Transcript Physical Science Gravity

Physical Science Gravity
Objectives
• Explain that gravitational force becomes
stronger as the masses increase and rapidly
become weaker as the distance between the
masses increases, F=G(m1m2/d2)
• Evaluate the concept that free-fall acceleration
near Earth’s 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
• The law of universal gravitation:
– All objects in the universe attract each other
through gravitational force (Sir Isaac Newton
1642-1727)
– Universal Gravitation Equation
• F=G(m1m2/d2)
• 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
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
• In a vacuum (an absence of air resistance)
a feather and an apple accelerate at the
same rate regardless of mass.
• Why?
• Newton’s second law of motion
(acceleration depends on both force and
mass)
• Weight- the force on an object due to
gravity
• Weight is equal to mass times free-fall
acceleration
• Weight=mass x free-fall acceleration
• w= mg
• Weight is a force, the SI unit of weight is
the newton, N.
• w=mg
• Ex)
• 1.0kg x 9.8m/s2 = 9.8 N
*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
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
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
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
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
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
Projectile motion
• Horizontal velocity + vertical downward
pull of gravity= curved path
• The two velocities are independent of
each other