ICNS 132 : Equilibrium Weerachai Siripunvaraporn Department of Physics, Faculty of Science Mahidol University &msn :

Static Equilibrium Equilibrium implies that the object moves with both constant velocity and constant angular velocity relative to an observer in an inertial reference frame. Will deal now with the special case in which both of these velocities are equal to zero –This is called static equilibrium. Static equilibrium is a common situation in engineering. The principles involved are of particular interest to civil engineers, architects, and mechanical engineers. Introduction CH12

F F F F If object has size, position of force must be considered.

3 factors -Size of force -Direction of force -Position of force

When a force is exerted on a rigid object pivoted about an axis, the object tends to rotate about that axis. Torque is the cause of changes in rotational motion, and is analogous to force, which cause changes in translational motion, but should not be confused with force. The tendency of a force to rotate an object about some axis is measured by a vector quantity called torque  High torque Easy to rotate High force Easy to move Unit is N-m but not equivalent to Joule as in Work.

where r is the distance between the pivot point and the point of application of F and d is the perpendicular distance from the pivot point to the line of action of F. (The line of action of a force is an imaginary line extending out both ends of the vector representing the force. Direction of rotation is only “clockwise” or “counterclockwise”. Direction of torque is perpendicular to r and F.

Direction of rotation will be determined by the net torque. When there are more than one forces, net force is used to determine direction of external force acting on the object.