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