DOCSLIB.ORG

Version 001 – Rolling Part I – Smith – (3102F16B1)

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Version 001 – Rolling Part I – Smith – (3102F16B1) Version001–RollingPartI–smith–(3102F16B1) 1 This print-out should have 28 questions. ∆v =7m/s , and Multiple-choice questions may continue on ∆t =3.4 s . the next column or page – find all choices before answering. AP M 1998 MC 6 v 001 10.0 points A wheel of mass M and radius R rolls on a ω level surface without slipping. If the angular velocity of the wheel about B its center is ω, what is its linear momentum Consider point B at the bottom of the relative to the surface? wheel. It is at rest since there is no slip- ping. Its velocity 2 1. p = MωR 2. p =0 vB = v − ωR =0 v ω = Mω2 R2 R 3. p = 2 2 4. p = Mω R ∆ω 1 ∆v α = = ∆t R ∆t 5. p = MωR correct 1 7m/s 2 = = 7.62527 rad/s . Explanation: 0.27 m 3.4 s First, we note that the wheel is rotating about its center at an angular velocity of ω, 003 (part 2 of 3) 10.0 points so the velocity difference between the center Through what angle does the wheel turn in of the wheel and the lowest point (the point of the 3.4 s? the wheel that touches the surface) is ωR in magnitude in the horizontal direction. Since Correct answer: 44.0741 rad. the wheel rolls without slipping (which means Explanation: the velocity of the lowest point is 0) the ve- locity v of the center of the wheel is ωR. 1 2 1 2 2 Obviously, the center of the wheel is also the θ = αt = (7.62527 rad/s )(3.4 s) center of the mass, so the linear momentum is 2 2 p = MωR. = 44.0741 rad . Bicycle Down a Hill 002 (part 1 of 3) 10.0 points 004 (part 3 of 3) 10.0 points A bicycle wheel of radius 0.27 m rolls down How many revolutions does it make? a hill without slipping. Its linear velocity increases constantly from 0 to 7 m/sin3.4 s. Correct answer: 7.01461 rev. What is its angular acceleration? Explanation: 2 1 rev =2 π rad, so Correct answer: 7.62527 rad/s . Explanation: 1 rev θ = 44.0741 rad · = 7.01461 rev . 2 π rad Let : R =0.27 m , Version001–RollingPartI–smith–(3102F16B1) 2 keywords: Explanation: If you roll them down an incline, the solid Concept 08 14 ball will reach the bottom first. (The hol- 005 10.0 points low ball has more rotational inertia compared Which will have the greater acceleration with its weight.) rolling down an incline: a bowling ball or a volleyball, and why? KE Ratio 01 007 10.0 points 1. the volleyball; mass A rigid body of radius R and mass M is rolling on a level floor without slipping. Suppose 2 2. no difference Icm = κMR . We can consider the point where the body toches the floor as a pivot, 3. the bowling ball; rotational inertia cor- and calculate the rotational kinetic energy rect Kr,p around that pivot. We usually define a kinetic energy that consists of the rotational 4. the volleyball; rotational inertia kinetic energy of the body around its center of mass, plus the kinetic energy of its center 5. the bowling ball; mass of mass motion; that is, K = Kr,cm + Kcm. Explanation: The bowling ball wins. A solid sphere of any Kr,p K mass and size beats both a solid cylinder and a hollow ball of any mass and size, because b b vcm a solid sphere has less rotational inertia per mass than the other shapes. A solid sphere b has the bulk of its mass nearer the rotational axis that extends through its center of mass, whereas a cylinder or hollow ball has more of What is the ratio of Kr,p to K, in general? its mass farther from the axis. The object with the least rotational inertia per mass is 1. κR the least lazy and will win races. 2. (κ + 1) by the parallel axis theorem Concept 08 15 006 10.0 points 3. Zero What technique would help you to distinguish between two heavy identical-looking spheres 4. (κ − 1. of the same weight, one solid and the other 2 1 hollow? 5. κ 1. Hit them and compare their reflective 6. 1.0 correct sounds. 2 7. κ 2. Plunge them into water and compare their 1 densities. 