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Notes: Chapter 11.1
Newton’s 1st & 2nd Laws of
Motion
Newton’s First Law of Motion
• An object at rest will remain at rest
and an object in motion will remain
in motion at a constant velocity
unless acted upon by an
unbalanced force
• Things don’t change their motion (or
rest) unless there are unbalanced
forces
• Also known as the Law of Inertia.
Inertia
• Inertia: tendency of object to not
change motion.
• It’s a property of matter, like density or
color or flammability. Not really a
number we measure, like forces.
• More mass you have, more inertia you
have e.g. takes more force to move
something heavy because it has more
inertia (resists that change)
Newton’s Second Law of Motion
• The unbalanced force acting on an
object equals its mass times its
acceleration.
• F = ma
• Really what it is saying is a =F/m
• acceleration is directly proportional to
force, inversely proportional to
mass.
Newton’s Second Law of Motion
• Force measured in kg x m/s2
• Also called a Newton (N)
• Mass and acceleration are opposites (if
force stays the same). As one gets
bigger, the other gets smaller.
Showing Motion
• Vector: an arrow drawn to show the direction
of a force and the magnitude (size) of a
force (in Newtons)
• Diagrams like this are called free body
diagrams:
– vectors are drawn from the center of an object,
even if that’s not where the force acts.
• Remember: if the forces are balanced, there
is no acceleration (still can have motion,
just is constant velocity or at rest)
• If forces are unbalanced, object will
accelerate
FN
Ff
Fa
Free Body Diagrams
Fg
• Fa is applied force (push/pull by you)
• Fg arrow is gravity, always towards
center of the Earth
• Ff arrow is friction/air resistance. It is
always opposite motion.
• FN arrow is normal force; the surface
supporting it. It is always
perpendicular to surface. If
something is “flying” there is no
normal force
Free Body Diagrams
FN
Ff
Fa
• Object is moving at constant
speed or sitting still
Fg
FN
Fa
Ff
Fg
FN
Ff
Fa
Fg
• Object is accelerating to the
right
• Object is slowing down while
moving to the right
(negative acceleration)
Free Body Diagrams
• What would something look
like falling?
Ff
– no normal…no surface
– no applied…no push
Fg
Ff
FN
Fg
• What would something look
like going down a ramp?
– normal…perpendicular to
surface
– no applied…no push
– gravity straight down
– friction opposite motion
“Ignore friction/air resistance”
• A lot of times, we’ll talk about “ignoring
friction” or ignoring “air resistance.”
This is “ideal world” where things are
perfect. It tells us what should
happen.
• When we do experiments, we may not
always see the same results, because
we are in the real world where there
is friction and air resistance.