143
Summary
Selected Vocabulary
repulsive............................describes a force that tends to push the two participating objects apart
attractive...........................describes a force that tends to pull the two participating objects together
oblique..............................describes a force that acts at some other angle, one that is not a direct
repulsion or attraction
normal force......................the force that keeps two objects from occupying the same space
static friction.....................a friction force between surfaces that are not slipping past each other
kinetic friction...................a friction force between surfaces that are slipping past each other
fluid..................................a gas or a liquid
fluid friction......................a friction force in which at least one of the object is is a fluid
spring constant..................the constant of proportionality between force and elongation of a spring
or other object under strain
Notation
F
N
.....................................a normal force
F
s
.......................................a static frictional force
F
k
......................................a kinetic frictional force
µ
s
......................................the coefficient of static friction; the constant of proportionality between
the maximum static frictional force and the normal force; depends on
what types of surfaces are involved
µ
k
......................................the coefficient of kinetic friction; the constant of proportionality between
the kinetic frictional force and the normal force; depends on what types of
surfaces are involved
k........................................the spring constant; the constant of proportionality between the force
exerted on an object and the amount by which the object is lengthened or
compressed
Summary
Newton’s third law states that forces occur in equal and opposite pairs. If object A exerts a force on object
B, then object B must simultaneously be exerting an equal and opposite force on object A. Each instance of
Newton’s third law involves exactly two objects, and exactly two forces, which are of the same type.
There are two systems for classifying forces. We are presently using the more practical but less
fundamental one. In this system, forces are classified by whether they are repulsive, attractive, or oblique;
whether they are contact or noncontact forces; and whether the two objects involved are solids or fluids.
Static friction adjusts itself to match the force that is trying to make the surfaces slide past each other, until
the maximum value is reached,
|F
s
|<
µ
s
|F
N
| .
Once this force is exceeded, the surfaces slip past one another, and kinetic friction applies,
|F
k
|=
µ
k
|F
N
| .
Both types of frictional force are nearly independent of surface area, and kinetic friction is usually
approximately independent of the speed at which the surfaces are slipping.
A good first step in applying Newton’s laws of motion to any physical situation is to pick an object of
interest, and then to list all the forces acting on that object. We classify each force by its type, and find its
Newton’s-third-law partner, which is exerted by the object on some other object.
When two objects are connected by a third low-mass object, their forces are transmitted to each other
nearly unchanged.
Objects under strain always obey Hooke’s law to a good approximation, as long as the force is small.
Hooke’s law states that the stretching or compression of the object is proportional to the force exerted on it,
F
˜
k(x-x
o
) .
Summary