Sections

Key Terms
# Key Terms

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

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

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

- free-body diagram
- a diagram showing all external forces acting on a body

- freefall
- a situation in which the only force acting on an object is the force of gravity

- friction
- an external force that acts in the direction opposite to the direction of motion

- inertia
- the tendency of an object at rest to remain at rest, or for a moving object to remain in motion in a straight line and at a constant speed

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

- mass
- the quantity of matter in a substance; measured in kilograms

- net external force
- the sum of all external forces acting on an object or system

- net force
- the sum of all forces acting on an object or system

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

- Newton’s second law of motion
- the net external force, ${F}_{\text{net}}\text{,}\text{\hspace{0.17em}}$on an object is proportional to and in the same direction as the acceleration of the object,
**a**, and also proportional to the object’s mass,*m*; defined mathematically as $\text{\hspace{0.17em}}{F}_{\text{net}}=ma\text{\hspace{0.17em}}$ or $\text{\Sigma}\text{F}=m\text{a}\text{.}$

- Newton’s third law of motion
- when 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
- the force that a surface applies to an object; acts perpendicular and away from the surface with which the object is in contact

- system
- one or more objects of interest for which only the forces acting on them from the outside are considered, but not the forces acting between them or inside them

- tension
- a pulling force that acts along a connecting medium, especially a stretched flexible connector, such as a rope or cable; when a rope supports the weight of an object, the force exerted on the object by the rope is called tension

- thrust
- a force that pushes an object forward in response to the backward ejection of mass by the object; rockets and airplanes are pushed forward by a thrust reaction force in response to ejecting gases backward

- weight
- the force of gravity,
**W**, acting on an object of mass*m*; defined mathematically as**W**=*m***g**, where**g**is the magnitude and direction of the acceleration due to gravity