Presentation on theme: "Physics 103: Lecture 8 Application of Newton's Laws"— Presentation transcript:
1Physics 103: Lecture 8 Application of Newton's Laws Survey resultsLeftovers from Chapter 4How to solve problems1
2Survey Results YES With 80% approval NO rating the web format is here tostay for this course.Are the preflights helpful?YESNOIs the interactive learning helpful?YESNOAre the homeworks on the web helpful?YESNOHas the course web site been problematic?
3But, all is not well! Most requests Please work out problems in class fullyWill address this to some extent in classBetter forum for that is the discussionPlease ask questionsSome requestsSlow down and explain moreMost complaintsThere is too much work for the courseHomework is too hard and takes too much timeSome complaintsWe need more problems assigned for practice!Some commentsDon’t go over pre-flights in lectureClarify the concepts (from pre-flights) in lectureThanks for taking time to answerI will try to balance the requests and address complaints
4From Lecture 1: Course Philosophy Basic Course Philosophyread about it (text)untangle it (lectures)play with it (labs)challenge yourself (homework)close the loop (discussion)Do make use of discussion, office hours, etc.Try the homework during the weekend so that youcan ask questions during the weekHomework is posted Friday and is due the following Friday.Exam worries:Will post practice exams by MondayYou can bring a single 8.5”x11” sheet with equations to exam
5Preflight 8: Question 1Consider a person standing in an elevator that is accelerating upward. The upward normal force N exerted by the elevator floor on the person isa) larger thanb) identical toc) less thanthe downward weight W of the person.Example 4.11 in the book!mgNPerson is accelerating upwards - net upwards force is non zero
6Preflight 8: Question 3You are pushing a wooden crate across the floor at constant speed. You decide to turn the crate on end, reducing by half the surface area in contact with the floor. In the new orientation, to push the same crate across the same floor with the same speed, the force that you apply must be abouta) four times as greatb) twice as greatc) equally as greatd) half as greate) one-fourth as greatas the force required before you changed the crate orientation.Frictional force does not depend on the area of contact. It depends only onthe normal force and the coefficient of friction for the contact.
7TensionTension can be transmitted around corners If there is no friction in the pulleys, T remains the sameTension is a force along the length of a medium
8Example 8: Pulley Problem I WExample 8: Pulley Problem IWhat is the tension in the string?A) T<WB) T=WC) W<T<2WD) T=2WPull withforce = WWSame answer
9Example 9: Pulley Problem II What is the tension in the string?A) T<WB) T=WC) W<T<2WD) T=2WW2Waa
10Preflight 8: Question 5In the 17th century, Otto von Guricke, a physicist in Magdeburg, fitted two hollow bronze hemispheres together and removed the air from the resulting sphere with a pump. Two eight-horse teams could not pull the halves apart even though the hemispheres fell apart when air was readmitted! Suppose von Guricke had tied both teams of horses to one side and bolted the other side to a heavy tree trunk. In this case, the tension on the hemisphere would bea) twice what it wasb) exactly what it wasc) half what it wasFhT2Fh2T
11Reminder: Procedure for Solving Problems Identify force using Free Body DiagramSet up axesWrite Fnet=ma for each axisCalculate acceleration componentsUse kinematic equationsSolve!the most crucial step!
12Problem 4.27What force does a trampoline have to apply to a 45 kg gymnast to accelerate her up at 7.5 m/s2?mgFt
13Problem 4.21A flea jumps by exerting a force of 1.2 x 10-5 N straight down on the ground. A breeze blowing on the flea parallel to ground exerts a force of 0.5 x N on the flea. Find the direction and magnitude of the acceleration of the flea if its mass is 6 x kg.mgFf
14Problem 4.33Suppose you have a 120 kg wooden crate resting on a wood floor. (a) What maximum force can you exert horizontally on the crate without moving it?What kind of friction is impeding movement?static kineticmgNFfk(b) If you continue to exert this force once the crate starts to slip what will its acceleration be?What is the net force acting on the crate?Force exertedForce exerted - Static frictionForce exerted - Kinetic friction
15Find acceleration of block =500 Example 3T=50 NM = 5 kgk = 0.2Find acceleration of block=500Similar to example 4.8 in bookHints:Draw FBDResolve T in x and y componentsNormal force is smaller than when pulling along horizontalTension component along horizontal is also smallerAnswer: 6.0 m/s2
16Find acceleration and normal force Example 6mg cos ()mg sin ()Find acceleration and normal forceChoose axis and calculate componentsAnswers: a= g sin() FN=mg cos ()
17Problems 4.59 and 4. 60 ms (ice-on-ice)=0.1 A contestant in a winter games event pushes a 45 kg block of ice across afrozen lake. Calculate the minimum force, F, he must exert to get the bockmoving. What is the force, F’, he needs to exert if he were to pull thebock with a rope over his shoulder at the same angle above the horizontal?Is F = F’?1) Yes2) No25oF cos25oF’ cos25o-F sin25oF’ sin25oF’Fms (ice-on-ice)=0.1
18Problem 4.59 and 4.60 Continued ms (ice-on-ice)=0.1 F’ 25o F cos25o -F sin25oF’ sin25oFms (ice-on-ice)=0.1NN’Case BCase Amgmg