Glossary

4.1 Forces

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-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

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 [latex]1 m/s^2[/latex]

4.2 Newton’s First Law

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

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

4.3 Newton’s Second Law

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

4.4 Newton’s Third Law

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

thrust

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

4.6 Mass and Weight

free fall

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

weight

force (W) due to gravity acting on an object of mass m

4.7 Normal, Tension and Other Forces

Hooke’s law

in a spring, a restoring force proportional to and in the opposite direction of the imposed displacement

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

4.8 Friction

friction

force that opposes relative motion or attempts at motion between systems in contact

kinetic friction

force that opposes the motion of two systems that are in contact and moving relative to each other

static friction

force that opposes the motion of two systems that are in contact and are not moving relative to each other

4.9 Drag Forces

drag force

[latex]{F}_{\text{D}}[/latex], found to be proportional to the square of the speed of the object; mathematically

[latex]{F}_{\text{D}}\propto {v}^{\text{2}}[/latex]

[latex]{F}_{\text{D}}=\frac{1}{2}\mathrm{C\rho }{\mathrm{Av}}^{2},[/latex]

where [latex]C[/latex] is the drag coefficient, [latex]A[/latex] is the area of the object facing the fluid, and [latex]\rho[/latex] is the density of the fluid

Stokes’ law

[latex]{F}_{s}=6\mathrm{\pi r\eta v}[/latex], where [latex]r[/latex] is the radius of the object, [latex]\eta[/latex] is the viscosity of the fluid, and [latex]v[/latex] is the object’s velocity

4.10 Elasticity: Stress and Strain

deformation

change in shape due to the application of force

Hooke’s law

proportional relationship between the force [latex]F[/latex] on a material and the deformation [latex]\Delta L[/latex] it causes, [latex]F=k\Delta L[/latex]

tensile strength

the breaking stress that will cause permanent deformation or fraction of a material

stress

ratio of force to area

strain

ratio of change in length to original length

shear deformation

deformation perpendicular to the original length of an object