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Newton’s
Laws of Motion
Force
• A force is a push or a pull.
• Net force refers to what you get when you
consider the total effect of all the forces acting
on an object.
• If the forces on an object are equal and
opposite, they are said to be balanced, and the
object experiences no change in motion.
• If the forces are not equal and opposite, then
the forces are unbalanced and the motion of
the object changes.
Acceleration
• Acceleration is any change in speed and/or
direction (a change in velocity) over some
period of time.
• A net force acting on an object causes it to
accelerate.
Object accelerates
Inertia
• It’s hard to change the motion of an
object that has lots of inertia & it’s easy
to change the motion of an object that
has little inertia.
• Mass can be defined as the measure of
an object’s inertia.
• The more mass an object has, the more
inertia it has, and the harder it is to
change its motion.
Applying a larger net force to an object
results in a larger acceleration.
A soccer ball sitting at rest takes an
unbalanced force of a kick to change its
motion.
leads to
small force
small acceleration
leads to
large force
large acceleration
If you apply equal net
forces to two objects,
the one with the smaller
mass will accelerate
more.
leads to
small mass
large acceleration
leads to
large mass
small acceleration
If you want to cause two
different objects to have the
same acceleration, the object
with the larger mass will
require a larger force.
can lead
to
small force
same acceleration
can lead
to
large force
same acceleration
Newton’s
net force acting on
an object
=
mass of
object
nd
2
acceleration of
object
F = ma
Those three letters, plus the equals sign, are a
statement of Newton’s 2nd Law of Motion.
Newton’s 3rd
Law of Motion
Imagine you are in a football game. You line
up facing your opponent, the ball’s snapped,
and the two of you crash together.
Who feels the force of the crash?
Action & Reaction
• Newton was the first to realize that all forces
occur in pairs.
– There is no such thing as an isolated force existing
by itself!
• Right now, gravity is pulling you down in your
seat, but Newton’s 3rd Law says your seat is
pushing up against you with equal force.
• You are not moving because there is a balanced
force acting on you.
– Gravity is pulling down, your seat is pushing
up.
Newton’s 3rd Law
• Whenever one object exerts a force
on a second object, the second
object exerts an equal and opposite
force on the first object.
• Simply put –
– For every action (force) there is an
equal, but opposite, reaction.
Review
• Newton’s 1st –
– Inertia- an object will resist a change in its
motion. (If not moving, it doesn’t want to
move. If moving, it wants to keep moving in
a straight line at a constant speed.)
• Newton’s 2nd –
– F = ma
– force is equal to mass times acceleration
• Newton’s 3rd –
– For every action there is an equal and
opposite reaction.