8. κ κ 3. Launch them and compare their trajecto- 9. ries. R 10. κ 4. Roll them down an incline and compare their speeds. correct Version001–RollingPartI–smith–(3102F16B1) 3 Explanation: Both K and K are standard representa- r,p b tions of the total kinetic energy of a rigid body F K c rolling with slipping, so r,p =1.0. K A B This is easy to see from the parallel axis f P 1 2 2 theorem: K = (κ + 1) MR ω . But r,p 2 Choose either A or P as the pivoting point. vcm = Rω for rolling without slipping, so Choose A: the torque about A is τ = f(c − b) 1 2 2 1 2 clockwise and the spool will rotate clockwise. K = κMR ω + M v r,p 2 2 cm Choose P : the torque about P is τ = F (c − b) 1 2 2 clockwise and spool will still rotate clockwise. = (κ + 1) MR ω . 2 The agreement in the direction between these two different points of view actually tells us that our choice of the direction of f is correct. Pulling a Spool 008 (part 1 of 2) 10.0 points Consider a spool with inner radius b and outer radius c. A string wraps around the inner 009 (part 2 of 2) 10.0 points stem in a counterclockwise manner. Starting Choose the correct set of equations that gov- from rest, the spool rolls when the string is ern the motion of the spool. Here f is the pulled horizontally along AB as shown below. magnitude of the frictional force between the The force F is applied in such way that there spool and the horizontal surface, a is the lin- is no slippage. ear acceleration of the center of mass, and I is the moment of inertia about the center of mass of the spool. The torque will be evalu- b ated with respect to the center of the spool. c F Be careful about the relative signs. A B 1. Fx : F + f = ma X a Determine the direction of rotation of the τ : f c − F b = I spool. X b 2. Fx : F − f = ma 1. counterclockwise X a τ : f c − F b = I correct X c 2. Cannot be determined 3. Fx : F − f = ma X 3. clockwise correct τ : f c − F b = Ia Explanation: X 4. Fx : F − f = ma X a Fnet = ma, and (1) τ : f c − F b = I a X b τnet = Iα = I , (2) r 5. Fx : F + f = ma X Under the action of force F , the spool tends τ : f c − F b = Ia to accelerate linearly to the right, and to ro- X tate counterclockwise. Therefore the contact 6. Fx : F − f = ma point (presently at P , see the picture below), X a τ : f c + F b = I will have a tendency to move to the right c against the surface. In turn, the friction force X must point to the left. Explanation: Version001–RollingPartI–smith–(3102F16B1) 4 Using equations (1) and (2), into the paper, and in case Q, there is no torque, so the spool in case P rolls to the Fnet = F − f = ma, right, while the spool in case Q does not roll, but instead slides along the table. and for clockwise torque about the center of mass Ramp Race a τnet = f c − Fb = Iα = I , 011 10.0 points c Consider four round bodies of the same mass a where we use the relation α = for rolling 9.7 kg and the same radius 0.34 m but of dif- c ferent geometries – a solid sphere, a spherical without slipping. So the correct pair of equa- shell, a solid cylinder, and a cylindrical shell. tions is All four bodies start with zero initial veloc- ity at the same distance from the bottom of Fx : F − f = ma X the incline and roll without slipping down the Ia same inclined plane. τ : f c − Fb = . Which body reaches the bottom first? X c 1. Depends on the steepness of the incline. keywords: 2. The spherical shell. Pulling a Spool 02 010 10.0 points 3. The solid sphere. correct Consider the two orientations for pulling on a string wrapped around the axle of a giant 4. All four take the same time. transparent spool, causing it to move across the table. 5. The solid cylinder. P Q 6. The cylindrical shell. What rolling motion is observed for P and Explanation: for Q, respectively? Consider a round body rolling down an in- clined plane. 1. To the right; not at all, it skids without rolling. correct 2. Not at all, it skids without rolling; to the left. 3. Both skid without rolling to the right. mg x 4. To the left; to the left. N f 5. To the right; to the right. The forward motion of the body is governed by Newton’s Second Law 6. To the left; to the right. Explanation: Take the pivot as the point where the spool net touches the table.
