Download Newtons laws of Motion

Newton’s Laws
8th Grade Science
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What will we be
examining?
• Newton’s Laws: Three laws that explain the
motion of objects caused by forces
• Inertia: The tendency of objects to keep on
doing what they are already doing (moving or
staying at rest
Why will we be examining
Sir Isaac Newton?
• S8P3. Students will investigate relationship
between force, mass, and the motion of
objects.
• b. Demonstrate the effect of balanced and
unbalanced forces on an object in terms of
gravity, inertia, and friction. (DOK 2)
But first… the scientists
who came before…
• Newton wasn’t the first
person to come up with
this idea…
• Aristotle (384-322 B.C.)
started the conversation
with the proposal that
force is required to keep
an object moving
• However, his concept
was proven incorrect…
• Galileo (1564-1642) was
next…
• Concluded that moving
objects not subjected
to friction or other
forces would continue
to move indefinitely
• Descartes (1596-1650)
was certainly on
Newton’s list of study.
• Published "Principles of
Philosophy”
• "that each thing, as far as
is in its power, always
remains in the same
state; and that
consequently, when it is
once moved, it always
continues to move."
Newton’s
st
1 Law of Motion
• The state of motion of an
object does not change
as long as the net force is
zero
• Known as the Law of
Inertia
• First Law - Law of
InertiaA moving object
moves in a straight line
with constant speed
unless a force acts on it.
• The tendency of an
object at rest to remain
at rest and an object in
motin to remain in motin
unless acted upon by an
unbalanced force.
•
What is Inertia?
• An object's tendency to resist a change in
motion is inertia .
• What is included what you discuss the motion
of an object?
Inertia and mass
• The more mass an object has, the
more inertia it has.
• Why?
This means that the more mass an object has, the
harder it is to move, stop, or change the speed or
direction of the object.
• An object will not start moving unless a force acts
on it
• An object will not stop moving unless a force acts
on it
• An object will not change speed unless a force acts
on it
• An object will not change direction unless a force
acts on it
• Inertia is the tendency of an object to not
change it's motion
• If it is moving, it keeps moving in the same
direction
• If it is at rest, it stays at rest
• Objects do not change their motion unless a
force acts on them
Recap…
• Let’s summarize…
• An object moving
wants to remain in
motion with the same
speed and direction
• An object at rest wants
to stay at rest
• Things want to keep
doing what they are
already doing!
nd
2
Newton’s
Law of
Motion
• If a ping pong ball and a basketball were both
dropped at the same time from the roof of our
school, which would hit the ground with a
greater force? Common sense tells us that the
basketball ball would. The difference in forces
would be caused by the different masses of
the balls. Newton stated this relationship in his
second law, the force of an object is equal to its
mass times its acceleration.
• Units for force, mass, and acceleration
• Units for F=ma
• Force is measured in Newtons, N.
• Mass is measured in kilograms, kg.
• Acceleration is measured in meters per second
squared, m/s2.
• It could be said that mass and acceleration
have a direct proportional relationship to the
force of an object. Look at the equation. What
does that mean?
• If acceleration is directly proportional to the
applied net force, then by whatever factor
acceleration changes, force changes by the
same factor. To see this we will need to
consider an object with constant mass. Let's
consider an object with a mass of 5 kg.
• Suppose this object has a mass of 5 kg and an
acceleration of 3 m/s2. Let's call this a1. So:
a1 = 3 m/s2
• What force is necessary to cause this
acceleration to our 5 kg object? Let's figure
that out and call it F1:
• F1 = ma1
• F1 = (5 kg)(3 m/s2)
• F1 = 15 N
• Now let's think about this object when it is
moving at twice this acceleration. That is, we
will change the acceleration by a factor of 2.
Let's call this new acceleration a2. So:
• What force is now necessary to cause this new
acceleration, a2? We will call this force F2, and
here is its calculation:
• Would the force increase?
• Would the force decrease?
• Why?
• F2 = ma2
• F2 = (5 kg)(6 m/s2)
• F2 = 30 N
• The second acceleration, 6 m/s2, is twice the
first acceleration, 3 m/s2, therefore,
• The second force, 30 N, is twice the first force,
15 N.
• As acceleration increases, so does force at the
same rate.
