Dynamics of Uniform Circular Motion

1. Dynamics of Uniform Circular Motion Chapter 5

2. Learning Objectives- Circular motion and rotation Uniform circular motion • Students should understand the uniform circular motion of a particle, so they can: • Relate the radius of the circle and the speed or rate of revolution of the particle to the magnitude of the centripetal acceleration. • Describe the direction of the particle’s velocity and acceleration at any instant during the motion. • Determine the components of the velocity and acceleration vectors at any instant, and sketch or identify graphs of these quantities. • Analyze situations in which an object moves with specified acceleration under the influence of one or more forces so they can determine the magnitude and direction of the net force, or of one of the forces that makes up the net force, in situations such as the following: • Motion in a horizontal circle (e.g., mass on a rotating merry-go-round, or car rounding a banked curve). • Motion in a vertical circle (e.g., mass swinging on the end of a string, cart rolling down a curved track, rider on a Ferris wheel).

3. Table Of Contents 5.1 Uniform Circular Motion 5.2 Centripetal Acceleration 5.3 Centripetal Force 5.4 Banked Curves 5.5 Satellites in Circular Orbits 5.6 Apparent Weightlessness and Artificial Gravity 5.7 Vertical Circular Motion

4. Chapter 5:Dynamics of Uniform Circular Motion Section 1: Uniform Circular Motion

5. Other Effects of Forces • Up until now, we’ve focused on forces that speed up or slow down an object. • We will now look at the third effect of a force • Turning • We need some other equations as the object will be accelerating without necessarily changing speed.

6. DEFINITION OF UNIFORM CIRCULAR MOTION Uniform circular motion is the motion of an object traveling at a constant speed on a circular path.

7. Let T be the time it takes for the object to travel once around the circle.

8. Example 1: A Tire-Balancing Machine The wheel of a car has a radius of 0.29m and it being rotated at 830 revolutions per minute on a tire-balancing machine. Determine the speed at which the outer edge of the wheel is moving.

9. Newton’s Laws • 1st • When objects move along a straight line the sideways/perpendicular forces must be balanced. • 2nd • When the forces directed perpendicular to velocity become unbalanced the object will curve. • 3rd • The force that pulls inward on the object, causing it to curve off line provides the action force that is centripetal in nature. The object will in return create a reaction force that is centrifugal in nature.

10. 5.1.1. An airplane flying at 115 m/s due east makes a gradual turn while maintaining its speed and follows a circular path to fly south. The turn takes 15 seconds to complete. What is the radius of the circular path? a) 410 m b) 830 m c) 1100 m d) 1600 m e) 2200 m

11. Chapter 5:Dynamics of Uniform Circular Motion Section 2: Centripetal Acceleration

12. In uniform circular motion, the speed is constant, but the direction of the velocity vector is not constant.

14. The direction of the centripetal acceleration is towards the center of the circle; in the same direction as the change in velocity.

15. Conceptual Example 2: Which Way Will the Object Go? An object is in uniform circular motion. At point O it is released from its circular path. Does the object move along the straight path between O and A or along the circular arc between points O and P ? Straight path

16. Example 3: The Effect of Radius on Centripetal Acceleration The bobsled track contains turns with radii of 33 m and 24 m. Find the centripetal acceleration at each turn for a speed of 34 m/s. Express answers as multiples of

17. 5.2.1. A ball is whirled on the end of a string in a horizontal circle of radius R at constant speed v. By which one of the following means can the centripetal acceleration of the ball be increased by a factor of two? a) Keep the radius fixed and increase the period by a factor of two. b) Keep the radius fixed and decrease the period by a factor of two. c) Keep the speed fixed and increase the radius by a factor of two. d) Keep the speed fixed and decrease the radius by a factor of two. e) Keep the radius fixed and increase the speed by a factor of two.

18. 5.2.2. A steel ball is whirled on the end of a chain in a horizontal circle of radius R with a constant period T. If the radius of the circle is then reduced to 0.75R, while the period remains T, what happens to the centripetal acceleration of the ball? a) The centripetal acceleration increases to 1.33 times its initial value. b) The centripetal acceleration increases to 1.78 times its initial value. c) The centripetal acceleration decreases to 0.75 times its initial value. d) The centripetal acceleration decreases to 0.56 times its initial value. e) The centripetal acceleration does not change.

