Announcements Inclined planes another of the simple machines 1 Exam 1 a Covers Ch 2 3 b Available in Testing center starting on Friday Sept 28 opening time up to Monday Oct 1 at 4 00 pm i Mon 4 pm to closing has a late fee c Sample Exam 1 is posted on the web site and Sample Exam 1 Solutions also i Don t look at the solutions until you ve worked the sample exam or you won t find out where you are weak It s easy to fool yourself d The exam is timed 3 hours max i Will likely be done by 2 hours Should probably plan on at least 1 5 hours 2 Exam Review on Thurs a Exam is computer graded we ll go over how to record answers in the review session on Thurs b The reading quiz questions will be taken from reading questions from past lectures A skiier is on a hill with no friction What is her acceleration Concept first What force is it that accelerates her Motion parallel and perpendicular to the slope What is the acceleration Two extremes level ground infinite slope a for any angle Colton Lecture 7 9 25 07 pg 1 Colton Lecture 7 9 25 07 pg 2 Worked Problem Same setup 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 If you push with the same force but horizontally what will the acceleration be hint qualitatively will it be more less or the same Colton Lecture 7 9 25 07 pg 3 Colton Lecture 7 9 25 07 pg 4 Kinetic and static friction Kinetic aka Moving Friction Q4 For the same magnitude of F is one easier to get moving Demo adjustable ramp F F a left is easier b right is easier c same Compare with the forces in breaking a rope Static Friction Tension vs pull force in rope tied to a wall Grows with sideways force to a point at slipping pt Ff N Tension T Ff N Demo Friction blocks Pull P Colton Lecture 7 9 25 07 pg 5 Friction Summary Static Ff max S N Kinetic Ff K N Colton Lecture 7 9 25 07 pg 6 Worked Problem Mary pulls a box of books with mass 25 kg to the right with a rope s 0 5 k 0 3 g 10 m s2 a First she pulls horizontally on the box with a force of 40 N The box doesn t move What is the frictional force of the floor on the box b What minimum force will she have to exert to get it moving Q5 If she pulls with force 175 N what will the acceleration be after it starts moving a 1 m s2 b 2 m s2 Colton Lecture 7 9 25 07 pg 7 c 4 m s2 d 6 m s2 e 6 m s2 Colton Lecture 7 9 25 07 pg 8 A 5000 kg truck has four tires with s 0 8 k 0 6 tire to road friction Worked Problem If Mary instead pulls the 25 kg box with force 180 N at an angle of 60 above the horizontal what will the acceleration be FBD What is the maximum stopping deceleration Strategy y direction stuff i ii iii x direction stuff i ii N2 for y What is the maximum stopping deceleration if the wheels are locked N2 for x What do anti lock brakes do Colton Lecture 7 9 25 07 pg 9 Colton Lecture 7 9 25 07 pg 10 Worked Problem Friction on slopes In the back of the open truck is a large box of mass 300 kg with s 0 4 k 0 3 Q6 For the same skis and snow as the slope angle increases the ski snow frictional force a increases b decreases c stays the same What is maximum acceleration the driver can give the truck if the box is not to slide out Problem If the kinetic coefficient of friction is find the frictional force f Colton Lecture 7 9 25 07 pg 11 Colton Lecture 7 9 25 07 pg 12 You invent a new olympic sport called pulley ski jumping If the kinetic coefficient of friction is what is the acceleration m M x direction y direction Q7 Did you discuss at least half of the discussion quiz questions today with a neighbor a Yes b No Colton Lecture 7 9 25 07 pg 13 Colton Lecture 7 9 25 07 pg 14