Chapter 8. Dynamics II: Motion in a Plane Chapter Goal: To learn how to solve problems about motion in a plane. В© 2013 Pearson Education, Inc. Slide 8-2 Chapter 8 Preview В© 2013 Pearson Education, Inc. Slide 8-3 Chapter 8 Preview В© 2013 Pearson Education, Inc. Slide 8-4 Chapter 8 Preview В© 2013 Pearson Education, Inc. Slide 8-5 Chapter 8 Preview В© 2013 Pearson Education, Inc. Slide 8-6 Chapter 8 Preview В© 2013 Pearson Education, Inc. Slide 8-7 Chapter 8 Preview В© 2013 Pearson Education, Inc. Slide 8-8 Dynamics in Two Dimensions п‚§ Suppose the x- and y-components of acceleration are independent of each other. п‚§ That is, ax does not depend on y or vy, and ay does not depend on x or vx . п‚§ Your problem-solving strategy is to: 1. Draw a pictorial representation: a motion diagram (if needed) and a free-body diagram. 2. Use NewtonвЂ™s second law in component form: The force components (including proper signs) are found from the free-body diagram В© 2013 Pearson Education, Inc. Slide 8-21 Dynamics in Two Dimensions Dynamics in Two Dimensions 3. Solve for the acceleration. If the acceleration is constant, use the two-dimensional kinematic equations of Chapter 4 to find velocities and positions: В© 2013 Pearson Education, Inc. Slide 8-22 Example 8.1 Rocketing in the Wind В© 2013 Pearson Education, Inc. Slide 8-23 Example 8.1 Rocketing in the Wind В© 2013 Pearson Education, Inc. Slide 8-24 Example 8.1 Rocketing in the Wind В© 2013 Pearson Education, Inc. Slide 8-25 Example 8.1 Rocketing in the Wind В© 2013 Pearson Education, Inc. Slide 8-26 Example 8.1 Rocketing in the Wind В© 2013 Pearson Education, Inc. Slide 8-27 Example 8.1 Rocketing in the Wind В© 2013 Pearson Education, Inc. Slide 8-28 Example 8.1 Rocketing in the Wind В© 2013 Pearson Education, Inc. Slide 8-29 Projectile Motion: Review п‚§ In the absence of air resistance, a projectile moves under the influence of only gravity. п‚§ If we choose a coordinate system with a vertical y-axis, then п‚§ Consequently, from NewtonвЂ™s second law, the acceleration is В© 2013 Pearson Education, Inc. Slide 8-30 Projectile Motion: Review п‚§ Consider a projectile with initial speed v0, and a launch angle of пЃ± above the horizontal. п‚§ In Chapter 4 (eq 4.16) we found that the distance it travels before it returns to the same elevation from which it was launched (the range) is: Trajectories of a projectile launched at different angles with a speed of 99 m/s. п‚§ The maximum range occurs for пЃ± пЂЅ 45п‚°. п‚§ All of these results neglect the effect of air resistance. В© 2013 Pearson Education, Inc. Slide 8-31 Projectile Motion п‚§ For low-mass projectiles on earth, the effects of air resistance, or drag, are too large to ignore. п‚§ When drag is included, the angle for maximum range of a projectile depends both on its size and mass. п‚§ The optimum angle is roughly 35п‚° for baseballs. п‚§ The flight of a golf ball is even more complex, because of the dimples and effects of spin. п‚§ Professional golfers achieve their maximum range at launch angles of barely 15п‚°! В© 2013 Pearson Education, Inc. Slide 8-32 Projectile Motion п‚§ The acceleration of a typical projectile subject to drag force from the air is: п‚§ The components of acceleration are not independent of each other. п‚§ These equations can only be solved numerically. п‚§ The figure shows the numerical solution for a 5-g plastic ball. В© 2013 Pearson Education, Inc. Slide 8-33 Uniform Circular Motion п‚§ When describing circular motion, it is convenient to define a moving rtz-coordinate system. п‚§ The origin moves along with a certain particle moving in a circular path. п‚§ The r-axis (radial) points from the particle toward the center of the circle. п‚§ The t-axis (tangential) is tangent to the circle, pointing in the ccw direction. п‚§ The z-axis is perpendicular to the plane of motion. В© 2013 Pearson Education, Inc. Slide 8-34 Uniform Circular Motion п‚§ A particle in uniform circular motion with angular velocity П‰ has velocity v = П‰r, in the tangential direction. п‚§ The acceleration of uniform circular motion points to the center of the circle. п‚§ The rtz-components of and are: В© 2013 Pearson Education, Inc. Slide 8-35 QuickCheck 8.1 The diagram shows three points of a motion diagram. The particle changes direction with no change of speed. What is the acceleration at point 2? В© 2013 Pearson Education, Inc. Slide 8-36 QuickCheck 8.1 The diagram shows three points of a motion diagram. The particle changes direction with no change of speed. What is the acceleration at point 2? В© 2013 Pearson Education, Inc. Acceleration of changing direction Slide 8-37 QuickCheck 8.2 A toy car moves around