Week 6 Assignments: - MasteringPhysics:
Transcription
Week 6 Assignments: - MasteringPhysics:
Lecture 11: Newton’s 3rd Law This Week’s Announcements: * HW #4 due today * Midterm I will be as scheduled (on tuesday 9/29) Week 6 Reading: Chapter 1-5 - Giancoli Due: 9/29 Week 6 Assignments: - HW #5 Chp 4: Q11, Q22, P9, P33, P83 || Q17, Q21 P13, P55, P82 - MasteringPhysics: - Assignment #5 * Midterm class period (09/29) is the last day to turn in late HW# 1 - 4 * Midterm I is tuesday 09/29: - 20 questions (~10 MC-TF-SA / 10 Word problems) - one 3x5” card (one side) of notes - Material to be tested: - I posted a copy of an old midterm (without solutions) to give an example (just studying it will be not be enough to do well) - no problems will directly require calculus, but vectors will be required - Chp 1 Chp 2 --- 2.2-2.7 Chp 3 --- 3.1-3.4; 3.5-3.9 Chp 4 --- all Chp 5.1-5.4 --- all -all clicker Qs; and any examples worked in class - no smart devices (even for listening to music) - arrive early so that seating arrangements can be worked and still provide you most of the time - If you need OCD accommodations make sure to get them arranged with me (privately) before the exam * Midterm class period (09/29) is the last day to turn in late HW# 1 - 4 Clicker Question: 4) You are dragging luggage up a ramp (making an angle α with the horizontal). There is friction between you and the ramp, with a rather large coefficient of kinetic friction, µk = 1. The luggage is heavy (as usual you’ve over-packed), and you wish to exert the least amount of force pulling the luggage (Fp). What is the optimum direction, θ, in which to pull the luggage such that it moves up the ramp at a constant speed with the least amount of effort (force)? Fp a) Straight up (θ = 90 – α) N θ Fk b) Along ramp (θ = 0) c) Somewhere in between α mg α Vector Forces T2 T2 T1 T1 sinθ1 θ1 T1 cosθ1 θ2 Fg T2 sinθ2 T2 cosθ2 Fg X: -T1cosθ1 + T2cosθ2 = 0 T1 Fnet = 0 - Mass isn’t accelerating - Equilibrium Y: T1sinθ1 + T2sinθ2 = mg Clicker Question: 6) You are in the process of designing a suspension bridge. At one junction six cables intersect and you measure the tension forces (direction and magnitude) listed. Assuming that you wish your bridge junction to remain at rest for a long while, should you be satisfied with the following cable tensions. 50 kN a) yes b) no 20 kN 20 kN 30o 10 kN 30o 10 kN 80 kN Clicker Question: 5) What is each piece of the string’s tension, T, caused by the 1 kg mass if we now have replace the joint with a pulley? Assume the strings are vertical. a) Each 4.9 N upward b) Each 4.9 N downward c) Each 9.8 N upward T d) Each 9.8 N downward e) Tension on the left string is 9.8 N upward and the tension in the right string is 9.8 N downward mg Clicker Question: 5) What is each piece of the string’s tension, T, caused by the 1 kg mass if we now have replace the joint with a pulley? Assume the strings are vertical. a) Each 4.9 N upward b) Each 4.9 N downward c) Each 9.8 N upward T d) Each 9.8 N downward e) Tension on the left string is 9.8 N upward and the tension in the right string is 9.8 N downward mg Centripetal Force Roads designed for high-speed travel are banked to give the normal force a component towards the center of the curve. A properly designed bank will permit a car to go around a circular curve without any need for frictional forces (steering). ---- same reason applies for why airplanes bank when they turn. What angle should a road with a 300 m radius of curvature be banked for travel at 30 m/s? θ N Fnet θ X: r mg N sin θ = Fnet = mv2/r Y: N cos θ = mg Fnet = mv2/r = (mg/cos θ) sin θ v2/r = g tan(θ) Clicker Question: 7) Now lets consider what happens if we rely on friction rather than banking. Lets say Jimmy Johnson enters a turn of 250 m (~0.15 mil) radius of curvature, at 70 m/s (155 mph). If we assume a NASCAR car (+ driver) has a mass of 1500 kg and a µs of 1.0 will the car be able to track the curve without wiping out in a massive crash? a) yes b) no Clicker Question: 7) Now lets consider what happens if we rely on friction rather than banking. Lets say Jimmy Johnson enters a turn of 250 m (~0.15 mil) radius of curvature, at 70 m/s (155 mph). If we assume a NASCAR car (+ driver) has a mass of 1500 kg and a µs of 1.0 will the car be able to track the curve without wiping out in a massive crash? a) yes b) no Newton’s 3rd Law For a force there is always an equal and opposite reaction: or the forces of two bodies on each other are always equal and are directed in opposite directions. -Newton - forces between two objects always come in pairs - forces equal in magnitude - directed in opposite directions - forces act of different objects Fr-f Ff-r Newton’s 3rd Law: Examples Hero of Alexandria: Circa: time of Christ reaction - Invented first steam engine: Action - Invented syringe - Invented forerunner of motion picture - Invented the vending machine - First known use of wind power to power an instrument - Many others Newton’s 3rd Law - Newton’s 3rd law: Fup = -Fdown ame = Fup/m me mme = 100 kg a⊕ m⊕ = 6.0 x 1024 kg m⊕ = Fdown/