Announcements Newton s 1st Law revisited 1 Exam 1 occurred a Scores i 75th percentile ii 50th percentile median iii 25th percentile b You will get the exam back Wed or Thurs c Pick up your exams in boxes by N357 ESC in hallway by Tutorial lab d They will be distributed according to the first two numbers of CID e Solutions will be posted in glass case in hallway near room N361 ESC where HW solutions are also posted 2 If you have questions on the exam a Look over your own exam b Look over the exam solutions see if you can figure out what you got problems wrong c Only then should you come talk to me about things you still don t understand Objects will continue to move at constant velocity unless acted upon by an outside force Colton Lecture 7 9 23 08 pg 1 Colton Lecture 7 9 23 08 pg 2 Newton s 3rd Law revisited F12 F21 For every force there is an equal and opposite partner force The demo we didn t do Force sensing carts http www oberlin edu physics catalog demonstrations mech thirdlaw html Clicker quiz Two carts run into each other Each has a force sensor How do the forces compare a They are the same b It depends which cart is heavier c It depends if they bounce or stick d It depends which direction they are accelerating Colton Lecture 7 9 23 08 pg 3 Objects at rest will remain at rest objects in motion will remain in constant straight line motion unless acted upon by an outside force Demo Inertia Card and Ball Demo Inertia Hoop and Pen Demo Tablecloth jerk Demo David and Goliath ball Newton s 2nd Law revisited F ma Different types of forces Gravity weight Normal force Regular push or pull Friction Rope tension Springs Clicker quiz To solve a Newton s second law problem the first thing you should do after reading the problem is a Draw a picture b Plug the forces into the left hand side c Determine the acceleration d Solve simultaneous equations e Day dream of Hawaii Colton Lecture 7 9 23 08 pg 4 Elevator Problem Review Accelerating Reference Frames Mary has a mass of 40 kg weight 392 N and stands on an SI unit scale in the elevator Demo Rotating chair a The elevator is at rest What is the scale reading To be able to ascribe accelerations to real forces you must be observing the motion from a non accelerating constant velocity point of view Physics lingo point of view reference frame b The elevator accelerates upward at 2 m s2 What is the scale reading Amusement Park Ride Floor drops out What are the forces c After a while the elevator moves up at a constant speed of 8 m s What is the scale reading In accelerating reference frames we tend to invent fictitious forces Colton Lecture 7 9 23 08 pg 5 Colton Lecture 7 9 23 08 pg 6 Clicker quiz A car rounds a curve while maintaining a constant speed Is there a net force on the car as it rounds the curve a No because its speed is constant b No because the normal force is balanced by gravity c Yes because it s changing direction d Yes because it s slowing down Ropes and pulleys When we pull on a rope we create tension T everywhere in it T T Clicker quiz A car hits an icy spot on the road at point 1 What is the path of the car if there is no friction on the ice rope Same on both ends so no net force on the rope What direction do ropes pull Always What do pulleys do frictionless massless 1 a e T m b c d Colton Lecture 7 9 23 08 pg 7 Does tension weight of object Colton Lecture 7 9 23 08 pg 8 Moveable pulleys m Image credit wikipedia One of six simple machines Gives mechanical advantage Tension required to hold or slowly lift is lower than lifting the mass directly Solving physics problems Draw FBD of the moveable pulley or sometimes of the mass pulley together See how many T vectors are pulling upward Solve Newton s 2nd law m Problem Assume frictionless massless string and pulleys and negligible acceleration What is the tension in the string you pull Demo Mechanical advantage 8 pulley demo Colton Lecture 7 9 23 08 pg 9 Clicker quiz A 100 kg man stands on a 25 kg platform He pulls on the rope that is attached to the platform via the frictionless pulley system shown here If he holds the platform stationary with how much force is he pulling on the rope Ignore friction and assume g 10 m s2 A 750 N B 500 N C 250 N D 125 N E 100 N Colton Lecture 7 9 23 08 pg 10 Worked Problem Gilbert 100 kg is lifting the 50 kg group of boxes over a frictionless pulley while on top of a building He then steps on some frictionless ice a If we treat Gilbert and the boxes as one group what is the magnitude of the force from outside that accelerates the group b What is the acceleration of the group Hint consider object as man platform one pulley c What is the tension in the rope above the two boxes Question What is he pulls the platform up at a steady rate Demo Atwood machine with airtrack Colton Lecture 7 9 23 08 pg 11 Colton Lecture 7 9 23 08 pg 12 Worked Problem Inclined planes another of the simple machines A skiier is on a hill with no friction What is her acceleration You push with a force of 200 N on a 25 kg frictionless ice block which is on a hill sloping 30 above the horizontal What is the acceleration of the block Concepts first What force is it that accelerates her What is the acceleration for two extremes level ground infinite slope What is acceleration for a given angle Colton Lecture 7 9 23 08 pg 13 Same setup If you push with the same force but horizontally what will the acceleration be Qualitatively will it be more less or the same Colton Lecture 7 9 23 08 pg 15 Colton Lecture 7 9 23 08 pg 14