• Clearly, both the acceleration and the force on
this object change by the same factor, 2. These
matching factor changes would occur for any
factor change and for any constant mass. So
the equation F=ma contains the direct
proportion between acceleration and the
applied net force.
Examine closely…
The inverse proportion between
acceleration and mass
• Now, what about the inverse proportion
between acceleration and mass, is that
contained within F=ma?
• Let's consider a situation in which an applied
net force is 6 N. That is, object 1 will have a net
force of 6 N applied to it.
• We will say our object has a mass of 3 kg. So:
• What will be the acceleration of this object?
Let's call that acceleration a1, and let's solve
for it using F=ma. So:
• Now let's cut the mass to one-third. That is, we
will consider a change in mass by a factor of
1/3. That would make the mass of object 2 to
be 1 kg, or m2 = 1 kg. If we apply the same
force, 6 N, to object 2, what will be its
acceleration?
Here is the answer
• So the second mass is 1/3 the first mass, since:
• m2 = (1/3)m1
• 1 kg = (1/3)(3 kg)
• 1 kg = 1 kg
• And the second acceleration is three times the
first acceleration, as in:
• a2 = (3)a1
• 6 m/s2 = (3)(2 m/s2)
• 6 m/s2 = 6 m/s2
• Clearly, the acceleration and mass change by
reciprocal (or inverse) factors. The factors are
3 and 1/3, respectively. Since these two
quantities change by reciprocal (inverse)
factors, these two quantities are in an inverse
proportion.
Let’s Examine closely…
rd
3
Newton’s
Law of
Motion
• Imagine a rocket is being launched from the earth.
Hot gases are pushed out from the bottom of the
rocket as the rocket is pushed upward. The force
of the gases pushing against the surface of the
earth is equal and opposite to the force with
which the rocket moves upward. The motion of
the rocket can be explained by Newton's third
law, for every action there is an equal and
opposite reaction. In other words, when one
object exerts a force on another object, the
second object exerts a force of equal strength in
the opposite direction on the first object.
rd
3
Newton’s
Law of
Motion
• The Third Law is concerned with how objects
push on each other and exchange momentum
when they interact.
• The third law states that for every force there
is an equal and opposite force. For example, if
you push on a wall, it will push back on you as
hard as you are pushing on it.
• Equal and opposite
forces
• Equal and opposite
forces are present
when two objects
collide, as shown in the
following animation.
• http://ezeducationora
ma.com/ezMedia/physi
cs/forces/newton/three
Laws/thirdLaw/forces/t
hirdLaw1.mp4
• Forces always act in pairs. The two forces act in
opposite directions. When you push on an object,
the object pushes back with an equal force. Think
of a pile of books on a table. The weight of the
books exerts a downward force on the table. This
is the action force. The table exerts an equal
upward force on the books. This is the reaction
force. Note that the two forces act on different
objects. The action force acts on the table. The
reaction force acts on the books.
Let’s look at another
example
• There is a force from the boy on the dog's toy,
and there is a force from the dog's toy on the
boy. Forces always come in pairs similar to this
example. Consider the boy (A) as one system
and the toy (B) as another.
• There can never be a single force acting alone,
without its action-reaction partner. Forces only
come in action-reaction pairs. Think carefully
about propelling a skateboard with your foot.
Your foot presses backward against the
ground. The force acts on the ground.
However, you move, so a force must act on
you, too. Why do you move? What force acts
on you?
• You move because the action force of your foot
against the ground creates a reaction force of the
ground against your foot. You 'feel' the ground
because you sense the reaction force pressing on your
foot. The reaction force is what makes you move
because it acts on you.
Draw Diagrams
• When sorting out action and reaction forces, it
is helpful to draw diagrams. Draw each object
apart from the other. Represent each force as
an arrow in the appropriate direction. Here are
some guidelines to help you sort out action
and reaction forces:
Drawing Diagrams
You try!
• Consider the situation of holding a book in
your hand. Draw one diagram for the example.
Are there any interaction pairs? When
identifying interaction pairs, keep in mind that
they always occur in two different diagrams
and they always will have the symmetry of
subscripts noted earlier. In this case, the
interaction pair is F(book)on hand and F(hand)
on book.
• In this case, the interaction pair is F(book)on
hand and F(hand) on book.