PHY 101 1nd Edition Lecture 21 Outline of Last Lecture I Summary Angular Linear Quantities II Forces Causing Centripetal Acceleration III Problem Solving Strategies IV 7 5 Newtonian Gravitation Revisited Outline of Current Lecture V Chapter 8 Rotational Equilibrium and Dynamics VI 8 1 Torque a Force vs Torque VII 8 2 Torque and Rotational Equilibrium Current Lecture Chapter 8 Rotational Equilibrium and Dynamics Physics behind opening a door Three factors that determine the effectiveness of the force in opening the door o The magnitude of the force o The position of the application of the force o The angle at which the force is applied Combining the 3 factors torque 8 1 Torque Torque measures the effectiveness of a force to rotate an object about an axis o rFsin SI Unit N m o R magnitude of the position vector of the point where force is applied measured from the rotational axis If the force is perpendicular to the position vector sin 1 rF Lever Arm It is sometimes convenient to introduce the concept of lever arm o d rsin so that These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute o rFsin dF Force vs Torque forces cause linear acceleration torques cause angular accelerations like force torque is a vector quantity o if the turning tendency of the force is counterclockwise the torque is positive o if the turning tendency is clockwise the torque is negative when two or more torques are acting on an object the torques are added as vectors can be positive or negative if the net torque applied to an object is zero the object s rate of rotation doesn t change it either does not rotate or does constant speed rotation the object is said to be in rotational equilibrium Example two people are pushing a revolving door at the same time o F1 625 N r1 1 2 m o F2 850N r2 0 8m Both forces are perpendicular to the position vectors Calculate the net torque on the revolving door Analysis torque can be positive CCW or negative CW Solution torque 1 negative CW o 1 r1F1 1 2 x 625 750 N m Torque 2 Positive CCW o 2 r2F2 0 8 x 850 680 N m total torque o 1 2 750 680 70N m 8 2 Torque Rotational Equilibrium we have discussed the conditions for equilibrium the net force applied on the object is zero actually this condition alone is not sufficient for an object in equilibrium consider the revolving door again suppose we have 2 Forces F1 F2 equal in magnitude opposite in direction applied to the door o F1 F2 0 The door keeps spinning faster and faster not in equilibrium
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