Forces and Motion Book Chapter 12

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Forces and MotionBook Chapter 12

• 12.1 - Forces
• 12.2 - Newtons 1st and 2nd Laws
• 12.3 - Newtons 3rd Law Momentum
• 12.4 - Universal Forces

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Forces

• A force is a push or pull that acts on an object.
• A force can cause a resting object to move, or it
  can accelerate a moving object by changing the
  objects speed or direction.

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Measuring Force

• Forces are measured in Newtons (named for Sir
  Issac Newton).
• One Newton is the force that causes a 1 kilogram
  mass to accelerate at 1 m/s2.

4
Combining Forces

• Forces are vectors, so you can use an arrow to
  represent it. The arrow has to be proportional
  to the force and in the same direction.
• When you combine forces by adding or subtracting,
  you are finding the Net Force.
• The net force is the overall force acting on an
  object after all forces have been combined.

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Balanced and Unbalanced Forces

• When the forces on an object are balanced, the
  net force is zero and there is no change in the
  objects motion.
• When an unbalanced force acts on an object, the
  object accelerates.

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Friction

• Friction is a force that affects motion by
  slowing an object down.
• It is always opposite to the direction of motion
  and affects how much an object can accelerate.
• Friction is present because of irregularities in
  the surfaces of objects that are in contact with
  one another.
• Fluids like water and air also exert a friction
  force on a moving object.

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Four Types of Friction

1. Static Friction friction force that acts on
   objects that are not moving
2. Sliding Friction Force that opposes the
   motion of an object as it slides over a
   surface
3. Rolling Friction-friction that acts on rolling
   objects
4. Fluid Friction-opposes motion as an object
   moves through a fluid

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Friction

• Direction of motion

FRICTION BETWEEN TIRES AND ROAD
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Gravity

• Gravity is an attractive force that acts between
  any two masses.
• Gravity causes objects to accelerate downward
  towards the center of the earth.

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Falling and Air Resistance
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Falling and Air Resistance

• As an object falls faster, the force of air
  resistance increases until it equals the weight
  of the object. At this time, the net force
  equals zero and the object stops accelerating.
• This final speed is called terminal velocity.

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Falling and Air Resistance
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Discovering the Reason for Motion

• Aristotle- Greek scientist who thought that
  force was necessary to maintain motion
• See Aristotles ideas on motion.

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Discovering the Reason for Motion

• Galileo Italian scientist who did experiments
  that helped correct misconceptions about force
  and motion

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Discovering the Reason for Motion

• Newton scientist who introduced several laws
  describing force and motion.

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Newtons 1st Law

• Law of Inertia
• Everything in motion stays in motion, and
  everything at rest stays at rest unless an
  outside force acts upon it.
• Everything in motion and at rest has inertia.

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Law of Inertia
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Law of Inertia
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Law of Inertia
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Mass

• Mass determines the amount of inertia possessed
  by an object.
• Mass is the amount of matter in an object.

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Mass is NOT Volume

• Volume is the amount of space an object takes up.
• EXAMPLE
• A pillow will have a volume larger than a car
  battery but less mass.

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Mass is NOT weight

• Weight is the force of gravity acting on a body
• Objects in space will still have mass but will
  not have weight.
• Metric Units mass - kilogram
• weight - Newton
• On the surface of the earth, mass and weight are
  proportional.
• 1 kg 9.8 Newtons

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Calculating the weight of an object

• Since weight depends on the mass of an object AND
  the gravitational pull of the earth, we can say
  that
• Weight mass x gravity
• Or
• Weight mg
• The weight of an object is measured in Newtons.
• Because weight depends on gravity, it is also a
  force.

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What is the weight of a 500 gram rock?

• 1. Convert to standard units
• 500 grams .5 kg
• 2. Remember the constant for gravity is 9.8
  m/s2.
• 3. Plug into the equation and solve.
• Weight mg
• Weight (.5 kg)(9.8 m/s2)
• Weight 4.9 N (Newtons)

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Questions

• 1. Your empty hand is not harmed if it bangs
  against the wall, but it is harmed if you are
  carrying a heavy load. Why?
• 2. Does a person diet to lose mass or to lose
  weight?
• 3. Can the force of gravity on a 1 kg mass ever
  be greater than on a 2 kg mass? Explain how.
• 4. A car at a junk yard is compressed until its
  volume is less than 1 cubic meter. Has its mass
  changed? Has its weight changed? Has its volume
  changed? Explain.
• 5. If you jump up in a bus that is moving at a
  constant velocity, will you land farther back in
  the bus? Explain.

