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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, $F net , F net ,$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 $F net =ma F net =ma$ or $ΣF=ma. ΣF=ma.$
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 = mg, where g is the magnitude and direction of the acceleration due to gravity