Load more

Recommended publications
  • Unit 1: Motion
    Macomb Intermediate School District High School Science Power Standards Document Physics The Michigan High School Science Content Expectations establish what every student is expected to know and be able to do by the end of high school. They also outline the parameters for receiving high school credit as dictated by state law. To aid teachers and administrators in meeting these expectations the Macomb ISD has undertaken the task of identifying those content expectations which can be considered power standards. The critical characteristics1 for selecting a power standard are: • Endurance – knowledge and skills of value beyond a single test date. • Leverage - knowledge and skills that will be of value in multiple disciplines. • Readiness - knowledge and skills necessary for the next level of learning. The selection of power standards is not intended to relieve teachers of the responsibility for teaching all content expectations. Rather, it gives the school district a common focus and acts as a safety net of standards that all students must learn prior to leaving their current level. The following document utilizes the unit design including the big ideas and real world contexts, as developed in the science companion documents for the Michigan High School Science Content Expectations. 1 Dr. Douglas Reeves, Center for Performance Assessment Unit 1: Motion Big Ideas The motion of an object may be described using a) motion diagrams, b) data, c) graphs, and d) mathematical functions. Conceptual Understandings A comparison can be made of the motion of a person attempting to walk at a constant velocity down a sidewalk to the motion of a person attempting to walk in a straight line with a constant acceleration.
    [Show full text]
  • Newton's Second
    Newton's Second Law INTRODUCTION Sir Isaac Newton1 put forth many important ideas in his famous book The Principia. His three laws of motion are the best known of these. The first law seems to be at odds with our everyday experience. Newton's first law states that any object at rest that is not acted upon by outside forces will remain at rest, and that any object in motion not acted upon by outside forces will continue its motion in a straight line at a constant velocity. If we roll a ball across the floor, we know that it will eventually come to a stop, seemingly contradicting the First Law. Our experience seems to agree with Aristotle's2 idea, that the \impetus"3 given to the ball is used up as it rolls. But Aristotle was wrong, as is our first impression of the ball's motion. The key is that the ball does experience an outside force, i.e., friction, as it rolls across the floor. This force causes the ball to decelerate (that is, it has a \negative" acceleration). According to Newton's second law an object will accelerate in the direction of the net force. Since the force of friction is opposite to the direction of travel, this acceleration causes the object to slow its forward motion, and eventually stop. The purpose of this laboratory exercise is to verify Newton's second law. DISCUSSION OF PRINCIPLES Newton's second law in vector form is X F~ = m~a or F~net = m~a (1) This force causes the ball rolling on the floor to decelerate (that is, it has a \negative" accelera- tion).
    [Show full text]
  • Rolling Rolling Condition for Rolling Without Slipping
    Rolling Rolling simulation We can view rolling motion as a superposition of pure rotation and pure translation. Pure rotation Pure translation Rolling +rω +rω v Rolling ω ω v -rω +=v -rω v For rolling without slipping, the net instantaneous velocity at the bottom of the wheel is zero. To achieve this condition, 0 = vnet = translational velocity + tangential velocity due to rotation. In other wards, v – rω = 0. When v = rω (i.e., rolling without slipping applies), the tangential velocity at the top of the wheel is twice the 1 translational velocity of the wheel (= v + rω = 2v). 2 Condition for Rolling Without Big yo-yo Slipping A large yo-yo stands on a table. A rope wrapped around the yo-yo's axle, which has a radius that’s half that of When a disc is rolling without slipping, the bottom the yo-yo, is pulled horizontally to the right, with the of the wheel is always at rest instantaneously. rope coming off the yo-yo above the axle. In which This leads to ω = v/r and α = a/r direction does the yo-yo move? There is friction between the table and the yo-yo. Suppose the yo-yo where v is the translational velocity and a is is pulled slowly enough that the yo-yo does not slip acceleration of the center of mass of the disc. on the table as it rolls. rω 1. to the right ω 2. to the left v 3. it won't move -rω X 3 4 Vnet at this point = v – rω Big yo-yo Big yo-yo, again Since the yo-yo rolls without slipping, the center of mass The situation is repeated but with the rope coming off the velocity of the yo-yo must satisfy, vcm = rω.
