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Newton’s Laws of Motion
Isaac Newton
There are “laws” that govern the motion of
objects in the universe.
• Isaac Newton was the scientist who identified
gravity the laws of motion.
I did NOT “create”
the laws of motion!
I merely identified
them!
• Newton found 3 laws of motion that are
true throughout the universe.
• Newton’s First Law of Motion:
– Objects at rest will remain at rest, and
objects in motion will remain in motion,
unless an unbalanced force acts on them.
The
st
1
Law of Motion
In OTHER words…
• Objects that are sitting still will stay still,
and…
• Objects that are moving will keep moving
until…
Something makes them
move faster or slower.
Newton’s
st
1
law can be described with
one word: inertia.
• Definition: inertia – the tendency of
an object at rest to remain at rest
OR
the tendency of a moving object to
keep moving.
If you were in outer space, and you threw a
baseball…what would happen to it?
It would keep moving in a straight line forever!
At least, until an unbalanced force acted on it…
• Mass is a measure of inertia.
–Inertia describe an object’s resistance
to change motion.
Which would be more difficult to push?
–Inertia increases as mass increases.
Even if the two vehicles are moving at the
same speed, the truck would be more
difficult to stop because it has more inertia.
Newton’s
nd
2
Law
• Newton’s Second Law:
– The unbalanced force acting on an object
equals the object’s mass times its
acceleration.
•F = ma
• Definition: Newton – the SI unit used to
measure force or weight.
•Weight is also a measure of force.
From Newton’s 2nd Law, we can infer that…
–As an object’s mass increases, it takes
more force to change its acceleration.
Which would you rather play
tennis with?
Which is easier to accelerate?
Forces are so common that we take many of
them for granted.
– Every push, pull, and any other movement
is related to Newton’s Laws of Motion.
– Understanding force is especially important
to automobile manufactures.
Why is Force important?
• An understanding of forces is also
important in every sport…
The 3rd Law of Motion
• Lets cut to the chase…what is
Newton’s 3rd Law?
–Newton’s 3rd Law states:
•For every action force, there is an equal
and opposite reaction force.
–In order for Newton's law to be true,
there must always be more than one
force present.
The 3rd Law of Motion
–Forces come in pairs that act on
different objects.
•When you kick a soccer ball, you exert a
force on the ball, and it does the same on
your foot.
The soccer ball is exerting
the same amount of force
on the man’s foot. He can
feel the ball exerting the
force!
The 3rd Law of Motion
• If forces are equal & opposite, why don’t
they cancel out?
– In a force pair, each force acts on a different
object.
– The force on one object may be enough to
accelerate it…
•But that same force may not be enough to
accelerate the other object.
•In the force pair below, the action force is acting on the
soccer ball.
•The reaction force is acting on the foot.
•The forces don’t cancel because they aren’t acting on the
same object!
6N
6N
Momentum
A small car and an 18-wheeler are traveling
beside each other at the same speed.
If both vehicles brake at the same time with the same
amount of brake force, which one will stop first?
The small car, of course. This is because it has less
momentum.
Momentum
–Definition: momentum – the amount of
motion an object contains.
– Momentum is measured by multiplying the
mass of the object by the velocity or speed
of the object.
(Mass x Velocity)
Momentum
• An unbalanced force is needed to change the
momentum off a moving object.
Momentum Conservation!
•The Law of Conservation of
Momentum states:
• The momentum of a moving object is never
destroyed – it is transferred.
Momentum Conservation
•Momentum is transferred!
–When a moving object hits another
object…
•The momentum of the moving object
is transferred to the other object.
Momentum Conservation!