AP Physics Unit 7 Part 2 Rotational Motion

  • Slides: 66
Download presentation
AP Physics Unit 7, Part 2 Rotational Motion and Equilibrium

AP Physics Unit 7, Part 2 Rotational Motion and Equilibrium

Chapter 8: Rotational Motion and Equilibrium 8. 1 8. 2 8. 3 8. 4

Chapter 8: Rotational Motion and Equilibrium 8. 1 8. 2 8. 3 8. 4 8. 5 Rigid Bodies, Translation, and Rotation Torque and Equilibrium Rotational Dynamics Rotational Kinetic Energy Angular Momentum

Homework for Chapter 8 • Read Chapter 8. • HW 7. B: Torque AP

Homework for Chapter 8 • Read Chapter 8. • HW 7. B: Torque AP Classroom MCQ Part A: 4 -6, 10 -12, 16, 17, 22. MCQ Part B: 1. • HW 7. C: Angular Momentum AP Classroom MCQ Part A: 1 -3, 13 -15, 18, 21. MCQ Part B: 2 -8. • HW 7. D: Conservation of Angular Momentum AP Classroom MCQ Part B: 10 -13, 15 -16, 18 -22. Click “Submit” on Parts A & B.

8. 1: Rigid Bodies, Translation, and Rotation

8. 1: Rigid Bodies, Translation, and Rotation

8. 1 Rigid Bodies, Translation, and Rotation Please Do Now How do you think

8. 1 Rigid Bodies, Translation, and Rotation Please Do Now How do you think the tight-rope walkers are able to keep from falling? Write three lines using full sentences.

8. 1 Rigid Bodies, Translation, and Rotation rigid body – an object or system

8. 1 Rigid Bodies, Translation, and Rotation rigid body – an object or system of particles in which the distances between particles are fixed and remain constant. Solids can be considered to be rigid bodies for the purpose of analyzing rotational motion. Fluids (gas or liquid) are not considered to be rigid bodies. translational motion – every particle in the object has the same instantaneous velocity. There is no rotation. Write two examples. rotational motion – motion about a fixed axis. All the particles of an object have the same instantaneous angular velocity and travel in circles about the axis of rotation. Write two examples. rigid body motion – is usually a combination of translational and rotational motion. Write two examples.

8. 2: Torque and Equilibrium

8. 2: Torque and Equilibrium

8. 2 Torque and Equilibrium τ = r F sinϕ On Gold Sheet

8. 2 Torque and Equilibrium τ = r F sinϕ On Gold Sheet

8. 2 Torque and Equilibrium The sign convention for torques is as follows: -The

8. 2 Torque and Equilibrium The sign convention for torques is as follows: -The torque, which rotates or tends to rotate the body clockwise (CW), is negative. -The torque, which rotates or tends to rotate the body counterclockwise (CCW), is positive. • The SI unit of torque is the m·N (meter-newton).

8. 2 Torque and Equilibrium The perpendicular distance r┴ from the axis of rotation

8. 2 Torque and Equilibrium The perpendicular distance r┴ from the axis of rotation to the line of action of a force is called the moment arm (or lever arm) and is equal to r sin θ.

8. 2 Torque and Equilibrium The same force in the opposite direction with a

8. 2 Torque and Equilibrium The same force in the opposite direction with a smaller moment arm produces a smaller torque in the opposite direction.

8. 2 Torque and Equilibrium When a force acts through the axis of rotation,

8. 2 Torque and Equilibrium When a force acts through the axis of rotation, the torque is zero.

8. 2 Torque and Equilibrium Think of a real-world example of torque. Turn and

8. 2 Torque and Equilibrium Think of a real-world example of torque. Turn and talk to your shoulder partner.

