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Transcript
N e w t o n’ s Unit 3 L a w s 3.1 Force and Mass Force – push or pull; required to change an object’s motion. Vector – so magnitude and direction Example of Contact Forces Examples of Field Forces Friction Tension Gravitational Applied Electric Magnetic Spring 3.1 Force and mass Mass – measurement of how difficult it is to change the objects velocity Inertia – resistance to change in velocity So mass is a measurement of an object’s inertia 3.1 Force and mass 3.2 Newton’s First Law of Motion 1st Law An object at rest remains at rest as long as no net force acts on it. An object moving with constant velocity continues to move with the same speed and in the same direction as long as no net force acts on it. 3.2 Newton’s First Law of Motion Sometimes called the Law of Inertia 3.2 Newton’s First Law of Motion 3.3 Newton’s Second Law of Motion 2nd Law A net force causes an acceleration in the direction of the net force. F ma Simulation 3.3 Newton’s Second Law of Motion Free body diagrams Show all the forces acting on an object For example an object sitting on a table W – weight = mg N N – Normal Force (perpendicular) to the surface W 3.3 Newton’s Second Law of Motion Free body diagrams If a rope pulls the object toward the right, then T = Tension N Practice Free Body T W 3.3 Newton’s Second Law of Motion Free body diagrams Steps in problems solving 1.Sketch the forces 2.Isolate the Object 3.Choose a Coordinate System 4.Resolve the Forces into Components 5.Apply Newton’s Second Law of Motion 3.3 Newton’s Second Law of Motion A 50 kg gopher has a string tied around his neck and pulled with a force of 80 N at an angle of 30o to the horizontal. What is his acceleration? 3.3 Newton’s Second Law of Motion A 50 kg gopher has a string tied around his neck and pulled with a force of 80 N at an angle of 30o to the horizontal. What is his acceleration? Free Body diagram 3.3 Newton’s Second Law of Motion A 50 kg gopher has a string tied around his neck and pulled with a force of 80 N at an angle of 30o to the horizontal. What is his acceleration? N T Free Body diagram W 3.3 Newton’s Second Law of Motion A 50 kg gopher has a string tied around his neck and pulled with a force of 80 N at an angle of 30o to the horizontal. What is his acceleration? N T Free Body diagram Axis W 3.3 Newton’s Second Law of Motion A 50 kg gopher has a string tied around his neck and pulled with a force of 80 N at an angle of 30o to the horizontal. What is his T acceleration? y N T Free Body diagram Tx Axis W 3.3 Newton’s Second Law of Motion A 50 kg gopher has a string tied around his neck and pulled with a force of 80 N at an angle of 30o to the horizontal. What is his T acceleration? F T ma y N Free Body x x x Fy Ty N W may diagram Tx Axis Equation W T cos 30 ma 80 cos 50a T sin a 1.39Nm2 W 0 s 3.3 Newton’s Second Law of Motion 3.4 Newton’s Third Law of Motion For every force that acts on an object, there is a reaction force acting on a different object that is equal in magnitude and opposite in direction. If object 1 exerts a force F on object 2, then object 2 exerts a force –F on object 1. 3.4 Newton’s Third Law of Motion What are the action reaction pairs in the following? 3.4 Newton’s Third Law of Motion A 60 kg man walks off a 3 m long canoe by walking from one end to the other. He applies a force of 20 N to the canoe, which has a mass of 150 kg. A.What is the acceleration of the man? B. What is the acceleration of the canoe? 