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Ch. 11 Motion & Forces I. Newton’s Laws of Motion “If I have seen far, it is because I have stood on the shoulders of giants.” - Sir Isaac Newton (referring to Galileo) A. Newton’s First Law Newton’s First Law of Motion An object at rest will remain at rest and an object in motion will continue moving at a constant velocity unless acted upon by a net force. B. Newton’s Second Law Newton’s Second Law of Motion The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. F = ma C. Newton’s Third Law Newton’s Third Law of Motion When one object exerts a force on a second object, the second object exerts an equal but opposite force on the first. Ch. 11 Motion & Forces II. Describing Motion Motion Speed & Velocity Acceleration Newton’s First Law Newton’s First Law of Motion An object at rest will remain at rest and an object in motion will continue moving at a constant velocity unless acted upon by a net force force. A. Motion Problem: Is your desk moving? We need a reference point... nonmoving point from which motion is measured A. Motion Motion Change in position in relation to a reference point. Reference point Motion A. Motion: Displacement The distance an object has been moved from one position to another. A. Motion Problem: You are a passenger in a car stopped at a stop sign. Out of the corner of your eye, you notice a tree on the side of the road begin to move forward. You have mistakenly set yourself as the reference point. B. Speed & Velocity Speed d rate of motion v t distance traveled per unit time distance speed time B. Speed & Velocity Instantaneous Speed speed at a given instant Average Speed total distance avg. speed total time B. Speed & Velocity Problem: A storm is 10 km away and is moving at a speed of 60 km/h. Should you be worried? It depends on the storm’s direction! B. Speed & Velocity Velocity speed in a given direction can change even when the speed is constant! C. Acceleration vf - vi a t Acceleration the rate of change of velocity change in speed or direction a v f vi t a: vf: vi: t: acceleration final velocity initial velocity time C. Acceleration Positive acceleration “speeding up” Negative acceleration “slowing down” D. Calculations Your neighbor skates at a speed of 4 m/s. You can skate 100 m in 20 s. Who skates faster? GIVEN: WORK: d = 100 m t = 20 s v=? d v t v=d÷t v = (100 m) ÷ (20 s) v = 5 m/s You skate faster! D. Calculations A roller coaster starts down a hill at 10 m/s. Three seconds later, its speed is 32 m/s. What is the roller coaster’s acceleration? GIVEN: WORK: vi = 10 m/s t=3s vf = 32 m/s vf - vi a=? a t a = (vf - vi) ÷ t a = (32m/s - 10m/s) ÷ (3s) a = 22 m/s ÷ 3 s a = 7.3 m/s2 D. Calculations Sound travels 330 m/s. If a lightning bolt strikes the ground 1 km away from you, how long will it take for you to hear it? GIVEN: WORK: v = 330 m/s t=d÷v d = 1km = 1000m t = (1000 m) ÷ (330 m/s) t=? t = 3.03 s d v t D. Calculations How long will it take a car traveling 30 m/s to come to a stop if its acceleration is -3 m/s2? GIVEN: WORK: t=? vi = 30 m/s vf = 0 m/s a = -3 m/s2 t = (vf - vi) ÷ a t = (0m/s-30m/s)÷(-3m/s2) vf - vi a t t = -30 m/s ÷ -3m/s2 t = 10 s E. Graphing Motion Distance-Time Graph A B slope = speed steeper slope = faster speed straight line = constant speed flat line = no motion E. Graphing Motion Distance-Time Graph A B Who started out faster? A (steeper slope) Who had a constant speed? A Describe B from 10-20 min. B stopped moving Find their average speeds. A = (2400m) ÷ (30min) A = 80 m/min B = (1200m) ÷ (30min) B = 40 m/min E. Graphing Motion Distance-Time Graph 400 Acceleration is indicated by a curve on a Distance-Time graph. Changing slope = changing velocity Distance (m) 300 200 100 0 0 5 10 Time (s) 15 20 E. Graphing Motion Speed-Time Graph 3 slope = acceleration +ve = speeds up -ve = slows down straight line = constant accel. flat line = no accel. (constant velocity) Speed (m/s) 2 1 0 0 2 4 6 Time (s) 8 10 E. Graphing Motion Speed-Time Graph Specify the time period when the object was... slowing down 5 to 10 seconds speeding up 0 to 3 seconds 3 Speed (m/s) 2 1 0 0 2 4 6 Time (s) 8 10 moving at a constant speed 3 to 5 seconds not moving 0 & 10 seconds Ch. 3 & 4 Motion & Forces III. Defining Force Force Newton’s First Law Friction A. Force Force a push or pull that one body exerts on another What forces are being exerted on the football? Fkick Fgrav A. Force Balanced Forces forces acting on an object that are opposite in direction and equal in size no change in velocity A. Force Net Force unbalanced forces that are not opposite and equal velocity changes (object accelerates) Fnet Ffriction Fpull N N W B. Newton’s First Law Newton’s First Law of Motion An object at rest will remain at rest and an object in motion will continue moving at a constant velocity unless acted upon by a net force. B. Newton’s First Law Newton’s First Law of Motion “Law of Inertia” Inertia tendency of an object to resist any change in its motion increases as mass increases Concept Test 1 TRUE or FALSE? The object shown in the diagram must be at rest since there is no net force acting on it. FALSE! A net force does not cause motion. A net force causes a change in motion, or