8. 2 Torque and Equilibrium equilibrium – a state in which things are in

8. 2 Torque and Equilibrium equilibrium – a state in which things are in balance or are stable. translational equilibrium – the sum of the forces on a body is zero • the body remains at rest or at constant velocity • Fi = F 1 + F 2 + F 3 + …. = 0 rotational equilibrium – the sum of torques on a body is zero • the body is rotationally at rest or rotates with a constant angular velocity • τi = τ 1 + τ 2 + τ 3 + …. = 0 F F The board is in translational and rotational equilibrium. The board is in translational but not rotational equilibrium.

8. 2 Torque and Equilibrium mechanical equilibrium – when the conditions for translational and

8. 2 Torque and Equilibrium mechanical equilibrium – when the conditions for translational and rotational equilibrium are satisfied. Fi = 0 (for translational equilibrium) τi = 0 (for rotational equilibrium) static equilibrium – the condition that exists when a rigid body remains at rest What are some examples of static translational equilibrium? What are some examples of static rotational equilibrium? Problem-Solving Hint: Use the convention counter-clockwise (ccw) is positive and clockwise (cw) is negative.

8. 2 Torque and Equilibrium Exit Ticket Write at least (3) lines to answer

8. 2 Torque and Equilibrium Exit Ticket Write at least (3) lines to answer the question: How did Philippe Petit demonstrate torque, translational and rotational equilibrium? Turn in your ticket when you are done.

8. 2 Torque and Equilibrium Please Do Now Answer the question: What is torque?

8. 2 Torque and Equilibrium Please Do Now Answer the question: What is torque? Describe it in your own words.

8. 2 Torque and Equilibrium Example 8. 1: The bolts on a car wheel

8. 2 Torque and Equilibrium Example 8. 1: The bolts on a car wheel require tightening to a torque of 90 m·N. If a 20 cm long wrench is used, what is the magnitude of the force required a) when the force is perpendicular to the wrench, b) when the force is 35° to the wrench as shown. c) Why does it need more force in (b) than in (a)?

8. 2 Torque and Equilibrium Example 8. 2: A uniform board of weight 40

8. 2 Torque and Equilibrium Example 8. 2: A uniform board of weight 40 N supports two children weighing 500 N and 350 N, respectively. If the support is at the center of the board and the 500 N child is 1. 5 m from the center, what is the position of the 350 N child?

8. 2 Torque and Equilibrium Example 8. 3: A 10 m long uniform beam

8. 2 Torque and Equilibrium Example 8. 3: A 10 m long uniform beam weighing 100 N is supported by two ropes at the ends as shown. If a 400 N person sits at 2. 0 m from one end of the beam, what are the tensions in the ropes?

8. 2 Check for Understanding 1.

8. 2 Check for Understanding 1.

8. 2 Check for Understanding 1.

8. 2 Check for Understanding 1.

8. 2 Check for Understanding 2.

8. 2 Check for Understanding 2.

8. 2 Check for Understanding 2.

8. 2 Check for Understanding 2.

8. 2 Check for Understanding 3. (Hint: Use worker A as the pivot point.

8. 2 Check for Understanding 3. (Hint: Use worker A as the pivot point. )

8. 2 Check for Understanding 3. a

8. 2 Check for Understanding 3. a

8. 3 Rotational Dynamics

8. 3 Rotational Dynamics

Warmup: Torque Your Way In Physics Warmup # 71 An object rotates because a

Warmup: Torque Your Way In Physics Warmup # 71 An object rotates because a torque acts on it. When you exert a force at the end of a wrench in order to rotate it (along with the bolt it is attached to), you may not have thought about the fact that you applied the force perpendicular to the handle. The longer the wrench’s handle, the less force you have to apply, because the amount of torque is equal to the product of the force times the distance to the axis about which it is rotating. _________________________________ Use the relationship of torque to force and distance to explain the following two small mysteries in your kitchen: 1. Why is it easier to open a cabinet door when the doorknob is at the end of the door than when it is in the middle of the door? 2. Why is a big doorknob easier to turn than a small one?

