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Transcript
Chapter 3 Section 1 Newton’s Second Law Newton’s First Law Law of Inertia (first stated by Galileo) • An object at rest will remain at rest unless it experiences a net external force. An object in motion will continue in motion at the same speed and in the same straight-line direction unless it experiences a net external force. • Corollary: If either the magnitude or the direction of the velocity of an object changes, someone or something has exerted a net force on the object. Newton’s 1st Law Law of Inertia • The motion of an object changes if and only if an unbalanced force acts on the object. Newton’s Second Law of Motion • The acceleration of an object is directly proportional to the net resultant force acting on the object and is inversely proportional to the mass of the object. The direction of the acceleration is in the direction of the net resultant force F aα m F=ma Acceleration and Force • For a given object (mass) the greater the force, the greater the acceleration. Acceleration and Mass • For a given force, the greater the mass, the smaller the acceleration. Units • Base Units – length – mass – time meter m kilogram kg second s • Derived Units – velocity = distance/time m/s – acceleration= ∆v/∆t = m/s/s – Force= ma = kg m/s2 Newton in honor of Isaac Newton Newton • 1 Newton (1N) is the force needed to accelerate a 1 kilogram (kg) mass at 1 m/s/s. September 23, 2011 • Read Page 68 and 69 • Page 69 Practice Problems 1,2,3,4 Page 69 Practice Problems 1-4 • You push a wagon that has a mass of 8 kg. If the net force on the wagon is 4 N, what is the wagon’s acceleration • Given • Asked • Formula • Given: • Asked: • Formula: • Substitute: • Answer: Problems September 24 • Read the last paragraph on page 70 and first paragraph on page 71 • Homework Assignment Page 74: 7-8 Schedule • • • • • • • Monday 9/23 Page 68-69 Newton’s 2nd Law – Page 69:1-4 Tuesday 9/24 Friction – P74: AM 7,8 Wednesday 9/25 Air Resistance Thursday 9/26 Friction Lab Friday 9/27 Gravity Monday 9/30 Projectile Motion Tuesday 10/1 Circular Motion, Centripetal Force – • • Wednesday 10/2 Thursday Section 3 Newton’s 3rd Law Thursday 10/3 Momentum – • Page 86:1-4 Friday 10/4 Conservation of Momentum – • • • Page 82: 6,7,8 Practice Problems Monday 10/1 NTWS Sections 1-2, DRCM 1 Tuesday 10/2 Chapter Review 1-14 and 24,25,28 Wednesday 10/3 Test Section 1-2 Chapter 2 Friction Cause- Microwelds (Figure 4 page 71) Friction Box Not Moving Applied Force Static Friction Friction Box Moving Applied Force Kinetic Friction Friction Rolling Static Friction Friction Sliding Kinetic Friction • Static Friction- object not moving • Kinetic friction- object sliding • Rotational –rolling- static friction –sliding- kinetic friction • Static Friction > Kinetic Friction Practical Application • Driving a car • Wheels rolling – Static Friction • Wheels sliding – Kinetic Friction • Static Friction > Kinetic Friction • Anti Lock Brakes (ABS)- prevents slamming on the brakes and locking the wheels Assignment Page 74: 7-8 • Given: • Asked: • Formula: • Substitute • Answer with Units: Air Resistance • Without air resistance, all objects would fall with the same acceleration. • The acceleration of gravity • g =- 9.8 m/s2 • Feather and ball - see page 73 Air resistance Air Resistance • • • • The amount of air resistance depends on the Speed Size Shape of the object. • The faster an object falls the greater the air resistance. Air Resistance (cont) • An object falling from an air plane starts with zero velocity but it is accelerating at -9.8 m/s2 • The object keeps falling faster and faster but the air resistance increases the faster it falls. Terminal Velocity • Finally the force of air resistance becomes equal to the force of gravity and the object no longer accelerates. • The velocity stays the same. • The object does not stop and float. • Terminal Velocity Review • • • • Newton’s 1st Law – law of inertia Newton’s 2nd Law - F= ma Unit of Force: kg m/s2 called a newton Friction – Static, Kinetic (sliding), Rolling – Static > kinetic • Air Resistance – Size, Shape, Speed Chapter 3 Section 2 Gravity Page 75-82 Gravity • A fundamental characteristic of all matter. • Einstein would say it is a fundamental characteristic of space and time. • Anything that has mass is