Physical Science Gravity

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Physical Science Gravity
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Objectives

• Explain that gravitational force becomes stronger
  as the masses increase and rapidly become weaker
  as the distance between the masses increases,
  FG(m1m2/d2)
• Evaluate the concept that free-fall acceleration
  near Earths surface is independent of the mass
  of the falling object
• Demonstrate mathematically how free-fall
  acceleration relates to weight
• Describe orbital motion as a combination of two
  motions

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• The law of universal gravitation
• All objects in the universe attract each other
  through gravitational force (Sir Isaac Newton
  1642-1727)
• Universal Gravitation Equation
• FG(m1m2/d2)

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• Gravitational force increases as one or both
  masses increases
• Gravitational distance decreases as the distance
  between the masses increases
• G is a constant
• If the distance between two objects is doubled,
  the gravitational force between them decreases to
  ¼ the original value
• If distance is tripled, gravitational force
  decreases by 1/9 the original value

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Free Fall

• Free fall- the motion of body when only the force
  of gravity is acting on the body
• Free fall results from gravity g 9.8m/s2
• Free fall acceleration of an object is directed
  toward the center of Earth

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• In a vacuum (an absence of air resistance) a
  feather and an apple accelerate at the same rate
  regardless of mass.
• Why?
• Newtons second law of motion (acceleration
  depends on both force and mass)

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• Weight- the force on an object due to gravity
• Weight is equal to mass times free-fall
  acceleration
• Weightmass x free-fall acceleration
• w mg

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• Weight is a force, the SI unit of weight is the
  newton, N.
• wmg
• Ex)
• 1.0kg x 9.8m/s2 9.8 N

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Recall the difference between mass and weight

• Mass is the measure of the amount of matter in an
  object (this does not change)
• Weight changes as gravitational force changes

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Space

• Planets in our solar system have different
  diameters and different masses. Therefore each
  planet has a unique value for g.
• Earth, g 9.8m/s2
• Venus, g 8.8m/s2
• Mars, g 3.7m/s2
• Neptune, g 11.8m/s2

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Velocity and air resistance

• A falling object stops accelerating when the
  force of air resistance the gravitational force
  on an object (the weight)
• Terminal velocity- the constant velocity of a
  falling object when the force of air resistance
  is equal in magnitude and opposite in direction
  to the force of gravity

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Free Fall

• An object is in free fall only if gravity is
  pulling it down and no other forces are acting on
  it
• Free fall can only occur in a vacuum- no air and
  no matter, or in space
• Orbiting objects are in free fall

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Orbiting

• Two motions combine to cause orbiting
• Orbiting- when an object is traveling in a
  circular or nearly circular path around another
  object
• Linear motion forward downward pull of gravity
  curve of Earths surface

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Projectile motion

• Two motions combine to form projectile motion
• Projectile motion- the curved path that an object
  follows when thrown, launched, or otherwise
  projected near the surface of Earth the motion
  of objects that are moving in 2-D under the
  influence of gravity

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Projectile motion

• Horizontal velocity vertical downward pull of
  gravity curved path
• The two velocities are independent of each other