    [Show full text]
  • Net Force Is Zero)
    Force & Motion Motion Motion is a change in the position of an object • Caused by force (a push or pull) Force A force is a a push or pull on an object • Measured in units called newtons (N) • Forces act in pairs Examples of Force: Ø Gravity Ø Magnetic Ø Friction Ø Centripetal Inertia Inertia is the tendency of an object to keep doing what it is doing • An object at rest will remain at rest until acted upon by an unbalanced force. • An object in motion will remain in motion until acted upon by an unbalanced force. Balanced Forces A balanced force is when all the forces acting on an object are equal (net force is zero) • Balanced forces do not cause a change in motion. How Can Balanced Forces Affect Objects? • Cause the shape of an object to change without changing its motion • Cause an object at rest to stay at rest or an object in motion to stay in motion (inertia) • Cause an object moving at a constant speed to continue at a constant speed What is an example of a balanced force? Unbalanced Forces An unbalanced force is when all the forces acting on an object are not equal • The forces can be in the same direction or in opposite directions. • Unbalanced forces cause a change in motion. How Can Unbalanced Forces Affect Objects? • Acceleration is caused by unbalanced forces: – slow down – speed up – stop – start – change direction – change shape What is an example of a unbalanced force? Net Force The Net force is he total of all forces acting on an object: – Forces in the same direction are added.
    [Show full text]
  • 1 Vectors: Geometric Approach
    c F. Waleffe, 2008/09/01 Vectors These are compact lecture notes for Math 321 at UW-Madison. Read them carefully, ideally before the lecture, and complete with your own class notes and pictures. Skipping the `theory' and jumping directly to the exercises is a tried-and-failed strategy that only leads to the typical question `I have no idea how to get started'. There are many explicit and implicit exercises within the text that complement the `theory'. Many of the `proofs' are actually good `solved exercises.' The objectives are to review the key concepts, emphasizing geometric understanding and visualization. 1 Vectors: Geometric Approach What's a vector? in elementary calculus and linear algebra you probably defined vectors as a list of numbers such as ~x = (4; 2; 5) with special algebraic manipulations rules, but in elementary physics vectors were probably defined as `quantities that have both a magnitude and a direction such as displacements, velocities and forces' as opposed to scalars, such as mass, temperature and pressure, which only have magnitude. We begin with the latter point of view because the algebraic version hides geometric issues that are important to physics, namely that physical laws are invariant under a change of coordinates - they do not depend on our particular choice of coordinates - and there is no special system of coordinates, everything is relative. Our other motivation is that to truly understand vectors, and math in general, you have to be able to visualize the concepts, so rather than developing the geometric interpretation as an after-thought, we start with it.
    [Show full text]
  • 5 | Newton's Laws of Motion 207 5 | NEWTON's LAWS of MOTION
    Chapter 5 | Newton's Laws of Motion 207 5 | NEWTON'S LAWS OF MOTION Figure 5.1 The Golden Gate Bridge, one of the greatest works of modern engineering, was the longest suspension bridge in the world in the year it opened, 1937. It is still among the 10 longest suspension bridges as of this writing. In designing and building a bridge, what physics must we consider? What forces act on the bridge? What forces keep the bridge from falling? How do the towers, cables, and ground interact to maintain stability? Chapter Outline 5.1 Forces 5.2 Newton's First Law 5.3 Newton's Second Law 5.4 Mass and Weight 5.5 Newton’s Third Law 5.6 Common Forces 5.7 Drawing Free-Body Diagrams Introduction When you drive across a bridge, you expect it to remain stable. You also expect to speed up or slow your car in response to traffic changes. In both cases, you deal with forces. The forces on the bridge are in equilibrium, so it stays in place. In contrast, the force produced by your car engine causes a change in motion. Isaac Newton discovered the laws of motion that describe these situations. Forces affect every moment of your life. Your body is held to Earth by force and held together by the forces of charged particles. When you open a door, walk down a street, lift your fork, or touch a baby’s face, you are applying forces. Zooming in deeper, your body’s atoms are held together by electrical forces, and the core of the atom, called the nucleus, is held together by the strongest force we know—strong nuclear force.