attracted by the force of gravity. • Anything that has mass attracts other things that have mass. • Newton found the formula for finding the force of gravity. FG = G m1m2 d2 G =6.67 X 10 -11N m2/kg2 Gravity Fundamental Forces • Electromagnetic force – Attracts or repels • Strong Nuclear force • Weak Nuclear force • Nuclear forces only over very short distances. • Gravitational Force – Always attraction – Long distance Earth’s Gravity The earth exerts a force of attraction on every object. At the same time that object exerts a force of attraction on the earth. Fg = m g g = -9.8 m/s2 W = m g = m *-9.8 m/s2 W = m g = kg*m/s2 = newtons Weight Varies Weight vs Mass • Weight and mass are not the same • Mass is a measure of the inertia of an object, how much the object resists a change in velocity. • Mass is a measure of the amount of substance • Weight is a force • Weight is the force of attraction between an object and the earth Weightlessness • If you are in an elevator that is in free fall (being accelerated toward the ground with the acceleration of g) then you would be weightless, but not massless. • See Figure 15, Page 79 • Elevators don’t free-fall ! Projectile motion • Projectile- anything that is thrown or shot through the air. • Projectile- an object moving in the horizontal direction at a constant velocity and under the influence of gravity in the vertical direction. • The object (projectile) follows a curved path, in fact the curve has a name- parabolic motion. Projectile Motion Page 80 Projectile Motion Parabolic Motion Time Constant horizontal velocity Accelerating vertical velocity Both balls hit ground at same time! Tuesday October 1 • Read Page 81 and 82 Motion in a Circle (p81) Velocity is constantly changing Changing Velocity • • • • • Velocity is constantly changing Changing velocity = acceleration Acceleration toward the center of the circle Acceleration means Force Force toward the center of the circle Motion in a Circle Centripetal Force/Acceleration v1 Fcenter v2 2 mV Fc = r Motion in a Circle Centrifugal Force/Acceleration v1 Fcenter Foutward v2 Centrifugal Force is a “made up” force that is really the inertia of the object. Centrifugal Force • Fugitive- some one fleeing the law • Centrifugal Force – the force fleeing the center • Experience going around a curve in a car“thrown” to the outside. • Centrifugal force is not a real force but is inertia of the body trying to stay moving in a straight line while the car is turning. Source of Centripetal Force • What provides the force that causes the moon to go around a curve rather than following inertia to keep going in a straight line? Source of Centripetal Force Gravity Source of Centripetal Force • What provides the force that causes a car to go around a curve rather than following inertia to keep going in a straight line? Chapter 3 Assignments • • • • • • Page 69: 1-4 Page 74: 7,8 Page 82: 6-8 Lab/Demonstration - Friday Note Taking Worksheet - Turn in Monday Chapter Review (Page94-95) 1-14, 24-28 Tuesday • Test- Wednesday Chapter 3 Section 3 Newton’s 3rd Law of Motion Page 83-88 Newton’s 3rd Law • When one object exerts a force on a second object, the second object exerts a force on the first object that is equal in strength and opposite in direction. • For every action force there is an equal and opposite reaction force. • The ladder law- if you jump off of a ladder the ladder exerts a force that moves you but you also exert a force on the ladder that may move the ladder. Examples of 3rd Law • When you push on a wall, the wall pushes back on you. • A skater exerts a force on a second skater and the second skater exerts an equal force on the first skater. • When you swim, you push the water back but the water pushes the swimmer forward. • A rocket engine exerts a force that pushes hot gases out the back of the rocket, but the hot gases exert a force that propels the rocket forward. Why don’t these opposite forces cancel each other? • Because the action force acts on one object and the reaction force acts on another object. – Force on wall - force on you – Force on you - force on ladder – Force on water - force on swimmer – Force on #1 skater - force on #2 skater – Force on gases - force on rocket Momentum (p86) • The Momentum is a property of a moving object that is related to the force that is