Torque and Equilibrium Lecture 8 Pre-reading : KJF §8.1 and 8.2.

Slides:



Advertisements
Similar presentations
Torque and Rotation Physics.
Advertisements

Torque Rotational Equilibrium Rotational Dynamics
Physics 111: Mechanics Lecture 12
Equilibrium and Torque
Torque Torque is defined as the tendency to produce a change in rotational motion.
Torque, Equilibrium, and Stability
Torque Physics 6A Prepared by Vince Zaccone
Chapter-9 Rotational Dynamics
Rotational Equilibrium and Dynamics
1 UCT PHY1025F: Mechanics Physics 1025F Mechanics Dr. Steve Peterson EQUILIBRIUM.
Physics Montwood High School R. Casao
For this review, answer each question then continue to the next page for the correct answer.
A ladder with length L weighing 400 N rests against a vertical frictionless wall as shown below. The center of gravity of the ladder is at the center of.
Force vs. Torque Forces cause accelerations
Rotational Equilibrium and Rotational Dynamics
Physics 201: Lecture 18, Pg 1 Lecture 18 Goals: Define and analyze torque Introduce the cross product Relate rotational dynamics to torque Discuss work.
Torque and Angular Momentum
Rotational Equilibrium and Rotational Dynamics
Chapter 9 Rotational Dynamics.
Rotational Dynamics and Static Equilibrium. Torque From experience, we know that the same force will be much more effective at rotating an object such.
Chapter 8 Rotational Equilibrium and Rotational Dynamics.
Chapter 8 Rotational Equilibrium and Rotational Dynamics.
Torque and Rotational Equilibrium
Rotational Equilibrium
Rotational Equilibrium and Rotational Dynamics
Torque.
Last Lecture Review 1 Two horizontal forces act on a block that is sliding on ice. Assume that a third horizontal force F also acts on the block. What.
Physics 106: Mechanics Lecture 07
Physics 106: Mechanics Lecture 08
Physics 106: Mechanics Lecture 02
D. Roberts PHYS 121 University of Maryland Physic² 121: Phundament°ls of Phy²ics I November 15, 2006.
Warm-Up: February 17, 2015 Write down a definition for equilibrium.
Rotational Equilibrium and Rotational Dynamics
Chapter-9 Rotational Dynamics. Translational and Rotational Motion.
Chapter 8: Torque and Angular Momentum
Chapter 9 Torque.
Chapter 9: Rotational Dynamics
Torque and Equilibrium
Torque and Rotation Physics. Torque Force is the action that creates changes in linear motion. For rotational motion, the same force can cause very different.
Copyright © 2010 Pearson Education, Inc. Lecture Outline Chapter 11 Physics, 4 th Edition James S. Walker.
Kinesiology Unit 8 1. Definition of Balance: An individual’s ability to control stability 2.
Chapter 8 Rotational Dynamics and Static Equilibrium
Centre of mass and Torque Lecture 7 Pre-reading : KJF §7.2 and 7.3.
Chapter 8 Rotational Motion.
Rotational Motion Chapter 6, 8 and 9. Acceleration in a Circle  Acceleration occurs when velocity changes  This means either speed OR direction changes.
Chapter 5: Turning effect of forces
Chapter 8 Statics Statics. Equilibrium An object either at rest or moving with a constant velocity is said to be in equilibrium An object either at rest.
Center of Mass Torque. Center of Mass When analyzing the motion of an extended object, we treat the entire object as if its mass were contained in a single.
Describe moment of force or torque as moment = force × perpendicular distance from pivot to the line of action of force;
Physics CHAPTER 8 ROTATIONAL MOTION. The Radian  The radian is a unit of angular measure  The radian can be defined as the arc length s along a circle.
Force in Mechanical Systems
Newton’s first and second laws Lecture 2 Pre-reading : KJF §4.3 and 4.4.
Torque 10.3 (pages 490 – 502). Friday Fun… Unravelled What did you learn? What does it mean to balance? –For an object to balance, you must use the centre.
Chapter 8: Rotational Equilibrium and Dynamics
Chapter 8 Rotational Equilibrium and Rotational Dynamics
Understand the principles of statics Graphical vectors Triangle of forces theorem Parallelogram of forces theorem Concept of equilibrium
Ying Yi PhD Chapter 9 Rotational Dynamics 1 PHYS HCC.
1 Rotational Dynamics The Action of Forces and Torques on Rigid Objects Chapter 9 Lesson 2 (a) Translation (b) Combined translation and rotation.
Newton’s third law of motion 1 Force 2
Chapter 9 Rotational Dynamics.
Physics 103: Lecture 14 Rotations of Extended Objects
Chapter 12. Rotation of a Rigid Body
Chapter 9: Statics Consider these four objects:
Torque and Rotation.
Rotational Dynamics Chapter 9.
HW #7 cancelled HW #8 available later today
Moments.
Torque and Rotation Physics.
Chapter 9 Torque.
Moment of a force or Torque
Presentation transcript:

Torque and Equilibrium Lecture 8 Pre-reading : KJF §8.1 and 8.2

2 Centre of mass, again

3 Archimedes’ Lever Rule At equilibrium (and with forces 90° to lever): r 1 F 1 = r 2 F 2

4 General Lever Rule For general angles r 1 F 1 sin θ 1 = r 2 F 2 sin θ 2 We call rF sinθ = τ torque S.I. unit of torque: newton metre (Nm) At equilibrium, the magnitude of torques exerted at each end of lever are equal KJF §7.2

5 Crudely speaking, torque is "twisting or turning ability" of a force that can: change the angular velocity of an object (i.e. speed up or slow down rotation) cause a twisting or bending distortion of an object A force with a "line of action" that does not cross the axis of rotation results in torque. What is torque?

