Chapter 3: Gravity, Friction, and Pressure

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Chapter 3 Gravity, Friction, and Pressure

• 12.1 Gravity is a force exerted by masses
• 12.2 Friction is a force that opposes motion
• 12.3 Pressure depends on force and area
• 12.4 Fluids can exert a force on objects

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12.1 Gravity is a force exerted by masses

• Masses attract each other
• Gravity is the force that objects exert on each
  other because of their masses
• Gravity accelerates all masses equally
• Gravity is a universal force acts between any
  two masses anywhere in the universe
• Between the earth and moon, sun and moon
• Between dust and gas particles in space helped
  form the solar system

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The force of gravity

• Recall the net force on you determines how your
  motion changes
• Force between you and the desk is extremely small
  compared with other forces constantly acting on
  you (friction, muscles, Earths gravity, other
  objects)
• Strength of the gravitational force between two
  objects depends on mass and distance
• Greater mass results in greater force
• Smaller distance results in greater force

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Gravity on Earth

• Gravity acts on both masses equally, even if the
  masses are very different
• Every action force has an equal and opposite
  reaction force
• Earths gravity exerts a force on a coin, the
  coin exerts an equal upward force on Earth
• Small coin mass means it can easily be
  accelerated
• Large Earth mass means it is much more difficult
  to accelerate
• g acceleration due to gravity 9.8 m/s2 at
  Earths surface (moon 1.6 m/s2)
• Newtons 2nd law Fma Fmg

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Gravity

• In a vacuum
• Quarter vs penny falling
• Quarter has more mass so gravity exerts more
  force on it
• But it also has more inertia, so the greater
  force does NOT produce a larger acceleration
• Therefore objects with different masses fall
  with the same acceleration

http//www.classzone.com/books/ml_science_share/vi
s_sim/mfm05_pg79_vacuum/mfm05_pg79_vacuum.html
                       At a given location on the earth and in the absence of air resistance, all objects fall with the same uniform acceleration. Thus, two objects of different sizes and weights, dropped from the same height, will hit the ground at the same time.
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Gravity

• In a vacuum
• Coin dropped falls at the same rate as one thrown
  forward
• Horizontal velocity does not affect acceleration
  due to gravity
• Gravity is directed downward so it changes only
  the downward velocity of the coin, not its
  forward velocity

An object is controlled by two independent motions. So an object projected horizontally will reach the ground in the same time as an object dropped vertically. No matter how large the horizontal velocity is, the downward pull of gravity is always the same.                                  
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Gravity

• At the instant a cannon fires a cannonball
  horizontally over a level range, another
  cannonball held at the side of the cannon is
  released and drops to the ground.
• Which strikes the ground first?

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Distance Fallen

• Distance fallen in one second is
• 5 meters.
• This distance fallen is the same whether falling
  straight down or in projectile motion.

0.5 s
5 meters
1.0 s
1.5 s
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• http//scienceblogs.com/dotphysics/2009/10/mythbus
  ters-bringing-on-the-physics-bullet-drop.php
• http//www.youtube.com/watch?vD_JbgP8mpsE

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Weight and Mass

• Mass is a measure of how much matter an object
  contains
• Same no matter where the object is located
• Weight is the force of gravity on that object
• Depends on the force of gravity acting upon it
• On Earth Mass 50 kg, Weight 490 N
• On Moon Mass 50 kg, Weight 82 N

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Gravity keeps objects in orbit

• Orbit elliptical path one body follows around
  another body due to the influence of gravity
• Centripetal force keeps one object in orbit
  around another due to the gravitational pull
• Between Earth and Moon, Earth and Sun
• falling around Earth

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Projectile Motion Curvature
For initial speeds that are faster and faster,
the range of the projectile is farther and
farther. For very large speeds, the curvature of
Earth starts to be noticeable.
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Earths Curvature

• Curvature of the Earth is about 5 meters over a
  distance of 8000 meters (which is about 5 yards
  over 5 miles).

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Missing the Ground
Suppose you throw a ball at a speed of 8000 m/s
(about 18,000 mph). After one second, ball
travels 8000 meters and falls 5 meters. In that
distance, Earth curves by same amount (5 meters).
If nothing stops the ball, what happens?
8000 m
5 m
Curvature
NOT to Scale
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Orbits and Centripetal Force

• Gravity provides the centripetal force required
  for a satellite to move in a circle.

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Getting into Orbit

• -Rocket needs to lift above
• the atmosphere and then
• fire thrusters to acquire the
• required orbital speed of
• about 8000 meters (8 km) per second.
• -path of the falling object
• matches the curve of Earths surface
• -greater than 11,000 m/s and
• the spacecraft will escape the
• pull of Earths gravity!

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Elliptical Orbits

• For speeds higher than 8 km/s, the orbit is
  elliptical instead of circular.

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Escape Speed
If speed exceeds 11.2 km/s then object escapes
Earth because gravity weakens (as object gets
further away) and never slows the object enough
to return it back towards Earth.
Hyperbolic
Circular
Elliptical
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People in Orbit

• Elevator on a scale during downward
  acceleration, you would appear to weigh less
  the scale is also moving downward, and you are
  pushing on it less
• If in free fall (fall entirely due to gravity),
  you would not press against the scale at all
• Spacecraft in orbit is in free fall (around
  Earth)
• Astronauts weight does not press against the
  floor of the spacecraft, so objects behave as if
  there were no gravity

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In-class activity p.394
Time (s) Velocity (m/s)
0 0
2 18
4 29
6 33
8 35
10 36
12 36
14 36
16 36
18 36

1. For both variables, decide the scale that each
   box on your graph will represent and what range
   you will show for each variable.
2. Determine the dependent and independent
   variables.
3. Plot the independent variable along the
   horizontal axis, and the dependent along the
   vertical axis.

Challenge alter the scale you chose to use in
1. Graph again. How do different scales give
different impressions of the data?
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• http//www.edinformatics.com/math_science/solar_sy
  stem/gravity_mass_weight.htm