Physics 101: Lecture 9, Pg 1 Physics 101: Lecture 9 Work and Kinetic Energy l Today’s lecture will be on Textbook Sections Exam II

Physics 101: Lecture 9, Pg 2 Furloughs / Budget

Physics 101: Lecture 9, Pg 3 Energy is Conserved l Energy is “Conserved” meaning it can not be created nor destroyed  Can change form  Can be transferred l Total Energy does not change with time. This is a BIG deal! 10

Physics 101: Lecture 9, Pg 4 Energy l Forms  Kinetic Energy Motion (Today)  Potential Energy Stored (Monday)  Heat later  Mass (E=mc 2 ) phys 102 l Units: Joules = kg m 2 / s 2 12

Physics 101: Lecture 9, Pg 5 Work: Energy Transfer due to Force l Force to lift trunk at constant speed  Case a T a – mg = 0 T = mg  Case b 2T b - mg =0 or T = ½ mg l But in case b, trunk only moves ½ distance you pull rope. l F * distance is same in both! TaTa mg TbTb TbTb 15 W = F dcos(  )

Physics 101: Lecture 9, Pg 6 Work by Constant Force l Only component of force parallel to direction of motion does work!  W = F  r cos  F  rr F W F < 0: 90<  < 180 : cos(  ) < 0 rr F W F = 0:  =90 : cos(  ) =0 rr F W F < 0: 90<  < 180 : cos(  ) < 0 rr F W F > 0: A) W>0B) W=0C) W<0 1) 2) 3) 4) Note Change in r! rr F  rr F 

Physics 101: Lecture 9, Pg 7 ACTS: Ball Toss You toss a ball in the air. What is the work done by gravity as the ball goes up? A) PositiveB) NegativeC) Zero What is the work done by gravity as the ball goes down? A) PositiveB) NegativeC) Zero 20

Physics 101: Lecture 9, Pg 8 Work by Constant Force l Example: You pull a 30 N chest 5 meters across the floor at a constant speed by applying a force of 50 N at an angle of 30 degrees. How much work is done by the 50 N force? N W = F  x cos   m) cos (30) = 217 Joules T mg N f 21

Physics 101: Lecture 9, Pg 9 Where did the energy go? l Example: You pull a 30 N chest 5 meters across the floor at a constant speed, by applying a force of 50 N at an angle of 30 degrees. l How much work did gravity do? l How much work did friction do? mg 90 rr W = F  r cos   30 x 5 cos(90)  T mg N f X-Direction:  F = ma T cos(30) – f = 0 f = T cos(30) f rr 180 W = F  r cos   50 cos(30) x 5 cos(180)  Joules 25

Physics 101: Lecture 9, Pg 10 Preflight 1 You are towing a car up a hill with constant velocity. The work done on the car by the normal force is: 1. positive 2. negative 3. zero T W FNFN V “The normal force is perpendicular to the surface so it is 0” correct 28

Physics 101: Lecture 9, Pg 11 Preflight 2 You are towing a car up a hill with constant velocity. The work done on the car by the gravitational force is: 1. positive 2. negative 3. zero “The work done by gravity is negative since it is opposite the uphill direction that the car is going in. ” W T FNFN V correct 30

Physics 101: Lecture 9, Pg 12 Preflight 3 You are towing a car up a hill with constant velocity. The work done on the car by the tension force is: 1. positive 2. negative 3. zero “The work done on the car by the tow rope is positive because it's parallel to the direction of motion” W T FNFN V correct 32

Physics 101: Lecture 9, Pg 13 Kinetic Energy: Motion l Apply constant force along x-direction to a point particle m. W = F x  x = m a x  x = ½ m (v f 2 – v 0 2 ) l Work changes ½ m v 2 l Define Kinetic Energy K = ½ m v 2 W =  K For Point Particles 35

Physics 101: Lecture 9, Pg 14 Preflight 4 You are towing a car up a hill with constant velocity. The total work done on the car by all forces is: 1. positive 2. negative 3. zero “the car is being towed up and thus, the net work done on the car should be positive.” (Not quite!) the net acceleration is zero which means the sum of the forces is zero and so the total work is also zero. W T FNFN V correct 37

Physics 101: Lecture 9, Pg 15 Example: Block w/ friction l A block is sliding on a surface with an initial speed of 5 m/s. If the coefficent of kinetic friction between the block and table is 0.4, how far does the block travel before stopping? 5 m/s mg N f x y Y direction:  F=ma N-mg = 0 N = mg Work W N = 0 W mg = 0 W f = f  x cos(180) = -  mg  x W =  K -  mg  x = ½ m (v f 2 – v 0 2 ) -  g  x = ½ (0 – v 0 2 )  g  x = ½ v 0 2  x = ½ v 0 2 /  g = 3.1 meters 44

Physics 101: Lecture 9, Pg 16 Falling Ball Example l Ball falls a distance 5 meters, What is final speed? Only force/work done by gravity  W =  KE W g = ½ m (v f 2 – v i 2 ) F g h cos(0) = ½m v f 2 mgh = ½m v f 2 V f = sqrt( 2 g h ) = 10 m/s mg 47

Physics 101: Lecture 9, Pg 17 Work by Variable Force W = F x  x  Work is area under F vs x plot  Spring F = k x »Area = ½ k x 2 =W spring Force Distance Work Force Distance F=kx Work 49

Physics 101: Lecture 9, Pg 18 Summary l Energy is Conserved l Work = transfer of energy using force  Can be positive, negative or zero  W = F d cos(  ) l Kinetic Energy (Motion)  K = ½ m v 2 l Work = Change in Kinetic Energy   W =  K 50