Exam 3 Physics 101 Lecture 14 Torque and

Exam 3 Physics 101: Lecture 14 Torque and Equilibrium l Review Tomorrow 9 -10 AM 144 Loomis Physics 101: Lecture 14, Pg 1

Torque What exactly is torque in normal English? l Rotational effect of force. Tells how effective force is at twisting or rotating an object. tnet = I α lt = +- r Fperpendicular = r F sin q è Units N m è Sign, CCW rotation is (usually) positive Why is there a sine theta in the Torque equation!? !!? ? ! Physics 101: Lecture 14, Pg 2 07

ACT The picture below shows three different ways of using a wrench to loosen a stuck nut. Assume the applied force F is the same in each case. In which of the cases is the torque on the nut the biggest? A. Case 1 B. Case 2 C. Case 3 Physics 101: Lecture 14, Pg 3 10

ACT 2 The picture below shows three different ways of using a wrench to loosen a stuck nut. Assume the applied force F is the same in each case. In which of the cases is the torque on the nut the smallest? A. Case 1 B. Case 2 C. Case 3 Physics 101: Lecture 14, Pg 4 12

Torque Example and ACT A person raises one leg to an angle of 30 degrees. An ankle weight (89 N) attached a distance of 0. 84 m from her hip. What is to torque due to this weight? 4 0. 8 r = 30 m F=89 N If she raises her leg higher, the torque due to the weight will A) Increase B) Same C) Decrease Physics 101: Lecture 14, Pg 5 17

Equilibrium Act l. A rod is lying on a table and has two equal but opposite forces acting on it, one on each end. What is the net force on the rod? A) Up B) Down C) Zero l Will the rod move? A) Yes B) No y F x F Physics 101: Lecture 14, Pg 6 21

Equilibrium l Conditions è F=0 S Mass) è t=0 S for Equilibrium Translational EQ (Center of Rotational EQ » Can choose any axis of rotation…. Choose Wisely! Physics 101: Lecture 14, Pg 7 25

Example: Equilibrium A meter stick is suspended at the center. If a 1 kg weight is placed at x=0. Where do you need to place a 2 kg weight to balance it? 50 cm 9. 8 N d 19. 6 N Physics 101: Lecture 14, Pg 8 25

Question The picture below shows two people lifting a heavy log. Which of the two people is supporting the greatest weight? 1. The person on the left is supporting the greatest weight 2. The person on the right is supporting the greatest weight 3. They are supporting the same weight Physics 101: Lecture 14, Pg 9 33

Example: hanging beam 2 m A 40 kg uniform beam is being supported by two ropes. What is the tension in each rope? 40 kg 3 m Physics 101: Lecture 14, Pg 10

Example: painter A painter of mass 50 kg uses two supports and a plank of mass 20 kg as a platform to paint his house. How far to the right of support B can the painter stand before the plank tips? 3 meter 1 meter 20 kg A B Just before board tips, force from A becomes zero Physics 101: Lecture 14, Pg 11 47

Example: continued 3 meter 1 meter 20 kg A B Physics 101: Lecture 14, Pg 12

Example: falling beam 2 m If the right rope breaks, what is the initial angular acceleration of the beam? 40 kg 3 m Physics 101: Lecture 14, Pg 13

Summary l Torque = Force that causes rotation è net = I α t è = F r sin q t è Work done by torque W = t q l Equilibrium è F=0 S è t=0 S » Can choose any axis. Physics 101: Lecture 14, Pg 14 50