康涅狄格州大学：《普通物理》课程PPT教学课件（英文版）Lecture 4 Average Velocity

Physics 121, Sections 9, 10, 11, and 12 Lecture 4 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 4, Pg 1

Physics 121: Lecture 4, Pg 1 Physics 121, Sections 9, 10, 11, and 12 Lecture 4 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 3 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 3: Forces and motion along a line Position Velocity: average instantaneous Acceleration average instantaneous Force and acceleration Motion with constant acceleration Falling objects Apparent weight Physics 121: Lecture 4, Pg 2

Physics 121: Lecture 4, Pg 2 Physics 121, Sections 9, 10, 11, and 12 Lecture 3 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 3: Forces and motion along a line Position Velocity: average & instantaneous Acceleration: average & instantaneous Force and acceleration Motion with constant acceleration Falling objects Apparent weight

Chapter 3: Position / Displacement Displacement is just change in position △X=X1-X 10 meters 15 meters Xf Joe O △x=5 meters Displacement is a vector quantity a vector quantity has both magnitude and direction a scalar quantity has only magnitude and no direction Physics 121: Lecture 4, Pg 3

Physics 121: Lecture 4, Pg 3 Chapter 3: Position / Displacement Displacement is just change in position. X = xf - xi Displacement is a vector quantity A vector quantity has both magnitude and direction A scalar quantity has only magnitude and no direction 10 meters 15 meters O xi Joe xf x = 5 meters {

Average Velocity The average velocity v during the time interval At is defined as the displacement△ k divided by△t 1 △t 孓t f Physics 121: Lecture 4, Pg 4

Physics 121: Lecture 4, Pg 4 Average Velocity The average velocity v during the time interval t is defined as the displacement x divided by t. f i f i t t x x t x v − − =   =

Instantaneous Velocity The instantaneous velocity v is defined as the limit of the average velocity as the time interval At becomes infinitesimally short ≡lim △t→>0 △△ Physics 121: Lecture 4, Pg 5

Physics 121: Lecture 4, Pg 5 Instantaneous Velocity The instantaneous velocity v is defined as the limit of the average velocity as the time interval t becomes infinitesimally short t x v t     →0 lim

Average Acceleration The average acceleration a during the time interval At, is defined as the change in velocity Av divided by the△t △t It is a vector Physics 121: Lecture 4, Pg 6

Physics 121: Lecture 4, Pg 6 Average Acceleration The average acceleration a during the time interval t, is defined as the change in velocity v divided by the t It is a vector … f i f i t t v v t v a − − =   =

Instantaneous Acceleration The instantaneous acceleration a is defined as the limit of the average acceleration as the time interva△ t goes to zero m av A→>0△t Physics 121: Lecture 4, Pg 7

Physics 121: Lecture 4, Pg 7 Instantaneous Acceleration The instantaneous acceleration a is defined as the limit of the average acceleration as the time interval t goes to zero t v a t     →0 lim

Kinematic variables Measured with respect to a reference frame (X-y axis) Measured using coordinates(having units) Many kinematic variables are vectors, which means they have a direction as well as a magnitude Vectors denoted by boldface v or arrow v Physics 121: Lecture 4, Pg 9

Physics 121: Lecture 4, Pg 9 Kinematic Variables Measured with respect to a reference frame. (x-y axis) Measured using coordinates (having units). Many kinematic variables are vectors, which means they have a direction as well as a magnitude. Vectors denoted by boldface v or arrow  v

Newton's 2nd law and acceleration It relates the net force to the acceleration net一 ∑码 ma A force is what changes the velocity of a particle No net force: no change in velocity The constant of proportionality is the mass For a given Fnet a more massive object will undergo a smaller acceleration (and vice versa) Physics 121: Lecture 4, Pg 10

Physics 121: Lecture 4, Pg 10 Newton’s 2nd law and acceleration It relates the net force to the acceleration A force is what changes the velocity of a particle No net force: no change in velocity The constant of proportionality is the mass For a given Fnet a more massive object will undergo a smaller acceleration (and vice versa)

Motion in 1 dimension In 1-D, we usually write position as X Since it's in 1-D. all we need to indicate direction is or Displacement in a time△t=t+-tis△X=x-X; X some particle's trajectory △X in 1-D △t Physics 121: Lecture 4, Pg 11

Physics 121: Lecture 4, Pg 11 Motion in 1 dimension In 1-D, we usually write position as x . Since it’s in 1-D, all we need to indicate direction is + or −. Displacement in a time t = tf - t i is x = xf - xi t x t i t f  x  t xi xf some particle’s trajectory in 1-D