19. 5.2.3. While we are in this classroom, the Earth is orbiting the Sun in an orbit that is nearly circular with an average radius of 1.50 × 1011 m. Assuming that the Earth is in uniform circular motion, what is the centripetal acceleration of the Earth in its orbit around the Sun? a) 5.9 × 103 m/s2 b) 1.9 × 105 m/s2 c) 3.2 × 107 m/s2 d) 7.0 × 102 m/s2 e) 9.8 m/s2

20. 5.2.4. A truck is traveling with a constant speed of 15 m/s. When the truck follows a curve in the road, its centripetal acceleration is 4.0 m/s2. What is the radius of the curve? a) 3.8 m b) 14 m c) 56 m d) 120 m e) 210 m

21. 5.2.5. Consider the following situations: (i) A minivan is following a hairpin turn on a mountain road at a constant speed of twenty miles per hour. (ii) A parachutist is descending at a constant speed 10 m/s. (iii) A heavy crate has been given a quick shove and is now sliding across the floor. (iv) Jenny is swinging back and forth on a swing at the park. (v) A football that was kicked is flying through the goal posts. (vi) A plucked guitar string vibrates at a constant frequency. In which one of these situations does the object or person experience zero acceleration? a) i only b) ii only c) iii and iv only d) iv, v, and vi only e) all of the situations

22. Chapter 5:Dynamics of Uniform Circular Motion Section 3: Centripetal Force

23. Recall Newton’s Second Law When a net external force acts on an object of mass m, the acceleration that results is directly proportional to the net force and has a magnitude that is inversely proportional to the mass. The direction of the acceleration is the same as the direction of the net force.

24. Recall Newton’s Second Law • Thus, in uniform circular motion there must be a net force to produce the centripetal acceleration. • The centripetal force is the name given to the net force required to keep an object moving on a circular path. • The direction of the centripetal force always points toward the center of the circle and continually changes direction as the object moves.

25.  Problem Solving Strategy – Horizontal Circles R • Draw a free-body diagram of the curving object(s). • Choose a coordinate system with the following two axes. a) One axis will point inward along the radius (inward is positive direction). b) One axis will point perpendicular to the circular path (up is positive direction). • Sum the forces along each axis to get two equations for two unknowns. a)  FRADIUS: +FIN FOUT = m(v2)/ r b)  F : FUP FDOWN = 0 • Do the math of two equations with two unknowns.

26.  Just in case… tan R • The third dimension in these problems would be a direction tangent to the circle and in the plane of the circle. • We choose to ignore this direction for objects moving at constant speed. • If an object moves along the circle with changing speed then the forces tangent to the circle have become unbalanced. • You can sum the tangential forces to find the rate at which speed changes with time, aTAN. • The linear kinematics equations can then be used to describe motion along or tangent to the circle. •  FTAN: FFORWARD FBACKWARD = m aTAN

27. Example 5: The Effect of Speed on Centripetal Force The model airplane has a mass of 0.90 kg and moves at constant speed on a circle that is parallel to the ground. The path of the airplane and the guideline lie in the same horizontal plane because the weight of the plane is balanced by the lift generated by its wings. Find the tension in the 17 m guideline for a speed of 19 m/s.

28. 5.3.1. A boy is whirling a stone at the end of a string around his head. The string makes one complete revolution every second, and the tension in the string is FT. The boy increases the speed of the stone, keeping the radius of the circle unchanged, so that the string makes two complete revolutions per second. What happens to the tension in the sting? a) The tension increases to four times its original value. b) The tension increases to twice its original value. c) The tension is unchanged. d) The tension is reduced to one half of its original value. e) The tension is reduced to one fourth of its original value.

29. 5.3.2. An aluminum rod is designed to break when it is under a tension of 600 N. One end of the rod is connected to a motor and a 12-kg spherical object is attached to the other end. When the motor is turned on, the object moves in a horizontal circle with a radius of 6.0 m. If the speed of the motor is continuously increased, at what speed will the rod break? Ignore the mass of the rod for this calculation. a) 11 m/s b) 17 m/s c) 34 m/s d) 88 m/s e) 3.0 × 102 m/s

30. 5.3.3. A ball is attached to a string and whirled in a horizontal circle. The ball is moving in uniform circular motion when the string separates from the ball (the knot wasn’t very tight). Which one of the following statements best describes the subsequent motion of the ball? a) The ball immediately flies in the direction radially outward from the center of the circular path the ball had been following. b) The ball continues to follow the circular path for a short time, but then it gradually falls away. c) The ball gradually curves away from the circular path it had been following. d) The ball immediately follows a linear path away from, but not tangent to the circular path it had been following. e) The ball immediately follows a line that is tangent to the circular path the ball had been following

31. 5.3.4. A rancher puts a hay bail into the back of her SUV. Later, she drives around an unbanked curve with a radius of 48 m at a speed of 16 m/s. What is the minimum coefficient of static friction for the hay bail on the floor of the SUV so that the hay bail does not slide while on the curve? a) This cannot be determined without knowing the mass of the hay bail. b) 0.17 c) 0.33 d) 0.42 e) 0.54

32. 5.3.5. Imagine you are swinging a bucket by the handle around in a circle that is nearly level with the ground (a horizontal circle). What is the force, the physical force, holding the bucket in a circular path? a) the centripetal force b) the centrifugal force c) your hand on the handle d) gravitational force e) None of the above are correct.

