Motion and Forces
Ch. 8.3 Newton’s Laws of Motion
Section 8.3 Objectives
 State Newton’s three laws of motion, and apply them to
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physical situations.
Calculate force, mass, and acceleration with Newton’s second
law.
Recognize that the free-fall acceleration near Earth’s surface
is independent of the mass of the falling object.
Explain the difference between mass and weight.
Identify paired forces on interacting objects.
Newton’s First Law
 Newton's 1st Law States :
An object at rest remains at rest and
an object in motion maintains its
velocity unless it experiences an
unbalanced force.
 Newton’s 1st Law is also known as
the Law of Inertia.
 Inertia is the tendency of an
object to remain at rest or in
motion with a constant velocity.
Newton’s Second Law
 Newton’s
2nd
Law States:
The unbalanced force acting
on an object equals the
object’s mass times its
acceleration.
 This means :
force = mass x acceleration
F = ma
Where :F is in Newton’s (N)
m is in kilograms ( kg)
a is in meters per second squared (m/s2)
Free fall and Weight
 Free fall is the motion of a
body when only the force of
gravity is acting on it.
 Weight is the force on an object
due to gravity.
 Weight equals mass times free
 Free fall acceleration near
fall acceleration or w = mg.
Earth’s surface constant.
 Weight is a force and has SI
 Disregarding air resistance, all
objects on Earth’s surface
(regardless of their mass)
accelerate at 9.8 m/s2.
units of Newtons (N).
Terminal Velocity
 Terminal velocity is the
maximum velocity reached
by a falling object that
occurs when the resistance
of the medium is equal to
the force due to gravity.
 At terminal velocity, air
resistance and force of
gravity are equal. Sky
divers reach a terminal
velocity of about 320 km/h
(200 mi/h).
Newton’s Third Law
 Newton’s Third Law states:
 For every action force,
there is an equal and
opposite reaction force.
 Forces always occur in
pairs but act on different
objects.
The upward push on the rocket
equals the downward push on the
exhaust gases.
Section 8.3 Summary
 An object at rest remains at
rest and an object in
motion maintains a
constant velocity unless it
experiences an unbalanced
force (Newton’s 1st law).
 The unbalanced force
acting on an object equals
the object’s mass times its
acceleration, or F = ma
(Newton’s 2nd Law).
 The SI unit for force is the
Newton (N). Weight equals
mass times free fall
acceleration, or W = mg.
 For every action force,
there is an equal and
opposite reaction force
(Newton’s 3rd law).