8. 3 Rotational Dynamics Rotational Inertia (also known as Moment of Inertia) On Gold

8. 3 Rotational Dynamics Rotational Inertia (also known as Moment of Inertia) On Gold Sheet Not On Gold Sheet Linear Analog: F = ma

8. 3 Rotational Dynamics

8. 3 Rotational Dynamics

Rotational inertia for some uniform objects

Rotational inertia for some uniform objects

8. 3 Rotational Dynamics Example 8. 5: A solid cylinder of mass 10 kg

8. 3 Rotational Dynamics Example 8. 5: A solid cylinder of mass 10 kg is pivoted about a frictionless axis through its center O. A rope wrapped around the outer radius R 1 = 1. 0 m, exerts a force of F 1 = 5. 0 N to the right. A second rope wrapped around another section of radius R 2 = 0. 50 m exerts a force of F 2 = 6. 0 N downward. a) What is the angular acceleration of the disk? b) If the disk starts from rest, how many radians does it rotate through in the first 5. 0 s? F 1 R 2 F 2 O

8. 3 Check for Understanding 1.

8. 3 Check for Understanding 1.

8. 3 Check for Understanding 1.

8. 3 Check for Understanding 1.

8. 3 Check for Understanding 2.

8. 3 Check for Understanding 2.

8. 3 Check for Understanding

8. 3 Check for Understanding

8. 3 Check for Understanding 3.

8. 3 Check for Understanding 3.

8. 3 Check for Understanding 3.

8. 3 Check for Understanding 3.

Please Do Now In full sentences, answer the question: Why does a disk beat

Please Do Now In full sentences, answer the question: Why does a disk beat a hoop of equal mass down the ramp every time? Use and underline the words moment of inertia, angular acceleration and angular velocity.

8. 4 Rotational Kinetic Energy

8. 4 Rotational Kinetic Energy

8. 4 Rotational Kinetic Energy in Rotation On Gold Sheet Linear Analog: K =

8. 4 Rotational Kinetic Energy in Rotation On Gold Sheet Linear Analog: K = ½ mv 2

8. 4 Rotational Work an Kinetic Energy

8. 4 Rotational Work an Kinetic Energy

8. 4 Rotational Work an Kinetic Energy Rotational Translational Kinetic Energy: K = ½

8. 4 Rotational Work an Kinetic Energy Rotational Translational Kinetic Energy: K = ½ I 2 K = ½ mv 2 Work-Energy Theorem: W = K = ½ I 2 - ½ I o 2 W = ½ mv 2 - ½ mvo 2 The kinetic energy of a rolling body (without slipping) relative to an axis through the contact point is the sum of the rotational kinetic energy about an axis through the center of mass and the translational kinetic energy of the center of mass. K = ½ ICM 2 + ½ mv. CM 2 total = rotational + translational KE KE + KE

8. 4 Rotational Work an Kinetic Energy Example 8. 7: A cylindrical hoop of

8. 4 Rotational Work an Kinetic Energy Example 8. 7: A cylindrical hoop of mass 10 kg and radius 0. 20 m is accelerated by a motor from rest to an angular speed of 20 rad/s during a 0. 40 s interval. How much work is required?

8. 4 Check for Understanding 1. A bowling ball rolls without slipping on a

8. 4 Check for Understanding 1. A bowling ball rolls without slipping on a flat surface. The ball has a. rotational kinetic energy b. translational kinetic energy c. both rotational and translational kinetic energies Answer: c

8. 4 Check for Understanding 2. A cylinder at rest is released from the

8. 4 Check for Understanding 2. A cylinder at rest is released from the top of a ramp, as shown above. The cylinder rolls down the ramp without slipping At the bottom of the ramp, the cylinder makes a smooth transition to a small section of a horizontal table and then travels over the edge, eventually landing on the floor at a horizontal distance of 1. 5 m from the table. After the cylinder leaves the table, but before it lands, how do the rotational kinetic energy and translational kinetic energy of the cylinder change, if at all? Rotational kinetic energy increases decreases stays the same Translational kinetic energy increases decreases stays the same

8. 4 Check for Understanding 3.

8. 4 Check for Understanding 3.