33. 5.3.6. Imagine you are swinging a bucket by the handle around in a circle that is nearly level with the ground (a horizontal circle). Now imagine there's a ball in the bucket. What keeps the ball moving in a circular path? a) contact force of the bucket on the ball b) contact force of the ball on the bucket c) gravitational force on the ball d) the centripetal force e) the centrifugal force

34. 5.3.7. The moon, which is approximately 4 × 109 m from Earth, has a mass of 7.4 × 1022 kg and a period of 27.3 days. What must is the magnitude of the gravitational force between the Earth and the moon? a) 1.8 × 1018 N b) 2.1 × 1022 N c) 1.7 × 1013 N d) 5.0 × 1022 N e) 4.2 × 1020 N

35. 5.3.8. The Rapid Rotor amusement ride is spinning fast enough that the floor beneath the rider drops away and the rider remains in place. If the Rotor speeds up until it is going twice as fast as it was previously, what is the effect on the frictional force on the rider? a) The frictional force is reduced to one-fourth of its previous value. b) The frictional force is the same as its previous value. c) The frictional force is reduced to one-half of its previous value. d) The frictional force is increased to twice its previous value. e) The frictional force is increased to four times its previous value.

36. Chapter 5:Dynamics of Uniform Circular Motion Section 4: Banked Curves

37. Unbanked curve On an unbanked curve, the static frictional force provides the centripetal force.

38. Banked Curve On a frictionless banked curve, the centripetal force is the horizontal component of the normal force. The vertical component of the normal force balances the car’s weight.

40. Example 8: The Daytona 500 The turns at the Daytona International Speedway have a maximum radius of 316 m and are steely banked at 31 degrees. Suppose these turns were frictionless. As what speed would the cars have to travel around them?

41. 5.4.1. Complete the following statement: The maximum speed at which a car can safely negotiate an unbanked curve depends on all of the following factors except a) the coefficient of kinetic friction between the road and the tires. b) the coefficient of static friction between the road and the tires. c) the acceleration due to gravity. d) the diameter of the curve. e) the ratio of the static frictional force between the road and the tires and the normal force exerted on the car.

42. 5.4.2. A 1000-kg car travels along a straight portion of highway at a constant velocity of 10 m/s, due east. The car then encounters an unbanked curve of radius 50 m. The car follows the curve traveling at a constant speed of 10 m/s while the direction of the car changes from east to south. What is the magnitude of the acceleration of the car as it travels the unbanked curve? a) zero m/s2 b) 2 m/s2 c) 5 m/s2 d) 10 m/s2 e) 20 m/s2

43. 5.4.3. A 1000-kg car travels along a straight portion of highway at a constant velocity of 10 m/s, due east. The car then encounters an unbanked curve of radius 50 m. The car follows the curve traveling at a constant speed of 10 m/s while the direction of the car changes from east to south. What is the magnitude of the frictional force between the tires and the road as the car negotiates the unbanked curve? a) 500 N b) 1000 N c) 2000 N d) 5000 N e) 10 000 N

44. 5.4.4. You are riding in the forward passenger seat of a car as it travels along a straight portion of highway. The car continues traveling at a constant speed as it follows a sharp, unbanked curve to the left. You feel the door pushing on the right side of your body. Which of the following forces in the horizontal direction are acting on you? a) a static frictional force between you and the seat b) a normal force of the door c) a force pushing you toward the door d) answers a and b e) answers a and c

45. Chapter 5:Dynamics of Uniform Circular Motion Section 5: Satellites in Circular Orbits

46. Don’t worry, it’s only rocket science There is only one speed that a satellite can have if the satellite is to remain in an orbit with a fixed radius.

48. Example 9: Orbital Speed of the Hubble Space Telescope Determine the speed of the Hubble Space Telescope orbiting at a height of 598 km above the earth’s surface.

49. Period to orbit the Earth

50. Geosynchronous Orbit