces Consider a system E.g. a baseball All atoms/particles inside interact with each other Atom 1 acts on atom 2 with F21 But atom 2 also acts on atom 1 with F12 Newton's 3rd law says that F12=-F21 So the net force is zero . same for all pairs of particles All interanl forces add up to zero Only external forces remains E.g., gravity or the contact of a stick We will deal with external forces mostly Physics 121: Lecture 3, Pg 10
Physics 121: Lecture 3, Pg 10 Internal and External Forces Consider a system E.g. a baseball All atoms/particles inside interact with each other Atom 1 acts on atom 2 with F21 But atom 2 also acts on atom 1 with F12 Newton’s 3rd law says that F12 = - F21 So the net force is zero … same for all pairs of particles All interanl forces add up to zero Only external forces remains E.g., gravity or the contact of a stick ! We will deal with external forces mostly