6 Note: torque is measured about a particular point. Usually this will be a hinge, pivot or axis torque has a sign. All forces that tend to rotate the object in the same direction produce torque with the same sign

7 Calculating torque (1) Choose a sign convention (e.g. anti-clockwise +ve), then decide in which direction force is pulling or pushing lever. Write that sign in front of your answer. Method 1: If you're given r and θ, use formula for torque (magnitude) τ = r F sinθ (Note: sinθ = sinφ, ∴ it doesn’t matter which angle you use) Example: Calculate torque on lever exerted by hand:

8 Calculating torque (2) Method 2: If you're given d the “perpendicular distance” from axis to the “line of action”, then use formula τ = d F If the “line of action” crosses the axis (i.e. d = 0) then τ = 0

9 Opening a door If r is perpendicular to F, then torque τ = r F If r is not perpendicular to F, then torque τ = r F sinθ where θ is the angle between r and F Axis of rotation r F What happens if you push in the middle of the door; do you need more or less force? Why? What happens if you push along a line passing through axis of rotation? Explain.

10 Problem The length of a bicycle pedal arm is r = m, and a downward force of F = 111 N is applied by the foot. What is the magnitude of torque about the pivot point when the angle θ between the arm & vertical is; 30.0°? 90.0°? 180.0°? [8.44 Nm, 16.9 Nm, 0.00 Nm]

11 Adding up Torques We will only consider torques acting in 2D (flat on page) Choose a sign convention (e.g. anti-clockwise is positive). Choose the rotation axis around which to calculate torque (unless it's already given). For each force, calculate the resulting torque (including sign). Add up all the torques. KJF §7.2, see p. 214

12 F1F1 F2F2

13 Adding up Torques: Example torque 1; τ 1 = –rF 1 sinθ = –0.5 × 10 × sin 30 = –2.50 Nm torque 2; τ 2 = +mgd = 1 × 9.8 × 0.25 = Nm ∴ net torque = ∑τ = τ 1 + τ 2 = (–2.50) = –0.05 Nm (i.e. clockwise)

14

15 Equilibrium KJF §8.1, 8.2

16 Conditions for Equilibrium For an object to be in static equilibrium Σ F = 0 no net force ⇒ Σ F x = 0, Σ F y = 0 Σ τ = 0 no net torque Because this is true for all pivot points, we are free to choose any point we like for calculating the torque ⇒ choose point where some torques disappear KJF §8.1

17 Hints for Statics Problems Draw a diagram! Decide on system Put in forces ON system only All forces in mechanics are either contact or gravity Define sign conventions Usually you're given some forces on a static body & need to find unknown forces or torques.

18 Solving Static Equilibrium Problems Decide on the “system” Choose a rotational axis and sign convention Best to choose one that causes some torques to disappear Remember nothing is rotating anyway so you're free to choose the axis. Calculate all horizontal components of forces acting on the system and write equation ∑F h = 0. Calculate all vertical components of forces acting on the system and write equation ∑F v = 0. Assume each object’s weight force is acting at its centre of mass. Calculate all torques and write equation ∑τ = 0. Remember that all external forces are possible sources of torque. Solve the equations simultaneously. KJF §8.1

19 Example A 65 kg woman is horizontal in a push-up position. What are the vertical forces acting on her hands and her feet? [hands 420 N, feet 210 N]

20 Example 2 Here the cargo is loaded correctly. Whatever rotation axis is chosen, there's always some normal forces opposing the torque due to the total system weight (treated as though it lies at the centre of mass) No net torque ∴ equilibrium. The “system” is the ass, the cart and the cargo.

21 But... Too much cargo is loaded at the back. If the wheel is chosen as the rotation axis, all resulting torques are acting in the clockwise direction. There is no torque opposing the torque due to the weight of the system, hence there is a net clockwise torque. The system will rotate until the cart hits the ground. The donkey will be lifted off the ground.

22

23

24 Types of Equilibrium Neutral: with a small displacement, remains at new position. Stable: with a small displacement, returns to original position. Unstable: with a small displacement, continues to move away from equilibrium position. KJF §8.2

25 Stable W N ↻ net torque around X X

26 W N NO net torque around X X Neutral

27 W N ↺ net torque around X X Unstable

28

Next lecture Momentum, impulse and energy Read: KJF §9.1, 9.2