needed to change the motion of the object. • Momentum is the product of the mass and the velocity of an object. • Momentum is given the symbol “p” • p (momentum) = m (mass) * v (velocity) Momentum and Force p= mv F= ma v f – vi ∆v a= = ∆t ∆t F = m v f – vi ∆t F = m ( vf – vi )/ ∆t F ∆t = m ( vf – vi ) Impulse = Change in Momentum Practice Problems Page 86 Assignment 1-4 • At the end of a race, a sprinter with a mass of 80.0 kg has a speed of 10.0 m/s. What is the sprinters momentum. • P = m*v • #1 asks for momentum • #2 asks for speed • #3 asks for mass Problems • Given: • Asked: • Formula: • Substitute: • Answer (with units): Units: Conservation of Momentum • If there is no outside force acting on an object or a group of objects then F= O so the change in momentum is zero. The momentum before the event is the same as the momentum after the even F ∆t = m vf – mvi F ∆t =0 and m vf – mvi = 0 m vf = mvi Examples of conservation of Momentum • A pool ball moving at a velocity strikes a group of pool balls at rest. Momentum is transferred from the moving ball to the balls at rest but after the collision, the total momentum must be the same. • An ice hockey puck moves across the ice and strikes a second puck. The first puck stops but the second puck moves away with the same momentum that the first puck had before the collision. • • • • • Review Newton’s 1st Law – law of inertia Newton’s 2nd Law - F= ma Newton’s 3rd Law- Action/Reaction Unit of Force: kg m/s2 called a newton Friction – Static, Kinetic (sliding), Rolling – Static > kinetic • Air Resistance – Size, Shape, Speed • Anything that has mass attracts other things that have mass. FG = G m1m2 d2 G =6.67 X 10 -11N m2/kg2 Fg = m g W = m g = kg*m/s2 = newtons Weight vs Mass • Weight and mass are not the same • Mass is a measure of the amount of substance • Weight is the force of attraction between an object and the earth Dear TJCA Parents, grades 7-12: This is an important reminder that any student who has a grade below 70 on midquarter or end of quarter report cards in a core subject area MUST attend the required after school tutoring from 3-4PM on the designated days for the respective courses, listed below, until the end of the next grading period. PLEASE note that students who do attend mandatory tutoring will be placed in In School Suspension the following day. We cannot stress enough how important it is to attend these tutoring sessions to help students to bring these grades up. Mondays: English, History (if failing both- teachers will rotate the schedule) Tuesdays: Science Thursdays: Math/computer Fridays: Latin, logic, foreign language We are all making sacrifices to help assure your child's success, so please make sure your child attends these mandatory sessions. • • • • • • • • Chapter 3 Assignments Page 69: 1-4 Page 74: 7-8 Page 82: 6-8 Page 86: 1-4 Note Taking Worksheet/Reinforcement Sheet Turn in Thursday Chapter Review (Page94-95) 1-14, 24-28 Test – Problem 24 • Find your mass if a scale on Earth reads 650N. • Given: • Asked: • Formula: • Substitute: • Answer (with units) Problem 25 • You weigh yourself at the top of a high mountain and the scale reads 720N. If your mass is 75 kg, what is the acceleration of gravity at your location? • Given: • Asked: • Formula: • Substitute: • Answer (with units) Problem 26 • The 2kg metal ball moving at a speed of 3m/s strikes a 1kg wooden ball that is at rest. After the collision, the speed of the metal ball is 1m/s. Assume momentum is conserved, what is the speed of the wooden ball after the collision? • Given: • Asked: • Formula: • Substitute: • Answer (with units) Problem 27 • Find the mass of a car that has a speed of 30m/s and a momentum of 45,000 kg m/s. • Given: • Asked: • Formula: • Substitute: • Answer (with units) Problem 28 • A box being pushed with a force of 85N slides along the floor at a constant speed. What is t • Given: • Asked: • Formula: • Substitute: • Answer (with units) he force of sliding friction on the box? Tuesday September 27, 2011 • • • • • Need your book, calculator, Reference Sheet Turn in all homework with your test Use the blue (gray) side of scantron Question 1: a=true, b=false Show all work on the problems – – – – – Given Asked Formula Substitution Answer with unit – After the test – Turn in all homework for chapter 3 – Read pages 100-102