 | Forces include gravity, friction, and forces from devices such as ropes or springs. Many forces may act on an object simultaneously. The net force is the vector sum of all forces acting on an object. The net force determines acceleration. Zero net force means zero acceleration. A nonzero net force causes acceleration equal to the net force divided by the object’s mass. Normal forces act between objects and supporting surfaces. A free-body diagram is a sketch showing the location and direction of all forces acting on an object. Forces always exist in action–reaction pairs. Each force in a pair is equal in magnitude and opposite in direction to its partner and the two forces in a pair always act on different objects. The force from a spring is proportional to its deformation according to Hooke’s law. Friction is a catch-all term for forces that act to oppose motion. Kinetic friction acts between moving surfaces, rolling friction acts between rolling surfaces, viscous friction applies to fluids, and static friction acts between surfaces with no relative motion.
|  | | By the end of the chapter you should be able to
|  | calculate the net force from any combination of one-, two-, or three-dimensional forces;
| | draw a free-body diagram of a two-dimensional system of forces;
| | identify and represent action–reaction, weight, friction, and normal forces on free-body diagrams;
| | solve problems including forces from static, kinetic, and rolling friction; and
| | solve problems involving forces from extension and compression springs.
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|  | 5A: Newton’s second law
5B: Hooke’s law
5C: Static and kinetic friction
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