Transcript 3. Both the same.

Review 5-7
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5 Newton’s 3rd Law
6 Momentum
7 Energy
Review Questions
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Forces and Interactions
• inter = between
• all forces are interactions between masses
• each mass feels same size force
Newton’s 3rd Law of Motion:
Whenever one object exerts a force on a second
object, the second object exerts an equal and opposite
force on the first.
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5 Newton’s Third Law
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What is the net force on the boxed
system? Explain with a diagram.
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What is the net force on the boxed
system? Explain with a diagram.
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Force Diagram each object. Which
has greater acceleration when
released?
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Vectors
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magnitude and direction
represented by arrows
examples: force, velocity, and displacement
vector addition is done tail-to-head
horizontal part is the “x component”
vertical part is the “y component”
Pythagorean theorem applies to x and y
components, i.e.
Size  (horizontal ) 2  (vertical) 2
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Example: Vector Sum of 1 and 1 is
the square root of 2.
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Example: A 30N force + 40N force could be
replaced with a single 50N force.
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The x-component of the 50N force is 40N.
The y-component of the 50N force is 30N.
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6 Momentum
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Impulse-Momentum
Momentum = mv
impulse = Ft
• Ft = D(mv)
SI Unit: kg·m/s
SI Unit: N·s = kg·m/s
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Conservation of Momentum
• Momentum of a System is constant when
the net external force on it is zero.
• This law is usefully applied in many
situations
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Conservation of Momentum
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Conservation of Momentum
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Conservation of Momentum
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Bouncing Motion
Dead stop: D(mv)= 0 – mv = – mv
Bouncing: D(mv) = –mv – mv = – 2mv
• bouncing object exerts/feels twice the
impulse as a dead stop
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Collisions
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any brief interaction between masses
inelastic (heat, sound, etc. are generated)
elastic (no heat, sound, etc. is created)
deformation is an important aspect of
collisions, e.g. ball being hit by club or bat
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7 Energy
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Work
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W = Fd (F parallel to d)
W = 0 (F perpendicular to d)
Work is change in energy
Energy is the capacity to do work
Energy can be positional or motional
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Energy
• Positional Energy is called Potential
Energy, e.g. stretched spring
• Motional Energy is called Kinetic Energy
• SI Units for Work and Energy:
J = Nm = kgm2/s2
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Work-Energy Theorem
• Work = D(KE)
• Examples: Positional-Motional
Bow and Arrow
Mass and Spring
Inclined Plane
Motor and Car
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Conservation of Energy
• Energy cannot be created or destroyed; it
may be transformed (by work) from one
form into another, but the total amount of
energy never changes.
• Energy Forms include: thermal, elastic,
chemical, electromagnetic, nuclear,
kinetic.
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Machine
• A device that changes an applied force by
increasing it, decreasing it, or changes its
direction.
• Examples of Machines: pulley, lever, jack,
inclined plane.
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Efficiency
• The percent of the work put into a machine
that is converted into useful work output.
efficiency = (work output)/(energy input)
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Review Questions
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What is different here if you held the scale from the right side? If
the scale were attached to a wall?
In the situation shown the scale reads
1. 100 N,
2. 200 N, or
3. Zero?
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scale reads 100. Nothing is different.
1. 100 N,
2. 200 N, or
3. Zero?
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Two identical rubber bands connect
masses A and B to a string over a
frictionless pulley of negligible
mass. The pulley is free and the
masses are accelerating.
The amount of stretch is greater in
the band that connects
1. A.
2. B.
3. Both the same.
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pulley is free to turn
does not affect size of
interaction between A and B
light frictionless pulley is
perfect machine for
redirecting a force without
changing its size
therefore 3rd Law applies:
1. A.
2. B.
3. Both the same.
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Two identical rubber bands connect
masses A and B to a string over a
frictionless pulley of negligible
mass.
The pulley is held and the masses
are motionless.
The amount of stretch is greater in
the band that connects?
1. A.
2. B.
3. Both the same.
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The pulley is held and the masses
are motionless.
The amount of stretch is greater in
the band that connects
1. A.
2. B.
3. Both the same.
The string is now interacting individually with
each side of the pulley – they are independently
hanging and each side holds the entire weight of
each object.
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Whenever an interaction occurs in a system, forces
occur in equal and opposite pairs. Which of the
following do not always occur in equal and opposite
pairs?
1. Impulses.
2. Accelerations.
3. Momentum changes.
4. All of these occur in equal and opposite pairs.
5. None of these do.
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Whenever an interaction occurs in a system, forces
occur in equal and opposite pairs. Which of the
following do not always occur in equal and opposite
pairs?
1. Impulses.
2. Accelerations.
3. Momentum changes.
4. All of these occur in equal and opposite pairs.
5. None of these do.
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In which car will you
be moving the fastest
at the very bottom of
the incline?
1. Front car
2. Middle car
3. Rear car
4. Other
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In which car will you
be moving the fastest
at the very bottom of
the incline?
1. Front car
2. Middle car
3. Rear car
4. Other
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Two smooth tracks of equal length
have “bumps”—A up, and B down, both of the
same curvature. If two balls start simultaneously
with the same initial speed, the ball to complete
the journey first is along
1. Track A.
2. Track B.
3. Same
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Two smooth tracks of equal length
have “bumps”—A up, and B down, both
of the same curvature. If two balls start
simultaneously with the same initial speed, the
ball to complete the journey first is along
1. Track A.
2. Track B.
3. Same
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Three baseballs are
thrown from the
top of the
cliff along paths
A, B, and C. If their
initial speeds are
the same and air
resistance is
negligible, the ball
that strikes the ground
below with the greatest speed will follow path
1. A.
2. B.
3. C.
5. All strike with the same speed.
4. Either A or C.
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Three baseballs are
thrown from the
top of the
cliff along paths
A, B, and C. If their
initial speeds are
the same and air
resistance is
negligible, the ball
that strikes the ground
below with the greatest speed will follow path
1. A.
2. B.
3. C.
4. Either A or C.
5. All strike with the same speed.
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