COLLEGE PHYSICS, Part I
Chapter 4:
FORCES AND NEWTON’S LAWS OF MOTION
Force and Mass
Newton’s First Law of Motion
Newton’s Second Law of Motion
The Vector Nature of Newton’s Second Law of Motion
Newton’s Third Law of Motion
Types of Forces
The Gravitational Force
Mass and Weight
The Normal Force
Apparent Weight

Force
Here we will be dealing with
dynamics
, i.e., the relationship between motion and
force. The principles of dynamics are described by three laws, known as
Newton’s
laws of motion
. While these are fundamental laws of Nature, they cannot be
deduced from or proved by any other principles.
Force can be defined in different ways, like “Force
is a push or a pull on an object,” or “
Force
is
whatever can cause an object with mass to
accelerate
.”

When a force involves direct contact
between objects, it is called a
contact force
.
When an object rests on a surface, there is a
component
of force perpendicular (normal)
to the surface, which is called a
normal
force
.
There may be a component of force parallel
to the surface, called
friction force
.
When a rope is attached to an object and
pulled, the force is called a
tension
.
Finally, the gravitational pull exerted by the
earth on an object is called the object’s
weight
.
Force is always a vector quantity
.

In the SI system
,
the unit of force is the
newton
, abbreviated N.
1 N = 0.2248 lb
The effect of any number of
forces exerted on an object is
the same as the effect of the
resultant of all forces, i.e., their
vector sum. This principle is
called
superposition of forces
.
1
2
R
F
F
=
+
r
r
r
F
1
and
F
2
are the
components
.
R
is the
resultant
, or the
net force
.
1 N = 1 kg
×
m/s
2

In this example, the component vectors of
F
are
F
x
and
F
y
,
and the corresponding components are
F
x
and
F
y
. The effect
of simultaneous actions of
F
x
and
F
y
is the same as the
effect of
F
.
Any force can be presented by 3 (
x
,
y
, and
z
) components,
but here we will consider only 2 dimensions. The
x
and
y
component do not have to be horizontal and vertical, but they
should be perpendicular to each other.
1
2
3
...
R
F
F
F
F
=
+
+
+
=
∑
r
r
r
r
r
The same is true for the components:
y
y
R
F
=
∑
,
x
x
R
F
=
∑
2
2
,
x
y
R
R
R
=
+
tan
/
y
x
R
R
θ
=
Any force can be replaced by its components, acting at the
same point.
As for other vector quantities, the magnitude and the
direction of the force can be found from:
(4.1)
(4.2)
(4.3)