acceleration. Taken from “The Physics Classroom” © Tom Henderson, 1996-2001. Concept Test 2 You are a passenger in a car and not wearing your seat belt. Without increasing or decreasing its speed, the car makes a sharp left turn, and you find yourself colliding with the right-hand door. Which is the correct analysis of the situation? ... Concept Test 2 1. Before and after the collision, there is a rightward force pushing you into the door. 2. Starting at the time of collision, the door exerts a leftward force on you. 3. both of the above 4. neither of the above C. Friction Friction force that opposes motion between 2 surfaces depends on the: • types of surfaces • force between the surfaces C. Friction Friction is greater... between rough surfaces when there’s a greater force between the surfaces (e.g. more weight) Pros and Cons? Ch. 3 & 4 Motion & Forces IV. Force & Acceleration Newton’s Second Law Gravity Air Resistance Calculations A. Newton’s Second Law Newton’s Second Law of Motion The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. F = ma A. Newton’s Second Law F a m F = ma F m a F: force (N) m: mass (kg) a: accel (m/s2) 1 N = 1 kg ·m/s2 B. Gravity Gravity force of attraction between any two objects in the universe increases as... • mass increases • distance decreases B. Gravity Who experiences more gravity - the astronaut or the politician? Which exerts more gravity the Earth or the moon? less distance more mass B. Gravity Weight the force of gravity on an object W = mg W: weight (N) m: mass (kg) g: acceleration due to gravity (m/s2) MASS WEIGHT always the same (kg) depends on gravity (N) B. Gravity Would you weigh more on Earth or Jupiter? Jupiter because... greater mass greater gravity greater weight B. Gravity Accel. due to gravity (g) In the absence of air resistance, all falling objects have the same acceleration! On Earth: g = 9.8 m/s2 W g m elephant g W m feather Animation from “Multimedia Physics Studios.” C. Air Resistance Air Resistance a.k.a. “fluid friction” or “drag” force that air exerts on a moving object to oppose its motion depends on: • speed • surface area • shape • density of fluid C. Air Resistance Terminal Velocity maximum velocity reached by a falling object F reached when… air Fgrav = Fair no net force no acceleration constant velocity Fgrav C. Air Resistance Terminal Velocity increasing speed increasing air resistance until… Fair = Fgrav Animation from “Multimedia Physics Studios.” C. Air Resistance Falling with air resistance heavier objects fall faster because they accelerate to higher speeds before reaching terminal velocity Fgrav = Fair larger Fgrav need larger Fair need higher speed Animation from “Multimedia Physics Studios.” D. Calculations What force would be required to accelerate a 40 kg mass by 4 m/s2? GIVEN: WORK: F=? m = 40 kg a = 4 m/s2 F = ma F m a F = (40 kg)(4 m/s2) F = 160 N D. Calculations A 4.0 kg shotput is thrown with 30 N of force. What is its acceleration? GIVEN: WORK: m = 4.0 kg F = 30 N a=? a=F÷m F m a a = (30 N) ÷ (4.0 kg) a = 7.5 m/s2 D. Calculations Mrs. J. weighs 557 N. What is her mass? GIVEN: WORK: F(W) = 557 N m=? a(g) = 9.8 m/s2 m=F÷a F m a m = (557 N) ÷ (9.8 m/s2) m = 56.8 kg Concept Test Is the following statement true or false? An astronaut has less mass on the moon since the moon exerts a weaker gravitational force. False! Mass does not depend on gravity, weight does. The astronaut has less weight on the moon. Motion & Forces V. Free-Fall Motion Free-fall Circular Motion Free-fall A. Free-Fall Free-Fall when an object is influenced only by the force of gravity Weightlessness sensation produced when an object and its surroundings are in free-fall object is not weightless! CUP DEMO A. Free-Fall Weightlessness surroundings are falling at the same rate so they don’t exert a force on the object Motion & Forces VI. Action and Reaction Newton’s Third Law Momentum Conservation of Momentum A. Newton’s Third Law Newton’s Third Law of Motion When one object exerts a force on a second object, the second object exerts an equal but opposite force on the first. A. Newton’s Third Law Problem: How can a horse pull a cart if the cart is pulling back on the horse with an equal but opposite force? Aren’t these “balanced forces” resulting in no acceleration? NO!!! A. Newton’s Third Law Explanation: forces are equal and opposite but act on different objects they are not “balanced forces” the movement of the horse depends on the forces acting on the horse A. Newton’s Third Law Action-Reaction Pairs The hammer exerts a force on the nail to the right. The nail exerts an equal but opposite force on the hammer to the left. A. Newton’s Third Law Action-Reaction Pairs The rocket exerts a downward force on the exhaust gases. The gases exert an equal but opposite upward force on the rocket. FG FR A. Newton’s Third Law Action-Reaction Pairs Both objects accelerate. The amount of acceleration depends on the mass of the object. F a m Small mass more acceleration Large mass less acceleration JET CAR CHALLENGE CHALLENGE: Construct a car that will travel as far as possible (at least 3 meters) using only the following materials. scissors tape 4 plastic lids 2 skewers 2 straws 1 balloon 1 tray How do each of Newton’s Laws apply?