    [Show full text]
  • Short Answers
    PHYS 185 Practice Final Exam Fall 2013 Name: You may answer the questions in the space provided here, or if you prefer, on your own notebook paper. Short answers 1. 2 points When an object is immersed in a liquid at rest, why is the net force on the object in the horizontal direction equal to zero? Solution: The pressure is the same on points that are at the same level but on opposite sides. 2. 2 points How would you determine the density of an irregularly shaped rock? Solution: Weigh it in air then weigh it in water. The difference in those weights is due to buoyancy. That buoyancy tells you the weight of a volume of water displaced by the rock whose volume is the same as the rock. Since you know the density of 3 water is ρw = 1000 kg/m , B = ρwgV and solve for V . Divide the weight you got for m the rock in air by 9.8 s2 to get the mass, and then divide the mass by the volume you figured out previously. This is the same as the golden crown problem we saw in class. 3. 2 points Can an object be in equilibrium if it is in motion? Explain. Solution: Yes, the condition of equilibrium is the all of the net forces and all of the net torques is equal to zero. If an object is in motion at a constant speed and constant direction, and/or is rotating at a constant rate, then that means there is no linear or angular acceleration which implies that the net force and net torque are equal to zero{which implies equilibrium.
    [Show full text]
  • Forces and Acceleration Review
    Forces, Motion, and Acceleration Investigation 5, 7, and 9 LT ––– 1 ––– I can use data to show what happens when a constantconstant net force is applied to an object. 1. What would a graph showing balanced forces look like? Draw a position vs. time Graph and a velocity vs. time graph that show balanced forces . (Constant motion) When forces are balanced, the motion is constant so the position vs. time graph can be drawn with a horizontal line (standing still) or with a straight diagonal line (constant motion). The velocity vs. time graph can be only a horizontal straight line (constant velocity). 2. What would a graph showing unbalanced forces (net force) look like? Draw a Position vs. time graph and a velocity vs. time graph that shows unbalanced forces.orces. The position vs. time graph should begin at zero and curve up to the right. (speeding up). The velocity vs. time graph should be a straight diagonal line going up to the right (speeding up). Your graphs could also show slowing down and be “unbalanced forces”. 3. Draw a sketch of a position vs. time graph and a velocity vs. time graph that shows the motion of a wheeled cart when a constant net force is applied. These two graphs will look just like the graphs you sketched in number 2. 4. Assume that the graphs you just sketched represent the motion of a wheeled cart that has one fan providing the constant net force. On the graphs below, draw three lines: one representing a one-fan cart, one representing a two-fan cart, and one representing a three-fan cart.
    [Show full text]
  • Simple Harmonic Motion and Springs
    Name Date AP Physics 1 Simple Harmonic Motion and Springs Hookean Spring Simple Harmonic Motion of Spring F kx m S T 2 W U 1 k(x2 ) k S S 2 x Acos(t) Acos(2ft) F kx a m m 1. What are the two criteria for simple harmonic motion? - Only restoring forces cause simple harmonic motion. A restoring force is a force that it proportional to the displacement from equilibrium and in the opposite direction. - Position, velocity and the other variables of simple harmonic motion are sinusoidal functions of time. 2. The diagram to the right shows a 2 kg block attached to a Hookean spring on a frictionless surface. The block experiences no net force when it is at position B. When the block is to the left of point B the spring pushes it to the right. When the block is to the right of point B, the spring pulls it to the left. The mass is pulled to the right from point B to point C and released at time t = 0. The block then oscillates between positions A and C. Assume that the system consists of the block and the spring and that no dissipative forces act. a) The block takes 40.0 s to make 20 oscillations. What is the “period of oscillation” for this system? T = 40s/20 oscillations = 2 sec/oscillation b) What is the frequency of this oscillating system? f = 1/T = ½ = 0.5 Hz c) What is the amplitude of vibration of this system? A = 0.2 m d) Determine the spring constant of the spring.
