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Chapter 13, Section 2
Gravity and Motion
Pages 376 - 382
Falling Objects
• Galileo - all objects fall
to the ground at the
same rate (9.8 m/s/s)
because the acceleration
due to gravity is the
same for all objects.
• Velocity of falling
objects: ∆v = g x t
g = 9.8 m/s/s
t=falling time
Example
• 1. If a stone at rest is dropped from
a cliff and it takes 3 seconds to hit
the ground, what is the stone’s
downward velocity when it hits the
ground?
• ∆v = g x t
• ∆v = 9.8 m/s/s x 3s
• ∆v = 29.4 m/s
Practice
• A penny at rest is dropped from the
top of a stairwell. What is the
penny’s velocity when it hits the
ground after falling for 4.5 seconds?
• ∆v = g x t
• ∆v = 9.8 m/s/s x 4.5 s
• ∆v = 44.2 m/s
Air Resistance
• Air resistance is a
type of fluid friction
that opposes the
motion of objects
moving through the
air.
• The amount of air
resistance on an
object depends on its
size and shape.
Terminal Velocity
• Eventually the gravitational force pulling
down on a falling object equals the force of
air resistance pushing up.
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
happen in a vacuum
or in space.
• The forces are then balanced, the
object quits accelerating and falls at a
constant velocity = terminal velocity.
Projectile Motion and Gravity
• Projectile motion is
the curved path an
object follows when
thrown or propelled
near the surface of
the Earth.
• Has two components
- horizontal and
vertical.
Horizontal Motion
• Horizontal
motion is the
motion that is
parallel to the
ground.
• When you throw
a ball, your hand
gives the ball its
horizontal
motion.
Vertical Motion
• Vertical motion is the motion that is
perpendicular to the ground.
• Gravity causes vertical motion.
Orbiting and Gravity
• An object is orbiting when it is moving
around another object in space.
• Centripetal force is the unbalanced
force that makes an object move in a
circular path around another object.
• Gravity provides the centripetal force
on the planets and their moons that
keeps them in orbit