Chapter 5 - University of North Floridan00006757/physicslectures/Knight 2e/05_lect_outline/ja... ·...
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1 © 2010 Pearson Education, Inc. PowerPoint ® Lectures for College Physics: A Strategic Approach, Second Edition Chapter 5 Applying Newton’s Laws © 2010 Pearson Education, Inc. Slide 5-2 5 Applying Newton’s Laws © 2010 Pearson Education, Inc. Slide 5-3
Transcript of Chapter 5 - University of North Floridan00006757/physicslectures/Knight 2e/05_lect_outline/ja... ·...
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© 2010 Pearson Education, Inc.
PowerPoint® Lectures forCollege Physics: A Strategic Approach, Second Edition
Chapter 5
ApplyingNewton’sLaws
© 2010 Pearson Education, Inc. Slide 5-2
5 Applying Newton’s Laws
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Reading Quiz1. Which of the following statements about mass and weight is
correct?
A. Your mass is a measure of the force gravity exerts onyou.
B. Your mass is the same everywhere in the universe.C. Your weight is the same everywhere in the universe.D. Your weight is a measure of your resistance of being
accelerated.
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Answer1. Which of the following statements about mass and weight is
correct?
A. Your mass is a measure of the force gravity exerts onyou.
B. Your mass is the same everywhere in the universe.C. Your weight is the same everywhere in the universe.D. Your weight is a measure of your resistance of being
accelerated.
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Reading Quiz2. The apparent weight of an object is
A. the pull of gravity on the object.B. the object’s mass times the acceleration of gravity.C. the magnitude of the contact force that supports the
object.D. the pull of gravity on an object that is accelerating
upward.
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Answer2. The apparent weight of an object is
A. the pull of gravity on the object.B. the object’s mass times the acceleration of gravity.C. the magnitude of the contact force that supports the
object.D. the pull of gravity on an object that is accelerating
upward.
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Reading Quiz3. The coefficient of static friction is
A. smaller than the coefficient of kinetic friction.B. equal to the coefficient of kinetic friction.C. larger than the coefficient of kinetic friction.D. not discussed in this chapter.
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Answer3. The coefficient of static friction is
A. smaller than the coefficient of kinetic friction.B. equal to the coefficient of kinetic friction.C. larger than the coefficient of kinetic friction.D. not discussed in this chapter.
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Reading Quiz4. The force of friction is described by
A. the law of friction.B. the theory of friction.C. a model of friction.D. the friction hypothesis.
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Answer4. The force of friction is described by
A. the law of friction.B. the theory of friction.C. a model of friction.D. the friction hypothesis.
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EquilibriumAn object is in equilibrium whenthe net force acting on it is zero.In component form, this is
The net force on eachman in the tower iszero.
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Example ProblemA 100.-kg block with a weight of 980 N hangs on a rope.Find the tension in the rope if
A. the block is stationary.
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Example ProblemA 100.-kg block with a weight of 980 N hangs on a rope.Find the tension in the rope if
A. the block is stationary.
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Example ProblemA 100.-kg block with a weight of 980 N hangs on a rope.Find the tension in the rope if
A. the block is stationary.B. it’s moving upward at a steady speed of 5.0 m/s.
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m
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Example ProblemA 100.-kg block with a weight of 980 N hangs on a rope.Find the tension in the rope if
A. the block is stationary.B. it’s moving upward at a steady speed of 5.0 m/s.C. it’s accelerating upward at 5.0 m/s2.
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A wooden box, with a mass of 22 kg, is pulled at a constantspeed with a rope that makes an angle of 25° with the woodenfloor. What is the tension in the rope?
Example Problem
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Checking UnderstandingA rod is suspended by a string as shown. The lower end ofthe rod slides on a frictionless surface. Which figure correctlyshows the equilibrium position of the rod?
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AnswerA rod is suspended by a string as shown. The lower end ofthe rod slides on a frictionless surface. Which figure correctlyshows the equilibrium position of the rod?
B
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Example ProblemA ball weighing 50 N is pulled back by a rope to an angle of 20°.What is the tension in the pulling rope?
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Example ProblemA ball weighing 50 N is pulled back by a rope to an angle of 20°.What is the tension in the pulling rope?
