Astrophysics: Origin of Modern Astronomy 4

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Astrophysics Origin of Modern Astronomy 4
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Newtons Force

• Force

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Types of Forces

• Frictional force
• Normal force
• Applied Force
• Air Resistance Force
• Tension Force
• Spring Force
• Gravitational Force
• acceleration due to gravity on Earth
• g 9.8 m/s2

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Force Diagram
FN
Fgrav
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Balanced vs. Unbalanced Forces
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Newtons Three Laws

• First Law (Law of Inertia)

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FORCE IS NOT NEEDED TO KEEP AN OBJECT IN MOTION!!!
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Newtons Second Law

• Fma

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This is an example of how Newton's Second Law
works
50 newtons
Mike's car, which weighs 1,000 kg, is out of gas.
Mike is trying to push the car to a gas station,
and he makes the car go 0.05 m/s/s. Using
Newton's Second Law, you can compute how much
force Mike is applying to the car.
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Newtons Second Law
Fma

• Force does not cause MOTIONit causes
  ACCELERATION!!!

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Newtons Second Law
What happens if
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F mg
Fgrav mg Wt mg
m Fgrav/g
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Speed

• Rate at which the position of an object changes
• Example
• An airplane traveling from Seattle, WA to Newark,
  NJ travels 3865 km in 5 hours. What is the
  average speed of the plane?

773 km/hr
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Velocity

• Speed with direction
• Speed 5 km/h
• Velocity 5 km/h North
• Example
• An airplane traveling from Seattle, WA to Newark,
  NJ travels 3865 km at a heading of 83 degrees in
  5 hours. What is the velocity of the plane?

773 km/h east
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Calculations (Alegbra!)

• S d/t
• d st
• t d/s
• V d/t
• d vt
• t d/v

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Acceleration
the change in velocity in a unit of time
Acceleration
Final velocity -
Original velocity
time
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Accelerate - change in velocity
a change in speed or change in direction
30 km/h
2 m/s
Objects spinning or orbiting at a constant speed
are accelerating because they are constantly
changing direction.
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Which of the following objects is NOT
accelerating?

• A ball being juggled
• ISS alpha circling the Earth
• A woman walking at 1.5 m/s down a straight road
• A car breaking for a deer ahead

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Fma

• Force does not cause MOTIONit causes
  ACCELERATION!!!

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• Since there is no vertical acceleration
• normal force gravity force.
• The mass can be found using the equation
• Fgrav m g.
• The Fnet is the vector sum of all the forces
• 80 N, up plus 80 N, down equals 0 N.
• 50 N, right plus 10 N, left 40 N, right.
• Finally, a Fnet / m (40 N) / (8.16 kg) 4.9
  m/s2.

• Fnorm 80 N
• m 8.16 kg
• Fnet 40 N, right
• a 4.9 m/s2, right

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• In a physics lab, Kate and Rob use a hanging mass
  and pulley system to exert a 2.45 N rightward
  force on a 0.500-kg cart to accelerate it across
  a low-friction track. If the total resistance
  force to the motion of the cart is 0.72 N, then
  what is the cart's acceleration?

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Fgrav 4.90 N Fnorm 4.90 N Fnet 1.73 N,
right a 3.46 m/s2, right

• The starting point for any problem such as this
  is the construction of a free-body diagram in
  which you show all the individual forces which
  are acting upon the book.
• There are two vertical forces - gravity and
  normal force.
• There are two horizontal forces - friction and
  the applied force.
• Since there is no vertical acceleration, normal
  force gravity force.
• Each of these forces could be determined using
  the equation Fgrav m g (0.500 kg)(9.8
  m/s2) 4.90 N.
• The Fnet is the sum of all the forces 2.45 N,
  right plus 0.72 N, left 1.73 N, right.
• Finally, a Fnet / m (1.73. N) / (0.500 kg)
  3.46 m/s2.  

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Newtons Third Law

• Law of Action and Reaction

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The baseball forces the bat to the right
Athlete pushes bar upwards
Bowling ball pushes pin rightwards
Compressed air pushes balloon wall outwards.
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