Physics 9: 1-4 Static Equilibrium
Text Reading: Giancoli, Physics - Principles with Applications, Chapter 9: Sections 1-4
- 9.1: Acceleration or change in motion can occur only if there is a net force acting. If we know forces are exerted on an object, but the object doesn't change its speed or direction, then the forces must be balanced, so that the net force is zero. In particular, objects at the Earth's surface experience gravitational acceleration. Objects in rest must be in a state of equilibrium, where other forces exist that are equal and opposite to the linear and angular acceleration of gravitational force and torque.
- 9.2: We can assess the forces acting on an object by breaking them into x, y, and z linear components, and clockwise or counterclockwise torques, and balancing the forces and torques in each direction. Our analysis depends on the fact that forces acting at right angles do not interfer with each other.
- 9.3: We can use the mechanical definitions of force and torque to analyze motions of the human body.
- 9.4: If we disturb a system in equilibrium slightly, it may return to its equlibrium state (stable equilibrium) or it may move to a new state further from its original state (unstable equilibrium). Consider a marble and a glass bowl. If we place the marble in the bottom of the upturned bowl and move it slightly up the walls, it will fall back to the bottom: this is a stable situation. However, if we turn the bowl over, and set the marble at the top of the curvature, slight deviations will cause the marble to move over the edge and slide down the outside of the bowl. Similarly, an object whose center of mass is no longer over its "footprint" will respond to torque forces that cause it to rotate and fall over.
Conditions for Static Equilibrium
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