### Key Terms

- dynamics
- study of how forces affect the motion of objects and systems

- external force
- force acting on an object or system that originates outside of the object or system

- force
- push or pull on an object with a specific magnitude and direction; can be represented by vectors or expressed as a multiple of a standard force

- free fall
- situation in which the only force acting on an object is gravity

- free-body diagram
- sketch showing all external forces acting on an object or system; the system is represented by a single isolated point, and the forces are represented by vectors extending outward from that point

- Hooke’s law
- in a spring, a restoring force proportional to and in the opposite direction of the imposed displacement

- inertia
- ability of an object to resist changes in its motion

- inertial reference frame
- reference frame moving at constant velocity relative to an inertial frame is also inertial; a reference frame accelerating relative to an inertial frame is not inertial

- law of inertia
- see Newton’s first law of motion

- net external force
- vector sum of all external forces acting on an object or system; causes a mass to accelerate

- newton
- SI unit of force; 1 N is the force needed to accelerate an object with a mass of 1 kg at a rate of $1\phantom{\rule{0.2em}{0ex}}{\text{m/s}}^{2}$

- Newton’s first law of motion
- body at rest remains at rest or, if in motion, remains in motion at constant velocity unless acted on by a net external force; also known as the law of inertia

- Newton’s second law of motion
- acceleration of a system is directly proportional to and in the same direction as the net external force acting on the system and is inversely proportional to its mass

- Newton’s third law of motion
- whenever one body exerts a force on a second body, the first body experiences a force that is equal in magnitude and opposite in direction to the force that it exerts

- normal force
- force supporting the weight of an object, or a load, that is perpendicular to the surface of contact between the load and its support; the surface applies this force to an object to support the weight of the object

- tension
- pulling force that acts along a stretched flexible connector, such as a rope or cable

- thrust
- reaction force that pushes a body forward in response to a backward force

- weight
- force $\overrightarrow{w}$ due to gravity acting on an object of mass
*m*