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Acceleration and Momentum
Chapter 3
May 17
General Science Chapter 3
1
Review
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Newton’s first law
Weight of one kilogram on Earth’s
surface
Gravity
May 17
General Science Chapter 3
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Newton’s second law
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A net force acting on an object causes
the object to accelerate in the direction
of the force.
The acceleration is affected by two
things
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size of force
mass of object
General Science Chapter 3
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Force
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If the force on an object increases,
acceleration will increase for that object
and if force decreases the acceleration
will decrease. IF THE MASS REMAINS
CONSTANT
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Mass
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A more massive object will take a larger
force to give it the same acceleration as
a smaller massed object.
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Equation for Newton’s 2nd law
force  mass  acceleration
F  ma
F  ma

We can also use the “magic” triangle
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General Science Chapter 3
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Units on force
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Last chapter we learned that force has
units of Newtons.
From Newton’s second law equation,
m
N  kg  2
s
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Example
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How much force is needed to accelerate
a 70-kg rider and the 200-kg
motorcycle the rider is on at 4 m/s2?
F = 270 kg x 4 m/s2
1080 N
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You try
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How much force is needed to accelerate
a 60-kg person and the 500-kg car the
person is in at 6 m/s2?
F = 560 kg x 6 m/s2
3360 N
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You try
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A 63-kg skater pushes off from a wall
with a force of 300 N. What is the
skater’s acceleration?
300 N = 63 kg • a
a = 300 N ÷ 63 kg
4.76 m/s2
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Review #1
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What is Newton’s 2nd Law of Motion?
What equation do we use with the 2nd
Law of Motion?
What 2 things will affect acceleration?
What are the units for Force?
True or False: A more massive object
will take less force to accelerate?
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Acceleration due to gravity
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Near Earth’s surface, gravity causes all
falling objects to accelerate at a rate of
9.8 m/s2, regardless of their mass.
Acceleration due to gravity is
represented by the letter g.
Does this number seem familiar?
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Weight
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The weight of an object is the force of
gravity acting on it.
F  ma
W  mg

W  1 kg  9.8 m/s
2

W  9.8 N
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Reality check
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Do all objects really fall with the same
acceleration?
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Air resistance
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The force air exerts on a moving object.
Acts in the opposite directions to which
an object is moving.
For falling objects, air resistance pushes
up while gravity pulls down.
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Air resistance
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Depends on the speed, size, and shape
of the object.
The larger the object, the more air
resistance affects it.
The faster an object is moving, the
more air resistance affects it.
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Terminal velocity
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As an object falls, air resistance
gradually increases until it equals the
pull of gravity.
At this point, the object stops
accelerating and moves with a constant
velocity – called its terminal velocity.
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Example
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A ball is dropped from the roof of a
50-m tall building. It hits the ground
3.2 s later. What is its velocity right
before it hits the ground?
a=v÷t
OR
v=a•t
v = 9.8 m/s2 x 3.2 s
31.36 m/s
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Review #2
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Define air resistance.
What is air resistance dependent upon?
What is terminal velocity?
What is the acceleration of gravity near
the surface of the earth?
How do we determine the weight of an
object?
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Projectiles
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Anything that’s shot or thrown through
the air.
Will always follow a curved path.
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Projectile motion
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Can be split into independent vertical
and horizontal motions.
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The horizontal part
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Once the object is released, there is no
force acting on it horizontally.
If there is no force, it must maintain a
constant horizontal velocity.
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The vertical part
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There is a force acting – gravity
The object is pulled downward with a
constant acceleration of 9.8 m/s2.
May 17
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Circular motion
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Objects moving in circles are constantly
accelerating.
This acceleration is towards the center
of the circle or curve.
It is called centripetal acceleration.
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Towards the center of a circle or a curved
path.
General Science Chapter 3
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Centripetal force
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The force that causes an object to
move in a curved or circular path.
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Weightlessness
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To be truly weightless, an object would
have to be free from gravity.
To feel weightless, something has to be
in free fall along with its surroundings.
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Weightlessness
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How does a scale measure weight?
If it can’t push back, it would read zero.
You would feel weightless.
May 17
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Review #3
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What is a projectile?
What are the 2 components of a
projectile?
What does centripetal mean?
Why are objects weightless during free
fall?
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Newton’s 3rd law
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To every action, there is an equal and
opposite reaction.
When one object exerts a force on a
second object, the second object exerts
a force on the first object that is equal
in size and opposite in direction.
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Examples
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Released balloon
Firing a gun
Jumping out of a boat
rockets
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Balanced forces?
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Action-reaction pairs of forces are not
balanced forces.
The forces act on different objects.
To be balanced forces, the equal and
opposite forces must act on the same
object
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Momentum
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A property a moving object has because
of its mass and velocity
momentum  mass  velocity
p  mv
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Momentum
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Has units of kg∙m/s
Has direction, because velocity has
direction
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Momentum
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Which has a higher momentum, a
50-kg dolphin swimming at 16.4 m/s or
a 6300-kg elephant walking at 0.11
m/s?
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Dolphin – (50 kg)(16.4 m/s) = 820 kg∙m/s
Elephant – (6300 kg)(0.11 m/s) = 693 kg∙m/s
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The dolphin has higher momentum.
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Example
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Which has higher momentum, a 65-kg
person running at 9 m/s or a 60-kg
person running at 11 m/s?
1st person – (65 kg)(9 m/s) = 585 kg∙m/s
2nd person – (60 kg)(11 m/s) = 660 kg∙m/s
The 60-kg person has higher momentum.
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Law of conservation of
momentum
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The total amount of momentum of a
group of objects does not change
unless outside forces act on the objects.
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Momentum transfer
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Momentum can be transferred from one
object to another.
Think about playing pool
The cue ball transfers its momentum to
the ball it hits.
The momentum lost by the cue ball is
gained by the other ball.
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Example
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A 5-kg object with a velocity of 6 m/s
strikes a motionless 10-kg ball. The 5
kg object stops moving. What is the
velocity of the 10 kg ball after the
collision?
Momentum of object before collision:
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May 17
(5 kg)(6 m/s) = 30 kg∙m/s
General Science Chapter 3
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Example continued
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When the object stops, it loses all its
momentum.
The momentum it lost is transferred to
the ball.
Momentum of the ball after collision:
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(10 kg)(velocity) = 30 kg∙m/s
Velocity = 3 m/s
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Example continued
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Notice that the velocity was not
conserved, but momentum was.
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Review #4
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What is Newton’s 3rd Law of Motion?
Define momentum.
What units does momentum have?
What is the law of conservation of
momentum?
May 17
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Chapter 3 Review
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State all 3 Laws of Motion
What 2 things affect acceleration on an
object
What is air resistance dependent upon?
How are the Law of Inertia and the Law
of conservation of momentum related?
May 17
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