Gravity in Motion

1
Gravity in Motion

• Section 6-1
• Pages 150 - 156

2
In the beginning.

• According to the Greek philosopher Aristotle, an
  objects mass affects how fast it will fall
• He said that objects with larger masses will fall
  faster
• Aristotle never actually tested his theory
• Italian scientist Galileo argued that the mass of
  an object des not affect the time it takes to
  fall to the ground
• He proved this by dropping two different massed
  canon balls from the Leaning Tower of Pisa and
  they landed at the same time

3
Gravity and Acceleration

• The reason the two cannon balls hit the ground at
  the same time is because acceleration due to
  gravity is the same for all objects
• As weve talked about before, acceleration
  depends on mass and force
• A heavier object experiences greater
  gravitational force but is harder to accelerate
  due to its increased mass
• The extra mass of the heavier object balances the
  additional gravitational force

4
Acceleration due to gravity

• Remember that acceleration is the rate at which
  velocity changes over time
• All objects accelerate toward Earth at a rate of
  9.8 m/s2
• This means that for every second an object falls
  toward the Earth, its velocity increases by 9.8
  m/s

5
Calculating change in velocity

• ?v g x t
• ? (delta) stands for change in
• ?v change in velocity
• g acceleration due to Gravity (always 9.8 m/s2)
• t time the object takes to fall (in seconds)
• Do Math focus on page 151 as a class

6
Air resistance

• Two pieces of paper, one flat
• and one crumpled
• They will not fall at the same rate
• This is because of air resistance
• The force that opposes motion of objects in the
  air
• Its pushes up against the object, going against
  the force of gravity
• The amount of air resistance depends on the size,
  shape and speed of the object
• The more surface area, the more air resistance
• The more mass, the less air resistance
• The faster the object moves, the more air
  resistance

7
We have reached Terminal Velocity

• Objects do not continue to accelerate (increase
  their velocity) forever until they hit the ground
• This is because air resistance continues to
  increase as speed increases
• Air resistance increases until it is equal to the
  force of gravity
• Results in a net force of 0N
• Is there any further change in motion (increase
  in speed) if net force is 0N?
• NO!!

8

• When net force is 0N, the object falls at a
  constant velocity which is called its terminal
  velocity
• Terminal means at the end
• Terminal velocity is a good thing
• Think about hail storms
• If large chunks of hail did not reach a terminal
  velocity (usually between 5 and 40 m/s), they
  could reach velocities of up to 350 m/s!
• Terminal velocity is reached at a faster pace
  when using a parachute allows sky divers and
  airmen to reach the ground at a safer speed

9
Free Falling

• When you ride the Free Fall at Six Flags, you are
  not really in free fall
• Free Fall occurs when gravity is the only force
  acting on an object
• Therefore, the only place you can experience free
  fall is in space where there is no air
• Why?
• If there is air, there is air resistance, which
  is a force

10
Weightless or not?

• Astronauts are said to be weightless when they
  are in space and that is why they float, but
• You can never truly be weightless
• Why not?
• Everything creates a gravitational force so there
  will always be something around you that causes
  you to have weight
• So why do astronauts float?
• They are in free fall as they orbit the Earth

11
Orbits not the gum

• An object is orbiting when it is traveling around
  another object in space
• It is caused when an object is moving forward,
  but it is also in free fall (being pulled towards
  the Earth or another object due to gravity)
• The two forces combine to create a circular path
  of motion
• Remember that any object traveling in a circular
  path is constantly changing direction
• A force that causes circular motion is called a
  centripetal force

12
Aim High

• Projectile motion is the curved path an object
  takes when it is thrown near the Earths surface
• Two parts to projectile motion
• Horizontal motion
• Ie Caused by the force of your hand throwing the
  ball
• Vertical motion
• Caused by gravity
• When these two parts combine, they cause a curved
  path of motion