F
ma
Force and Motion
NEWTON'S LAWS OF MOTION
Rev. 0
Page 1
CP-03
NEWTON'S LAWS OF MOTION
The study of Newton's laws of motion allows us to understand and accurately
describe the motion of objects and the forces that act on those objects.
EO 1.1
STATE Newton's first law of motion.
EO 1.2
STATE Newton's second law of motion.
EO 1.3
STATE Newton's third law of motion.
EO 1.4
STATE Newton's law of universal gravitation.
The basis for modern mechanics was developed in the seventeenth century by Sir Isaac Newton.
From his studies of objects in motion, he formulated three fundamental laws.
Newton's first law of motion states "an object remains at rest (if originally at rest) or moves
in a straight line with constant velocity if the net force on it is zero."
Newton's second law states "the acceleration of a body is proportional to the net (i.e., sum or
resultant) force acting on it and in the direction of that net force." This law establishes the
relationship between force, mass, and acceleration and can be written mathematically as shown
in Equation 3-1.
(3-1)
where:
F
=
force (Newton = 1 Kg-m/sec , or lbf)
2
m
=
mass (Kg or lbm)
a
=
acceleration (m/sec or ft/sec )
2
2
This law is used to define force units and is one of the most important laws in physics. Also,
Newton's first law is actually a consequence of this second law, since there is no acceleration when
the force is zero, and the object is either at rest or moving with a constant velocity. Equation 3-1
can be used to calculate an objects weight at the surface of the earth. In this special case, F is the
force, or weight, caused by the gravitational acceleration of the earth acting on the mass, m, of the
object. When dealing with this type of problem, we designate the acceleration, g, which equals
9.8m/sec or 32.17 ft/sec (g is called gravitational acceleration constant). Thus, equation 3-1
2
2
becomes F = mg for this case.