8. 4 Check for Understanding

8. 4 Check for Understanding

HW 7. B: Torque AP Classroom MCQ Part A: 4 -6, 10 -12, 16,

HW 7. B: Torque AP Classroom MCQ Part A: 4 -6, 10 -12, 16, 17, 22. MCQ Part B: 1.

8. 5 Angular Momentum

8. 5 Angular Momentum

8. 5 Angular Momentum Newton’s 2 nd law can be written as: F =

8. 5 Angular Momentum Newton’s 2 nd law can be written as: F = ma F = m v or t F= p t The rotational form of Newton’s 2 nd law: On Gold Sheet Not On Gold Sheet = mvr = L t = net torque L = change in angular momentum t = change in time Translational Analog: p = mv On Gold Sheet

8. 4 Angular Momentum The SI unit of angular momentum is kg m 2/s.

8. 4 Angular Momentum The SI unit of angular momentum is kg m 2/s. Net Torque on a Rotating Rigid Body The net torque of a rotating rigid body is equal to the time rate of change of angular momentum, L / t. In other words, a net torque causes a change in angular momentum. The SI units of torque is m N.

8. 4 Angular Momentum Problem #86 from textbook: What is the angular momentum of

8. 4 Angular Momentum Problem #86 from textbook: What is the angular momentum of a 2. 0 gram particle moving counterclockwise with an angular speed of 5 rad/s in a horizontal circle of radius 15 cm?

8. 4 Angular Momentum Problem #87 from textbook: A 10 kg rotating disk of

8. 4 Angular Momentum Problem #87 from textbook: A 10 kg rotating disk of radius 0. 25 m has an angular momentum of 0. 45 kg·m 2/s. What is its angular speed?

HW 7. C: Angular Momentum AP Classroom MCQ Part A: 1 -3, 13 -15,

HW 7. C: Angular Momentum AP Classroom MCQ Part A: 1 -3, 13 -15, 18, 21. MCQ Part B: 2 -8.

8. 5 Angular Momentum

8. 5 Angular Momentum

8. 5 Angular Momentum Examples of Conservation of Angular Momentum: - enormous wind speeds

8. 5 Angular Momentum Examples of Conservation of Angular Momentum: - enormous wind speeds of hurricanes and tornados - passing a football; spin of a bullet; gyroscope - others:

8. 5 Angular Momentum Example 8. 8: A figure skater rotating at 4. 00

8. 5 Angular Momentum Example 8. 8: A figure skater rotating at 4. 00 rad/s with arms extended has a moment of inertia of 2. 25 kg m 2. If she pulls her arms in so the moment of inertia decreases to 1. 80 kg m 2, what is the magnitude of her final angular speed?

8. 5 Check for Understanding 1. The units of angular momentum are a) N

8. 5 Check for Understanding 1. The units of angular momentum are a) N m b) kg m/s 2 c) kg m 2/s d) J m Answer: c

8. 5 Check for Understanding 2.

8. 5 Check for Understanding 2.

8. 5 Check for Understanding 2.

8. 5 Check for Understanding 2.

8. 5 Check for Understanding 3. The release of vast amounts of carbon dioxide

8. 5 Check for Understanding 3. The release of vast amounts of carbon dioxide may increase the Earth’s average temperature through the greenhouse effect and cause the polar ice caps to melt. If this occurred and the ocean level rose substantially, what effect would it have on the Earth’s rotation and on the length of the day? Answer: The polar ice caps (with almost zero moment of inertia) will go to the ocean and increase the moment of inertia of the Earth. This results in a slower rotational speed or a longer day.

8. 5 Check for Understanding 4.

8. 5 Check for Understanding 4.

8. 5 Check for Understanding 4.

8. 5 Check for Understanding 4.

HW 7. D: Conservation of Angular Momentum AP Classroom MCQ Part B: 10 -13,

HW 7. D: Conservation of Angular Momentum AP Classroom MCQ Part B: 10 -13, 15 -16, 18 -22. Click “Submit” on Parts A & B.