3.4 Newton’s Third Law of Motion A 60 kg man walks off a 3 m long canoe by walking from one end to the other. He applies a force of 20 N to the canoe, which has a mass of 150 kg. Free Body Diagrams Nc Nm P P Wm Wc 3.4 Newton’s Third Law of Motion A 60 kg man walks off a 3 m long canoe by walking from one end to the other. He applies a force of 20 N to the canoe, which has a mass of 150 kg. Fx P mm a Equations Nc Fx P mc a Fy N c Wc 0 Fy N m Wm 0 Nm P P Wm Wc 3.4 Newton’s Third Law of Motion A 60 kg man walks off a 3 m long canoe by walking from one end to the other. He applies a force of 20 N to the canoe, which has a mass of 150 kg. P mm a 20 60a A-acceleration of man Nc Fx P mc a Fy N c Wc 0 Nm a 0.33 sm2 P P Wm Wc 3.4 Newton’s Third Law of Motion A 60 kg man walks off a 3 m long canoe by walking from one end to the other. He applies a force of 20 N to the canoe, which has a mass of 150 kg. P mm a 20 60a A – acceleration of canoe Nc P mc a Nm 20 150a P a 0.13 sm2 a 0.33 sm2 P Wm Wc 3.4 Newton’s Third Law of Motion Two boxes are tied together with a rope, and the first one is pulled by a second rope. Both boxes accelerate at 2.0 m/s2. If the front box has a mass of 25 kg, and the second a mass of 50 kg, what is the tension on each rope? a 3.4 Newton’s Third Law of Motion Free body diagrams Nb Nf T2 T1 T2 Wb Wf 3.4 Newton’s Third Law of Motion Equations Fx T2 mb a Fx T1 T2 m f a Fy N b Wb 0 Fy N f W f 0 Solve (add) T1 (m f mb )a T2 mb a T1 T2 m f a T2 (50kg)2 100 N m Nb Nf T1 (25 50)2 T1 150 N T2 s2 T1 T2 Wb Wf 3.4 Newton’s Third Law of Motion 3.5 The Vector Nature of Forces Forces are vectors, so they can be treated using vectors rules 3.5 The Vector Nature of Forces Two men are carrying a 1.3 kg pail of water, the first dude (Bob) exerts a force of 7N, and the second one (Leon) exerts a force of 11N @ 28o. What is the angle of Bob’s force? Free Body Diagram? L B W 3.5 The Vector Nature of Forces Two men are carrying a 1.3 kg pail of water, the first dude (Bob) exerts a force of 7N, and the second one (Leon) exerts a force of 11N @ 28o. What is the angle of Bob’s force? Components? B L By Bx W 3.5 The Vector Nature of Forces Two men are carrying a 1.3 kg pail of water, the first dude (Bob) exerts a force of 7N, and the second one (Leon) exerts a force of 11N @ 28o. What is the angle of Bob’s force? L y Components? L By Bx Lx W 3.5 The Vector Nature of Forces Two men are carrying a 1.3 kg pail of water, the first dude (Bob) exerts a force of 7N, and the second one (Leon) exerts a force of 11N @ 28o. What is the angle of Bob’s force? L y Equations? Fx Lx Bx max Fy Ly By W may By Bx Lx W 3.5 The Vector Nature of Forces Two men are carrying a 1.3 kg pail of water, the first dude (Bob) exerts a force of 7N, and the second one (Leon) exerts a force of 11N @ 28o. What is the angle of Bob’s force? L y Values? Fx Lx Bx max Fy Ly By W may By Bx Lx W 3.5 The Vector Nature of Forces Two men are carrying a 1.3 kg pail of water, the first dude (Bob) exerts a force of 7N, and the second one (Leon) exerts a force of 11N @ 28o. What is the angle of Bob’s force? L y Values? L cos Bx 0 L sin By mg 0 By Bx Lx W 3.5 The Vector Nature of Forces Two men are carrying a 1.3 kg pail of water, the first dude (Bob) exerts a force of 7N, and the second one (Leon) exerts a force of 11N @ 28o. What is the angle of Bob’s force? L y Solve? By L cos Bx 0 L sin By mg 0 Bx Lx W 3.5 The Vector Nature of Forces Two men are carrying a 1.3 kg pail of water, the first dude (Bob) exerts a force of 12.3N, and the second one (Leon) exerts a force of 11N @ 28o. What is the angle of Bob’s force? L y Solve? 11cos 28 Bx 11sin 28 (1.3)(9.8) By Bx 9.7 By 7.6 By Bx Lx W 7.6 o tan 38 3.5 The Vector Natureof Forces 9 .7 1 3.6 Frictional Forces Friction – force that opposes motion Caused by microscopic irregularities of a surface Increases as pushing force increases 3.6 Frictional Forces Depends on the normal force and the type of surface f mN f – force of friction (N) N – normal force m – coefficient of friction (1 or less) 3.6 Frictional Forces Three types of friction 1.Static – object at rest 2.Kinetic – object in motion Surfaces µ (static) µ (kinetic) Steel on steel 0.74 0.57 Glass on glass 0.94 0.4 Metal on Metal (lubricated) 0.15 0.06 0.1 0.03 Teflon on Teflon 0.04 0.04 Tire on concrete 1 0.8 Tire on wet road 0.6 0.4 Tire on snow 0.3 0.2 Ice on ice 3.Rolling – just like it sounds 3.6 Frictional Forces Example 3.6 Frictional Forces