    [Show full text]
  • 15. Parallel Axis Theorem and Torque A) Overview B) Parallel Axis Theorem
    15. Parallel Axis Theorem and Torque A) Overview In this unit we will continue our study of rotational motion. In particular, we will first prove a very useful theorem that relates moments of inertia about parallel axes. We will then move on to develop the equation that determines the dynamics for rotational motion. In so doing, we will introduce a new quantity, the torque, that plays the role for rotational motion that force does for translational motion. We will find, once again, that the rotational analog of mass will be the moment of inertia. B) Parallel Axis Theorem We have shown earlier that the total kinetic energy of a system of particles in any reference frame is equal to the kinetic energy of the center of mass of the system, defined to be one half times the total mass times the square of the center of mass velocity, plus the kinetic energy of the motion of all of the parts relative to the center of mass. = + K system ,lab K REL KCM For a solid object the only possible motion relative to the center of mass is rotation. Therefore, the kinetic energy relative to the center of mass is just equal to one half times the moment of inertia about a rotation axis through the center of mass times the square of the angular velocity about the center of mass. = 1 ω 2 + 1 2 K system ,lab 2 ICM CM 2 Mv CM We can use this result to calculate the moment of inertia about a chosen axis if the moment of inertia about a parallel axis that passes through the center of mass is known.
    [Show full text]
  • Forces in One Dimension Vocabulary - Section 1
    Forces in One Dimension Vocabulary - Section 1 ▪ Force • Equilibrium ▪ Balanced forces • Newton’s first law ▪ Unbalanced forces • Newton’s second law ▪ Free body diagrams • Newton’s third law ▪ Net force • system ▪ Contact force ▪ Field force ▪ Inertia Force and motion ▪ In physics, a force is a push or pull ▪ Unbalanced forces change motion ▪ Objects at rest, or not moving, still have forces acting upon them. These are known as balanced forces ▪ Contact forces – forces that directly touch the object (ex. Your hand pushing an object or simply holding an object) ▪ Field forces – forces that are exerted without contact. (ex. Gravity) Force and Motion ▪ Free body diagrams – a pictorial representation of forces acting on an object – the object is depicted by a dot – Each force is represented by a vector (arrow) showing the direction of the force on the object – The length of the vector must be proportionate to the force acting on the object (use the magnitude of the vector) – Each force must be labeled. Ex. Draw a Free body diagram of a person holding a ball in their hand: Fhand on ball Fgravity Force and motion ▪ Forces are measured in Newtons ▪ 1 N = kgㆍm/s2 ▪ The force exerted by an apple on your hand is approximately 1 Newton. Force and Motion ▪ Net force – the vector sum of the forces acting on an object – To calculate net force: ▪ If forces are being applied in the same direction, then add forces together ▪ If forces are being applied in opposite directions, then subtract forces. Newton’s 2nd law ▪ Newton’s 2nd law states the force is directly proportional to the mass of an object and its acceleration.
    [Show full text]
  • MOMENT of a COUPLE Today’S Objectives: Students Will Be Able to A) Define a Couple, And, In-Class Activities: B) Determine the Moment of a Couple
    ENGR-1100 Introduction to Engineering Analysis Lecture 11 MOMENT OF A COUPLE Today’s Objectives: Students will be able to a) define a couple, and, In-Class activities: b) determine the moment of a couple. • Reading Quiz • Applications • Moment of a Couple • Concept Quiz • Group Problem Solving • Attention Quiz APPLICATIONS A torque or moment of 12 N·m is required to rotate the wheel. Why does one of the two grips of the wheel above require less force to rotate the wheel? APPLICATIONS (continued) When you grip a vehicle’s steering wheel with both hands and turn, a couple moment is applied to the wheel. Would older vehicles without power steering need larger or smaller steering wheels? MOMENT OF A COUPLE A couple is defined as two parallel forces with the same magnitude but opposite in direction separated by a perpendicular distance “d.” The moment of a couple is defined as MO = F d (using a scalar analysis) or as MO = r F (using a vector analysis). Here r is any position vector from the line of action of F to the line of action of F. MOMENT OF A COUPLE (continued) The net external effect of a couple is that the net force equals zero and the magnitude of the net moment equals F *d. Since the moment of a couple depends only on the distance between the forces, the moment of a couple is a free vector. It can be moved anywhere on the body and have the same external effect on the body. Moments due to couples can be added together using the same rules as adding any vectors.
    [Show full text]