Download Newton`s Laws

Taken from - Chapter 10, sec. 3 and 4
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Your word is Inertia
 chapter
10 , sec. 3
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Sir Isaac Newton proposed the three basic laws
of motion in the late 1600s.
His laws were based on Galileo’s suggestions
about motion.
‘Galileo suggested that, once an object is in
motion, no force is needed to keep it in motion.
Force is needed only to change the motion of
the object.’
Prentice Hall Science Explorer, Grade 8, p. 349
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Newton’s first law is a restatement of
Galileo’s theory.
The First Law states that an object at rest
will remain at rest, and an object in
motion will remain in motion, unless
acted upon by an unbalanced force.
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This means that you have to apply some sort of
force (a push or pull) to make an object move if it
is still.
This also means that you have to apply a force to
make an object stop moving.
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Otherwise the object in motion will continue moving
forever.
The force that we know of that helps stop objects
that are in motion is friction.
On Earth both gravity and friction are unbalanced
forces that can change an objects motion.
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Galileo introduced the idea of ‘inertia’.
‘Inertia is the tendency of an object to resist a
change in motion’.
Newton’s first law is also called the law of inertia.
Inertia explains why you move forward after
coming to a sudden stop on a carnival ride.
Prentice Hall Science Explorer, Grade 8, p. 350
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In this instance, a force is required to keep you from
continuing to move forward, such as a seat belt or a bar.
Inertia depends on mass – the greater the mass of
an object, the greater its inertia!
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Newton’s second law states that acceleration
depends on the object’s mass and on the net force
acting on the object.
An object with a larger mass requires more force to cause
a change in its motion.
 An object with a smaller mass requires less force to cause
a change in its motion.
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Acceleration = Net force
Mass
– if you rearrange this formula to solve for
Net force you get mass x acceleration = Net
force which gives us Kg-m/s2 = newton
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You can calculate the net force even if you
don’t know what the forces are that are acting
on an object as long as you know the mass and
the object’s acceleration rate.
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Remember if an object is not falling, it is not
accelerating at 9.8 m/s2
What is the net force on a 1,000 kg object
accelerating at 3 m/s2 ?
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Fnet= 1,000 kg x 3 m/s2 = 3,000 N
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1.
2.
What are two ways to increase the
acceleration of an object?
You can decrease the mass of the object.
You can increase the force applied to the object.
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Your word is Momentum.
 Chapter
10, sec. 4
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‘Newton’s third law state that if one
object exerts a force on another object,
then the second object exerts a force of
equal strength in the opposite direction
on the first object.’
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Prentice Hall Science Explorer Grade 8, p. 353
Basically, this means that for every action
there is an equal but opposite reaction.
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Action-Reaction Pairs
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The third law explains action-reaction pairs – when
you jump, you exert a downward force on the floor
and then the floor exerts an equal force back up to
you.
See p. 354 for examples of action-reaction pairs.
Name a few action-reaction pairs that you have
observed!
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Can we always detect motion in an action-reaction
pair?
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No – sometimes the reaction to the action is so small that
it is undetectable.
Do Action- Reaction forces ever cancel out?
No – because they are acting on different objects
 Unlike balanced forces acting on the same object, an
action-reaction pair has forces acting on two different
objects so they cannot cancel each other out.
 If you hit a volleyball with your arms, the volleyball
exerts an equal force back to you, not back to the ball.
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Momentum is a characteristic of a moving
object that is related to the mass and velocity of
that object.
Momentum is calculated by multiplying the
mass times the velocity.
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Momentum = Mass x Velocity
Momentum = kg x m/s (NOTE: this is not equal to a
newton – remember a newton = 1kg-m/s2)
The unit for momentum is simply kg-m/s
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What is the momentum of a bird with a mass of
0.018 kg flying at 15m/s?
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Answer = 0.27 kg-m/s
Momentum involves direction of motion so it is
calculated using velocity, not speed.
The more momentum and object has, the
harder it is to stop.
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The Law of Conservation of Momentum states
that, without outside forces, the total
momentum of objects that interact does not
change.
This means that momentum is conserved (or
saved) – momentum is neither lost or gained
when two objects collide – it is simply
transferred from one object to the other.
The total momentum of any group of objects
remains the same unless outside forces act on
the group.
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Collisions with 2 moving objects – the momentum of
one object decreases while the momentum of the other
object increases but total momentum stays the same.
Collisions with 1 moving object – the momentum from
the moving object is transferred to the stationary object
which causes the moving object to stop and the
stationary object to start moving, but the total
momentum stays the same.
Collisions with connected objects – when two objects
collide and become connected as a result of that
collision, ½ of the momentum of the moving object is
transferred to the nonmoving object when they connect
making the total momentum stay the same.
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This is taken from chap. 10, sec. 5.
Centripetal force is what makes objects move in
a circle.
Centripetal means center-seeking.
Something traveling in a circle is still
accelerating because it is constantly changing
direction.
Satellites (any object that orbits another object
in space) travel in a circle around the Earth
because of the Earth’s gravity pulling the
satellites toward the center of the Earth.
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Since an object’s momentum depends on its
mass and velocity, if two dogs are running at
the same velocity, but one dog is large and one
dog is small, which dog will have the greatest
momentum?
Answer – the dog with the greater mass will
have the greater momentum.