a circular track at constant speed. It suddenly doubles its speed вЂ” a change of a factor of 2. As a result, the centripetal acceleration changes by a factor of A. 1/4. B. 1/2. C. No change since the radius doesnвЂ™t change. D. 2. E. 4. В© 2013 Pearson Education, Inc. Slide 8-38 QuickCheck 8.2 A toy car moves around a circular track at constant speed. It suddenly doubles its speedвЂ”a change of a factor of 2. As a result, the centripetal acceleration changes by a factor of A. 1/4. B. 1/2. C. No change since the radius doesnвЂ™t change. D. 2. E. 4. В© 2013 Pearson Education, Inc. Slide 8-39 Example 8.2 The Ultracentrifuge В© 2013 Pearson Education, Inc. Slide 8-40 Example 8.2 The Ultracentrifuge В© 2013 Pearson Education, Inc. Slide 8-41 Dynamics of Uniform Circular Motion п‚§ An object in uniform circular motion is not traveling at a constant velocity in a straight line. п‚§ Consequently, the particle must have a net force acting on it п‚§ Without such a force, the object would move off in a straight line tangent to the circle. п‚§ The car would end up in the ditch! В© 2013 Pearson Education, Inc. Highway and racetrack curves are banked to allow the normal force of the road to provide the centripetal acceleration of the turn. Slide 8-42 Dynamics of Uniform Circular Motion п‚§ The figure shows a particle in uniform circular motion. п‚§ The net force must point in the radial direction, toward the center of the circle. п‚§ This centripetal force is not a new force; it must be provided by familiar forces. В© 2013 Pearson Education, Inc. Slide 8-43 QuickCheck 8.3 An ice hockey puck is tied by a string to a stake in the ice. The puck is then swung in a circle. What force or forces does the puck feel? A. B. C. D. E. A new force: the centripetal force. A new force: the centrifugal force. One or more of our familiar forces pushing outward. One or more of our familiar forces pulling inward. I have no clue. В© 2013 Pearson Education, Inc. Slide 8-44 QuickCheck 8.3 An ice hockey puck is tied by a string to a stake in the ice. The puck is then swung in a circle. What force or forces does the puck feel? A. B. C. D. E. A new force: the centripetal force. A new force: the centrifugal force. One or more of our familiar forces pushing outward. One or more of our familiar forces pulling inward. I have no clue. The rules about what is or is not a force havenвЂ™t changed. 1. Force must be exerted at a point of contact (except for gravity). 2. Force must have an identifiable agent doing the pushing or pulling. 3. The net force must point in the direction of acceleration (NewtonвЂ™s second law). В© 2013 Pearson Education, Inc. Slide 8-45 QuickCheck 8.4 An ice hockey puck is tied by a string to a stake in the ice. The puck is then swung in a circle. What force is producing the centripetal acceleration of the puck? A. Gravity B. Air resistance C. Friction D. Normal force E. Tension in the string В© 2013 Pearson Education, Inc. Slide 8-46 QuickCheck 8.4 An ice hockey puck is tied by a string to a stake in the ice. The puck is then swung in a circle. What force is producing the centripetal acceleration of the puck? A. Gravity B. Air resistance C. Friction D. Normal force E. Tension in the string В© 2013 Pearson Education, Inc. Slide 8-47 Example 8.4 Turning the Corner I В© 2013 Pearson Education, Inc. Slide 8-48 Example 8.4 Turning the Corner I VISUALIZE п‚§ The second figure below shows the top view of a tire as it turns a corner. п‚§ The force that prevents the tire from sliding across a surface is static friction. п‚§ Static friction pushes sideways on the tire, perpendicular to the velocity, since the car is not speeding up or slowing down. п‚§ The free-body diagram, drawn from behind the car, shows the static friction pointing toward the center of the circle. В© 2013 Pearson Education, Inc. Slide 8-49 Example 8.4 Turning the Corner I В© 2013 Pearson Education, Inc. Slide 8-50 Example 8.4 Turning the Corner I В© 2013 Pearson Education, Inc. Slide 8-51 Example 8.4 Turning the Corner I В© 2013 Pearson Education, Inc. Slide 8-52 Banked Curves п‚§ Real highway curves are banked by being tilted up at the outside edge of the curve. п‚§ The radial component of the normal force can provide centripetal acceleration needed to turn the car. (Ex 8.5) п‚§ For a curve of radius r banked at an angle пЃ±, the exact speed at which a car must take the curve without assistance from friction is . В© 2013 Pearson Education, Inc. Slide 8-53 Banked Curves п‚§ Consider a car going around a banked curve at a speed higher than . п‚§ In this case, static friction must prevent the car from slipping up the hill. В© 2013 Pearson Education, Inc. Slide 8-54 Banked Curves п‚§ Consider a car going around a banked curve at a speed slower than . п‚§ In this case, static friction must prevent the car from slipping down the hill. В© 2013 Pearson Education, Inc. Slide 8-55 Circular Orbits п‚§ The figure shows a perfectly smooth, spherical, airless planet with one tower of height h. п‚§ A projectile is launched parallel to the ground with speed v0 . п‚§ If v0 is very small, as in trajectory A, it simply falls to the ground along a parabolic trajectory. п‚§ This is the вЂњflat-earth approximation.