52
F
F
12
21
= −
Magnitude of force is mathematically explained as:
F
F
G
m m
r
12
21
1 2
2
= =
Where
G is Newton’s constant = 6.67 × 10
−11
 Nm
2
/kg
2
.
For a body of mass near the surface of earth, magnitude of gravitational
force will be
F
G
m m
R
gravity
e
=
1
2
Where
m e  = mass of earth = 5.98 × 10
24
 kg
R = radius of earth = 6.38 × 10
6
 m
Many forces may act upon a body during which resultant force can be identified
in accordance with polygon law of forces (Fig. 2.2). If many forces are acting at a
point that can be represented in direction and magnitude by the sides of open polygon in order, then their resultant shall be closing side of the polygon in the opposite
direction.
Motion occurs in both natural and anthropogenic environment. Dynamics is the
specialization of mechanics that is concerned with the motion of bodies due to the
action of forces. Some of the common terms used in dynamics are given Table 2.2.
m 1
F 12
F 21
r
m 2
Fig. 2.1 Schematic diagram explaining Newton’s universal law of gravitation
2 Fundamentals of Physics for Environmental and Medical Professionals