Physics 121 Tuesday January 22 2008 Credit Brookhaven National Laboratory NY USA http www bnl gov bnlweb history Omega minus asp Frank L H Wolfs The bubble chamber picture of the first omega minus An incoming K meson interacts with a proton in the liquid hydrogen of the bubble chamber and produces an omega minus a K0 and a K meson which all decay into other particles Neutral particles which produce no tracks in the chamber are shown by dashed lines The presence and properties of the neutral particles are established by analysis of the tracks of their charged decay products and application of the laws of conservation of mass and energy Department of Physics and Astronomy University of Rochester Physics 121 Tuesday January 22 2008 Today s topics Physics 121 website and homework Introduction to PRS Chapter 2 Describing One Dimensional Motion Position Velocity Acceleration Frank L H Wolfs Department of Physics and Astronomy University of Rochester Physics 121 Course Information Our web site provides up to date course information All course handouts are posted in pdf format on our web site The web site contains links to workshop and laboratory information The web site contains a link to our homework system and homework solutions the solutions are password protected The web site contains links to practice exams and their solutions All lecture presentations including audio recordings will be available on the web site shortly before or after class http teacher pas rochester edu phy121 Phy121HomePage html Frank L H Wolfs Department of Physics and Astronomy University of Rochester 1 Physics 121 Course Information Additional comments Homework set 0 is now available it is due on Saturday 1 26 at 8 30 am This set is to make you familiar with WeBWorK and does not count toward your final grade Make sure you verify that you can log in to WeBWorK Homework set 1 will be available later this week and is due on Saturday 2 2 at 8 30 am This set will be the first set that counts towards your final grade Frank L H Wolfs Department of Physics and Astronomy University of Rochester Introduction to the Personal Response System PRS Frank L H Wolfs Department of Physics and Astronomy University of Rochester Describing motion in one dimension There are different ways in which we can describe the motion of an object In Chapter 2 we will focus on describing motion along a straight line or one dimensional motion The direction of motion is not limited to the horizontal direction but for example can also be in the vertical direction e g free fall One dimensional motion can be simply translational but may also include rotational motion Frank L H Wolfs Department of Physics and Astronomy University of Rochester 2 Describing motion in one dimension When we limit ourselves to pure translational motion we in general can describe the motion in terms of three parameters The position x t units m The velocity v t units m s The acceleration a t units m s2 x 0 x 0 x 0 To specify the position x of an object we need to define the origin the point where x 0 m of our coordinate system Frank L H Wolfs Department of Physics and Astronomy University of Rochester Position Two terms often confused in in describing the motion of an object are Distance traveled the total distance the object moved during the motion from its starting point to its end point Note depends on the exact path followed and is always positive Displacement the change in the position of an object Note depends only on its starting point and its end point Frank L H Wolfs Department of Physics and Astronomy University of Rochester Position Do we understand position versus time graphs and do we really understand the difference between distance traveled and displacement Let s see Concept Tests 2 1 and 2 2 Frank L H Wolfs Department of Physics and Astronomy University of Rochester 3 Velocity All information about the motion of an object is in principle contained in the time dependence of its position x t Often it is useful to talk about the velocity v of the object which is defined as the ratio of the change in position x and the change in time t The velocity calculated in this manner is the average velocity over the time interval t Frank L H Wolfs Department of Physics and Astronomy University of Rochester Velocity Some remarks about velocity Velocity can be positive and negative In our choice of coordinate system a positive velocity corresponds to motion towards the right and negative velocity corresponds to motion towards the left The sign of the velocity is does not depend on the sign of the position The speed and velocity of an object are often confused The speed of an objective is the magnitude of the velocity of the object It is thus always positive Frank L H Wolfs Department of Physics and Astronomy University of Rochester Velocity When the time interval t decreases the average velocity approaches the instantaneous velocity The velocity of the object is related to the slope of the position versus time graph A positive slope correspond to a positive velocity a negative slope corresponds to a negative velocity When the slope increases the velocity increases Frank L H Wolfs Department of Physics and Astronomy University of Rochester 4 Velocity Understanding velocity Do you get it Concept Tests 2 3 2 6 and 2 7 Frank L H Wolfs Department of Physics and Astronomy University of Rochester Acceleration The acceleration of an object is defined at the ratio of the change of the velocity of an object v and the change in time t The acceleration calculated in this manner is the average acceleration over the time interval t The acceleration can be positive or negative depending on whether v 0 m s or v 0 m s Frank L H Wolfs Department of Physics and Astronomy University of Rochester Acceleration Some remarks about acceleration Acceleration can be positive and negative In our every day life we often use the term acceleration when we speed up and deceleration when we slow down This leads to the assumption that a positive acceleration implies an increase in speed while a negative acceleration corresponds to a decrease in speed THIS IS ABSOLUTELY WRONG Negative acceleration implies that v 0 m s This can be achieved in a number of different ways If v1 30 m s and v2 10 m s a reduction in speed If v1 10 m s and v2 30 m s an increase in speed Frank L H Wolfs Department of Physics and Astronomy University of Rochester 5 Constant acceleration Many important physics effects involve