вЂќ В© 2013 Pearson Education, Inc. Slide 8-58 Circular Orbits п‚§ As the initial speed v0 is increased, the range of the projectile increases as the ground curves away from it. п‚§ Trajectories B and C are of this type. п‚§ If v0 is sufficiently large, there comes a point where the trajectory and the curve of the earth are parallel. п‚§ In this case, the projectile вЂњfallsвЂќ but it never gets any closer to the ground! п‚§ This is trajectory D, called an orbit. В© 2013 Pearson Education, Inc. Slide 8-59 Circular Orbits п‚§ In the flat-earth approximation, shown in figure (a), the gravitational force on an object of mass m is: п‚§ Since actual planets are spherical, the real force of gravity is toward the center of the planet, as shown in figure (b). В© 2013 Pearson Education, Inc. Slide 8-60 Circular Orbits п‚§ An object in a low circular orbit has acceleration: п‚§ If the object moves in a circle of radius r at speed vorbit the centripetal acceleration is: п‚§ The required speed for a circular orbit near a planetвЂ™s surface, neglecting air resistance, is: В© 2013 Pearson Education, Inc. Slide 8-61 Circular Orbits п‚§ The period of a low-earth-orbit satellite is: п‚§ If r is approximately the radius of the earth Re = 6400 km, then T is about 90 minutes. п‚§ An orbiting spacecraft is constantly in free fall, falling under the influence only of the gravitational force. п‚§ This is why astronauts feel weightless in space. В© 2013 Pearson Education, Inc. Slide 8-62 Fictitious Forces п‚§ If you are riding in a car that makes a sudden stop, you seem to be hurled forward. п‚§ You can describe your experience in terms of fictitious forces. п‚§ Fictitious forces are not real because no agent is exerting them. п‚§ Fictitious forces describe your motion relative to a noninertial reference frame. В© 2013 Pearson Education, Inc. Slide 8-64 Centrifugal Force? п‚§ The figure shows a birdвЂ™seye view of you riding in a car as it makes a left turn. п‚§ From the perspective of an inertial reference frame, the normal force from the door points inward, keeping you on the road with the car. п‚§ Relative to the noninertial reference frame of the car, you feel pushed toward the outside of the curve. п‚§ The fictitious force which seems to push an object to the outside of a circle is called the centrifugal force. п‚§ There really is no such force in an inertial reference frame. В© 2013 Pearson Education, Inc. Slide 8-65 QuickCheck 8.6 A coin sits on a turntable as the table steadily rotates ccw. The free-body diagrams below show the coin from behind, moving away from you. Which is the correct diagram? В© 2013 Pearson Education, Inc. Slide 8-66 QuickCheck 8.6 A coin sits on a turntable as the table steadily rotates ccw. The free-body diagrams below show the coin from behind, moving away from you. Which is the correct diagram? В© 2013 Pearson Education, Inc. Slide 8-67 QuickCheck 8.7 A coin sits on a turntable as the table steadily rotates ccw. What force or forces act in the plane of the turntable? В© 2013 Pearson Education, Inc. Slide 8-68 QuickCheck 8.7 A coin sits on a turntable as the table steadily rotates ccw. What force or forces act in the plane of the turntable? В© 2013 Pearson Education, Inc. Slide 8-69 QuickCheck 8.8 Two coins are on a turntable that steadily speeds up, starting from rest, with a ccw rotation. Which coin flies off the turntable first? A. Coin 1 flies off first. B. Coin 2 flies off first. C. Both coins fly off at the same time. D. We canвЂ™t say without knowing their masses. В© 2013 Pearson Education, Inc. Slide 8-70 QuickCheck 8.8 Two coins are on a turntable that steadily speeds up, starting from rest, with a ccw rotation. Which coin flies off the turntable first? A. Coin 1 flies off first. B. Coin 2 flies off first. C. Both coins fly off at the same time. D. We canвЂ™t say without knowing their masses. В© 2013 Pearson Education, Inc. Slide 8-71 Loop-the-Loop п‚§ The figure shows a rollercoaster going around a vertical loop-the-loop of radius r. п‚§ Note this is not uniform circular motion; the car slows down going up one side, and speeds up going down the other. п‚§ At the very top and very bottom points, only the carвЂ™s direction is changing, so the acceleration is purely centripetal. п‚§ Because the car is moving in a circle, there must be a net force toward the center of the circle. В© 2013 Pearson Education, Inc. Slide 8-73 Loop-the-Loop п‚§ The figure shows the roller-coaster free-body diagram at the bottom of the loop. п‚§ Since the net force is toward the center (upward at this point), n > FG . п‚§ This is why you вЂњfeel heavyвЂќ at the bottom of the valley on a roller coaster. п‚§ The normal force at the bottom is larger than mg. В© 2013 Pearson Education, Inc. Slide 8-74 Loop-the-Loop п‚§ The figure shows the roller-coaster free-body diagram at the top of the loop. п‚§ The track can only push on the wheels of the car, it cannot pull, therefore presses downward. п‚§ The car is still moving in a circle, so the net force is also downward: п‚§ The normal force at the at the top can exceed mg if vtop is large enough. В© 2013 Pearson Education, Inc. Slide 8-75 Loop-the-Loop п‚§ At the top of the roller coaster, the normal force of the track on the car is: п‚§ As vtop decreases, there comes a point when n reaches zero. п‚§ The speed at which n = 0 is called the critical speed: п‚§ This is the slowest speed at which the car can complete the circle without falling off the track near the top. В© 2013 Pearson Education, Inc. Slide 8-76 Loop-the-Loop A roller-coaster car at the top of the loop. В© 2013 Pearson Education, Inc. Slide 8-77 QuickCheck 8.9 A physics textbook swings back and forth as a pendulum. Which is the correct free-body diagram when the book is at the bottom and moving to the right? В© 2013 Pearson Education, Inc. Slide 8-78 QuickCheck 8.9 A physics textbook swings back and forth as a pendulum. Which is the correct free-body diagram when the book is at the bottom and moving to the right? В© 2013 Pearson Education, Inc. Centripetal acceleration requires an upward force. Slide 8-79 QuickCheck 8.10 A car thatвЂ™s out of gas coasts over the top of a hill at a steady 20 m/s. Assume air resistance is negligible. Which free-body diagram describes the car at this instant? В© 2013 Pearson Education, Inc. Slide 8-80 QuickCheck 8.10 A car thatвЂ™s out of gas coasts over the top of a hill at a steady 20 m/s. Assume air resistance is negligible. Which free-body diagram describes the car at this instant? В© 2013 Pearson Education, Inc. Now the centripetal acceleration points down. Slide 8-81 QuickCheck 8.11 A roller coaster car does a loopthe-loop. Which of the free-body diagrams shows the forces on the car at the top of the loop? Rolling friction can be neglected. В© 2013 Pearson Education, Inc. Slide 8-82 QuickCheck 8.11 A roller coaster car does a loopthe-loop. Which of the free-body diagrams shows the forces on the car at the top of the loop? Rolling friction can be neglected. В© 2013 Pearson Education, Inc. The track is above the car, so the normal force of the track pushes down. Slide 8-83 Nonuniform Circular Motion: Review п‚§ The particle in the figure is speeding up as it moves around the circle. п‚§ The tangential acceleration is: п‚§ The centripetal acceleration is: ar = v2/r = П‰2r В© 2013 Pearson Education, Inc. Slide 8-84 Nonuniform Circular Motion: Review п‚§ In terms of angular quantities, the equations of constant-acceleration kinematics are: В© 2013 Pearson Education, Inc. Slide 8-85 Dynamics of Nonuniform Circular Motion п‚§ A net force is applied to a particle moving in a circle. п‚§ is likely to be a superposition of several forces, such as tension, thrust, friction, etc. п‚§ The tangential component of the net force (Fnet)t creates a tangential acceleration and causes the particle to change speed. п‚§ The radial component (Fnet)r is directed toward the center, creates a centripetal acceleration, and causes the particle to change direction. В© 2013 Pearson Education, Inc. Slide 8-86 Dynamics of Nonuniform Circular Motion п‚§ Force and acceleration are related through NewtonвЂ™s second law: В© 2013 Pearson Education, Inc. Slide 8-87 QuickCheck 8.12 A ball rolls ccw around the inside of a horizontal pipe. The ball is fastest at the lowest point, slowest at the highest point. At the point shown, with the ball moving down, what is the direction of the net force on the ball? В© 2013 Pearson Education, Inc. Slide 8-90 QuickCheck 8.12 A ball rolls ccw around the inside of a horizontal pipe. The ball is fastest at the lowest point, slowest at the highest point. At the point shown, with the ball moving down, what is the direction of the net force on the ball? В© 2013 Pearson Education, Inc. Slide 8-91

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