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Newtons 2nd Law of Motion

• Law of Acceleration
• What causes an object to accelerate?
• -or change its state of motion?
• NET FORCE

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What is a Net Force?

• A net force is the combination of all forces
  acting on an object
• Applied Forces Net Force

15 N
5 N
10 N
5 N
10 N
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Equilibrium

• Applied Forces Net Force

0 N
5 N
If the net force is zero, the motion of the
object will not change. This is a condition
called EQUILIBRIUM. If an object is in
equilibrium, we have to go back to the Law of
Inertia.
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Acceleration

• Acceleration is directly related to the net
  force.
• Acceleration Net Force
• The larger the net force is acting on an object,
  the greater the acceleration of the object.

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Acceleration

• Acceleration is inversely proportional to the
  mass of an object
• Acceleration
• Try to push a 250 pound crate. Now try to push a
  25 pound crate. Which could you move faster?

1 mass
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Newtons 2nd Law of Motion

• The acceleration of an object is directly
  proportional to the net force acting on it, is in
  the same direction as the net force, and
  inversely proportional to the mass of the object.
• a

F m
OR F ma
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F
a
m
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Newtons 3rd Law of Motion

• Action / Reaction
• Law of Interaction
• Newton realized that force is not a thing in
  itself, but part of an interaction between one
  thing and another.

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Forces always occur in pairs

• One force is called the action force and the
  other the reaction force.
• Action Object A exerts a force on Object B.
• Reaction Object B exerts an equal and
  opposite force on Object A.

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Newtons 3rd Law

• For each action force, there is an equal and
  opposite reaction force.
• Whenever one object exerts a force on a second
  object, the second object exerts an
• equal and opposite force on the first object.

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Action / Reaction Forces
Action Tire pushes road Reaction Road pushes
tire
Action Rocket pushes gas Reaction Gas pushes
rocket
Action Earth pulls ball Reaction Ball pulls
Earth
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Action / Reaction Forces
Consider the apple at rest on the table. If we
call the gravitational force exerted on the apple
action, what is the reaction force? Are there
any other action/reaction forces present?
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Action / Reaction Forces
If a cement truck and a car have a head-on
collision, which vehicle will experience the
greater impact force? A. The cement truck B.
The car C. Both the same D. .it depends
on other factors
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Momentum

• Momentum is inertia in motion.
• Momentum (mass)(velocity)
• It is harder to stop a large truck than a small
  car when both are moving at the same speed
  because the truck has a larger mass and larger
  momentum

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Calculating Momentum
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Example

• Which would be harder to stop.a .25 kg ball
  thrown at you at 40 m/s or a 7 kg bowling ball
  rolled at 1 m/s?

It would be harder to stop the baseball it has
more momentum.
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Law of Conservation of Momentum

• In the absence of an external force, the total
  momentum of a system remains unchanged.
• This is an extension of Newtons 3rd Law
• (Mass)(Velocity before) (Mass)(Velocity after)
• See an example of the Law of Conservation of
  Momentum (and Energy)

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Conservation of Momentum
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Conservation of Momentum
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Universal Forces

• Electromagnetic forces, Nuclear forces, and
  Gravitational forces are all universal forces.
• Electromagnetic force is associated with charged
  particles.
• Electric force and magnetic force are the only
  forces that can both attract and repel.

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Nuclear Forces

• Two forces, the strong nuclear force and the weak
  nuclear force, act within the nucleus to hold it
  together.
• The strong nuclear force acts only on the
  neutrons and protons in the nucleus.
• The weak nuclear force acts over a short range
  and affects all particles, not just protons and
  neutrons.

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Gravitational Force

• Gravitational force is an attractive force that
  acts between two masses.
• The bigger the masses and the closer they are,
  the more the attraction force.
• Newtons Law of Universal Gravitation states that
  every object in the universe attracts every other
  object.

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Gravitational Force

• Gravity is the weakest universal force, but it is
  the most effective over long distances.
• Earths gravitational force keeps the moon in a
  nearly circular orbit.
• The gravitational pull of the moon on the Earth
  causes ocean tides.

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Satellites

• When an artificial satellite is put into orbit,
  its inertia and the pull of the Earth keep it in
  orbit. If the satellite is too close to the
  atmosphere, friction slows it down and it
  crashes into the Earth.

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Satellite without Gravity
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Launch speed less than 8000 m/s
Launch speed equals 8000 m/s