Forces and the Laws of Motion

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Forces and the Laws of Motion
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Forces

• Vocabulary
• Force
• Balanced Force
• Unbalanced Force
• Net Force
• Newton
• Friction

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Forces

• A force is a push or a pull that starts, stops,
  or changes the direction of motion of an object.
• A force is also the cause of acceleration, or
  change in an objects velocity.
• Direction is important.
• You are constantly affected by some kind of force.

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Forces

• The combination, or sum, of all forces acting on
  an object determines whether the object will
  experience a change in velocity (acceleration).
• This combination of forces is known as the net
  force.
• If the net force zero, the forces are balanced.
• If the net force ? zero, the forces are
  unbalanced.

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Forces

• When the net force equals zero, the forces are
  balanced.
• Balanced forces do not change the motion of an
  object.
• The object will continue to move with constant
  velocity (zero acceleration) or stay at rest.

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Forces

• When the net force does not equal zero, the
  forces are unbalanced.
• Unbalanced forces cause an object to change its
  velocity (acceleration) or change directions.
• The direction the object moves is in the same
  direction as the net force.

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Force

• The SI unit for force is the Newton (N).
• 1 N 1 kg x 1 m/s2
• The pound is the British unit of measurement for
  force.
• Some common forces include friction, weight, and
  gravity.

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Forces

• The force of friction allows motion to start and
  stop.
• Friction occurs when the surfaces of any kind of
  matter move past each other.
• Friction depends on the types of surfaces in
  contact, and the surface area in contact.
• Friction may be either static or kinetic.

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Forces

• How are weight and mass different?
• Weight is a force.
• The weight of an object depends on the force that
  pulls the object toward the earth.
• This is known as the force of gravity.
• The force of gravity pulls objects toward the
  earth with an acceleration of 9.8 m/s2.
• This is often represented as g.

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Forces

• The mass of an object never changes.
• The weight of an object can change depending on
  the force of gravity exerted. (You may be lighter
  or heavier on other planets.)

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Forces

• Homework
• Read pages 52-56 in your textbook. (Be ready for
  a possible pop-quiz.)
• List five examples of balanced and unbalanced
  forces that you observe at home.

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Newtons Laws of Motion

• Vocabulary
• Inertia
• Gravity
• Mass
• Weight
• Newtons 1st Law
• Newtons 2nd Law
• Newtons 3rd Law

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Newtons Laws of Motion

• Without friction an object would keep moving or
  stay at rest forever.
• Newton summarized the relationship between motion
  and force in three laws.
• Newtons 1st Law of Motion states that an object
  at rest remains at rest or an object in motion
  remains in motion unless acted upon by an
  unbalanced force.

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Newtons Laws of Motion

• According to Newtons 1st law, an object in
  motion will continue along a straight path. Only
  an unbalanced force can change its direction.
• Newtons 1st law of motion is also known as the
  Law of Inertia.
• Inertia is the tendency of an object to resist a
  change in motion. Therefore, if an object is
  moving it will continue moving in a straight
  line, or if an object is at rest it will continue
  resting.
• The amount of inertia an object has is directly
  related to its mass.

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Newtons Laws of Motion

• Homework
• Enrichment Worksheet, Section 3 page 32
• Read 68 78 in your book. Answer questions 1-5
  on page 74.

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Newtons Laws of Motion

• Newtons 2nd law of motion describes the effect
  of an unbalanced force acting on an object.
• Newtons 2nd law of motion states that the net
  force (or unbalanced force) acting on an object
  is equal to the objects mass times its
  acceleration.
• The direction of the acceleration is in the same
  direction as the net force.

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Newtons Laws of Motion

• In mathematical terms, Newtons 2nd law states
• F ma. (Force mass x acceleration)
• The F in this equation is the net force or sum of
  all forces acting on an object.
• Are force and acceleration directly or inversely
  related?

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Newtons Laws of Motion

• Homework
• Newtons 2nd Law Practice Problems
• Enrichment Worksheet page 32
• Review Worksheet page 14

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Newtons Laws of Motion

• According to Newtons 3rd law of motion, forces
  occur in pairs.
• Newtons 3rd law of motion states that for every
  action force, there is an equal and opposite
  reaction force.
• The action and reaction force of a force pair act
  on different objects.

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Newtons Laws of Motion

• When you kick a soccer ball, your foot exerts a
  force on the ball. At the same time, the ball
  exerts an equal and opposite force on your foot.
• This is not a case of balanced forces. Each force
  is acting on a different object.
• Newtons 3rd law is used to study rocket
  propulsion.

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Newtons Laws of Motion

• Homework
• Section Activities Worksheet Review Page 16
• Skills Worksheet pages 12-13

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Applications of Forces

• Vocabulary
• Centripetal Force
• Centripetal Acceleration
• Buoyancy
• Pressure
• Archimedes Principle
• Bernoullis Principle
• Pascals Principle

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Applications of Forces

• Newtons law are used to analyze several types of
  motion including free-fall, projectile, and
  circular motion.
• When gravity is the only force acting on an
  object, it is said to be in free fall.
• In the absence of air resistance (friction) all
  objects accelerate toward the earth at the same
  rate regardless of their mass. (g)

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Applications of Forces

• Recall that weight is the force of gravity acting
  on an object. Therefore, even if you are not
  freely falling, your weight can be treated as
  such.
• Based on Newtons 2nd law, weight is equal to
  mass times acceleration due to gravity.
• W mg

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Applications of Forces

• The arrow will follow a curved path to the
  ground.
• Anything that is thrown or shot through the air
  is called a projectile.
• Projectiles follow a curved path because of
  Earths gravitational pull.
• Projectiles have both horizontal and vertical
  velocities.

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Applications of Forces

• Both balls would it the ground at the same time.
• Imagine if you could throw a ball, and it kept
  going? What type of path would the ball follow?
• Would the ball maintain this path? If so, what
  type of force is keeping the ball in this
  direction?

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Applications of Forces

• Centripetal force is any force that allows an
  object to follow a circular path.
• The centripetal force is always directed toward
  the center of the circular path.
• Imagine driving a car around a race track. As you
  turn the curves, the centripetal force is the
  force of friction between the car and the road.
  In which direction is your body pushed while
  turning the curves?

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Applications of Forces

• If there is a centripetal force acting on an
  object, the object is experiencing centripetal
  acceleration.
• Centripetal acceleration is acceleration directed
  toward the center of the circular path.
• Centripetal force explains how the planets orbit
  the sun, and why the moon orbits the Earth.
• Satellites are constantly being pulled toward the
  Earth due to centripetal acceleration.

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Applications of Forces

• Homework
• Read pages 79-82, Answer questions 1-4 in page
  82.

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References

• Glencoe, Integrated Physics and Chemistry. 2002
• Holt, Science Spectrum A Physical Approach. 2002
• Addison-Wesley, Science Insights Exploring
  Matter and Energy. 1997