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Using Newton’s Second Law
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Example ProblemA sled with a mass of 20 kg slides along frictionless ice at 4.5 m/s.It then crosses a rough patch of snow which exerts a friction forceof 12 N. How far does it slide on the snow before coming to rest?
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Macie pulls a 40 kg rolling trunk by a strap angled at 30° from thehorizontal. She pulls with a force of 40 N, and there is a 30 Nrolling friction force acting on trunk. What is the trunk’sacceleration?
Example Problem
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Mass and Weight
–w = may = m(–g)
w = mg
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Apparent Weight
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Apparent Weight
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A 50 kg student gets in a 1000 kg elevator at rest. As the elevatorbegins to move, she has an apparent weight of 600 N for the first 3s. How far has the elevator moved, and in which direction, at theend of 3 s?
Example Problem
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Find the x- and y-components of w in each of these threecoordinate systems.
Example Problem
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Find the x- and y-components of w in each of these threecoordinate systems.
Example Problem
Slide 5-26b
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Find the x- and y-components of w in each of these threecoordinate systems.
Example Problem
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A 75 kg skier starts down a 50-m-high, 10° slope on frictionlessskis. What is his speed at the bottom?
Example Problem
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Burglars are trying tohaul a 1000 kg safeup a frictionless rampto their getaway truck.The ramp is tilted atangle θ. What is thetension in the rope ifthe safe is at rest? Ifthe safe is moving upthe ramp at a steady 1m/s? If the safe isaccelerating up theramp at 1 m/s2? Dothese answers havethe expected behaviorin the limit θ → 0° andθ → 90°?
Example Problem
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The same burglars push the 1000 kg safe up a 20° frictionlessslope with a force of 4000 N. What is the safe’s acceleration?
Example Problem
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Static Friction
fs max = µsnSlide 5-30
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Kinetic Friction
fk = µkn
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Working with Friction Forces
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A car traveling at 20m/s stops in a distanceof 50 m. Assume thatthe deceleration isconstant. Thecoefficients of frictionbetween a passengerand the seat are μs =0.5 and μk = 0.3. Will a70 kg passenger slideoff the seat if notwearing a seat belt?
Example Problem
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DragAn object moving in a gas or liquid experiences a drag force
Drag coefficient. Depends ondetails of the object’s shape.“Streamlining” reduces drag bymaking CD smaller. For a typicalobject, CD 0.5.
Density of gas or liquid. Air hasa density of 1.29 kg/m3.
A is the object’s cross section areawhen facing into the wind.
Drag depends on the square of the speed.This is a really important factor that limits thetop speed of cars and bicycles. Going twiceas fast requires 4 times as much force and,as we’ll see later, 8 times as much power.
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Cross-Section Area
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Terminal SpeedA falling object speeds upuntil reaching terminal speed,then falls at that speedwithout further change.
If two objects have the samesize and shape, the moremassive object has a largerterminal speed.
At terminal speed, the netforce is zero and the objectfalls at constant speed withzero acceleration.
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What is the terminal speed of a lacrosse ball with adiameter of 6.4 cm and a mass of 150 g?
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Applying Newton’s Third Law: Interacting Objects
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Example ProblemBlock A has a massof 1 kg; block B’smass is 4 kg. Theyare pushed with aforce of magnitude10 N.a. What is the
acceleration ofthe blocks?
b. With what forcedoes A push onB? B push onA?
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Checking Understanding
A. The string tension and the friction force acting on A.B. The normal force on A due to B and the weight of A.C. The normal force on A due to B and the weight of B.D. The friction force acting on A and the friction force acting on
B.
Which pair of forces is an action/reaction pair?
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Example ProblemWhat is the acceleration of block B?
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Example ProblemWhat is the acceleration of block B?
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Ropes and Pulleys
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Example ProblemBlock A, with mass4.0 kg, sits on africtionless table.Block B, with mass2.0 kg, hangs from arope connectedthrough a pulley toblock A. What is theacceleration of blockA?
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Summary
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Summary
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Summary
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Additional Example ProblemA wooden box, with a mass of 22 kg, is pulled at a constantspeed with a rope that makes an angle of 25° with the woodenfloor. What is the tension in the rope?
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