Astronomy%20100%20Tuesday,%20Thursday%202:30%20-%203:[e-mail]%20www.xanga.com/astronomy100

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Astronomy 100Tuesday, Thursday 230 - 345
pmTom Burbinetburbine_at_mtholyoke.eduwww.xanga.
com/astronomy100
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Help Desk

• There is an Astronomy Help Desk in HAS 205.  It
  will be open from Monday through Thursday from
  7-9 pm.

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Another Homework Assignment(Due Thursday March
3rd)

• Make up a test question
• Multiple Choice
• A-E possible answers
• 1 point for handing it in
• 1 point for me using it on test

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Homework assignments due today (or it is late)

• Two due today
• One is from this sheet that I handed out
• Two calculations
• Worth 1 HW point
• I will discuss them next class

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2nd HW assignment due today (or it is late)

• I want you to pick an element
• Write its symbol
• Is it a solid, metal, or gas?
• Atomic Number
• Atomic Mass
• Its isotopes
• What this element is used for?
• http//www.webelements.com/
• Worth 1 HW point

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Next assignment

• Handed out today
• More calculations
• Due Feb. 24 (Thursday) by 345 pm
• You need to show work to get credit

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PRS

• You need to make sure that you have a clicker
  registered
• And your name appears on screen
• If your name does not appear onscreen, I dont
  know you are when you use the clicker

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Monkey Drop Experiment

• A hunter shoots at a monkey in the tree
• The monkey drops the second that the hunter pulls
  the trigger
• What happens?

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Show video

• http//www.ap.stmarys.ca/demos/content/mechanics/m
  onkey_and_hunter/monkey_and_hunter.html

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So what is happening

• The monkey and the bullet are accelerating at the
  same speed downwards
• g 9.8 m/s2 acceleration due to gravity
• Objects fall faster by 9.8 m/s with every second
• Even though the bullet has a velocity in the
  horizontal direction, it is also moving downwards
  in the vertical direction

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Acceleration

• Acceleration is when your velocity is changing
• Velocity not changing, no acceleration

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Difference between mass and weight

• Mass is the amount of matter in your body
• Weight is the amount of force acting on your body
• So on the Moon, you would have the same mass as
  on Earth but weigh less on the Moon since the
  Moon is less massive than Earth

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Who came up with the first understanding of how
gravity and forces really work?
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Isaac Newton (1642-1727)
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Isaac Newton (1642-1727)

• Supposedly saw an apple fall to the ground
• He then understood that gravity was universal,
  meaning it affected both the planets and us on
  Earth
• Came up with 3 Laws of Motion

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1st Law

• In the absence of a net (overall) force acting
  upon it, an object moves with a constant velocity
• An object at rest remains at rest
• An object in motion tends to remain in motion
  unless a force is acting upon it

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2nd Law

• Force mass x acceleration
• Units of Force kg?m/s2 newton
• So much do you weigh
• Say your mass is 100 kg
• F 100 kg x 9.8 m/s2
• F 980 Newtons

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Show video
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3rd Law

• For any force, there is an equal and opposite
  reaction force
• Gravity is holding you on the ground
• The ground is also pushing back up on you with
  the same amount of force

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(No Transcript)
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Conservation of Momentum
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Conservation of Angular Momentum

• Angular momentun m x v x r
• M mass
• V velocity
• R radius
• In the absence of a net torque (twisting force),
  the angular momentum of a system remains constant

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(No Transcript)
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Universal Law of Gravitation

• Every mass attracts every other mass through the
  force called gravity
• Newton came up with this formula
• F G M1 M2
• d2
• M1, M2 are the masses of the two objects
• d is the distance between the objects
• G constant 6.67 x 10-11 m3/(kg?s2)

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So what should you know about this formula

• F G M1 M2
• d2
• The force of attraction between any two objects
  is directly proportional to the product of their
  masses
• The force of attraction between two objects
  decreases with the square of the distance between
  their centers
• G is a very small number

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What is the attraction of two people in this room?

• F G M1 M2
• d2
• Say their masses are both 100 kg
• Their distances are 10 meters apart
• F 6.67 x 10-11 m3/(kg?s2) 100100 kg2/(1010
  m2)
• F 6.67 x 10-9 N 0.0000000067 N
• Remember the person weighs 980 N

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PRS question

• F G M1 M2
• d2
• How would the force between the two people change
  if they were only 5 meters apart instead of 10
  meters?
• A) Stay the same
• B) Double (Increase by a Factor of 2)
• C) Quadrupul (Increase by a Factor of 4)
• D) halve (decrease by a factor of 2)

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PRS question

• F G M1 M2 G M1 M2 4 G M1 M2
• (d/2)2 d2/4
  d2
• How would the force between the two people change
  if they were only 5 meters apart instead of 10
  meters?
• A) Stay the same
• B) Double (Increase by a Factor of 2)
• C) Quadrupul (Increase by a Factor of 4)
• D) halve (decrease by a factor of 2)

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Can you use this formula to calculate
acceleration of gravity?

• F M2a G M1 M2 you are M2
• d2
  M1 is the Earths radius
• a G M1
  d is the Earths diameter
• d2
• a 6.67 x 10-11 m3/(kg?s2) (6.0 x 1024 kg)
• (6.4 x 106 m) (6.4 x 106 m)
• a 9.8 m/s2

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Escape velocity

• Velocity above this will allow an object to
  escape Earths gravity
• v square root(2 x G x M)/R
• v square root(2 x 6.67 x 10-11 m3/(kg?s2) x
  (6.0 x 1024 kg)
• (6.4 x 106 m)
• v square root 1.25 x 108 m2/s2
• v 11.2 x 103 m/s 11.2 km/s

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Show animation
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• Why do we have tides?

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Remember

• Rock is stretching too
• However, water is easier to stretch than rock

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Force due to Moon on side closest to Earth

• F G M1 M2
• d2
• F 6.67 x 10-11 m3/(kg?s2) (6.0 x 1024 kg)
  (7.4 x 1022 kg)
• (3.84 x 108 m) (3.84 x 108 m)
• F 2 x 1020 N

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Force due to Moon on side farthest from Earth

• F G M1 M2
• d2
• F 6.67 x 10-11 m3/(kg?s2) (6.0 x 1024 kg)
  (7.4 x 1022 kg)
• (3.904 x 108 m) (3.904 x 108
  m)
• F 1.94 x 1020 N
• Difference is 6 x 1018 N

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Is the force due to the Sun stronger or weaker on
the Earth?

• F G M1 M2
• d2
• F 6.67 x 10-11 m3/(kg?s2) (6.0 x 1024 kg)
  (1.99 x 1030 kg)
• (1.5 x 1011 m) (1.5 x 1011 m)
• F 3.54 x 1022 N
• This is approximately 184 times stronger than the
  force due to the Moon

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PRS Question

• Why arent the tides due mainly to the Sun than
  the Moon?
• A) they are mainly due to the Sun
• B) the difference in the Suns pull on the
  different sides of the Earth is smaller than the
  Moons
• C) the forces due to the Moon and Sun contribute
  equally to the tides
• D) the tides are not due to forces from either
  the Sun or the Moon

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PRS Question

• Why arent the tides due mainly to the Sun than
  the Moon?
• A) they are mainly due to the Sun
• B) the difference in the Suns pull on the
  different sides of the Earth is smaller than the
  Moons
• C) the forces due to the Moon and Sun contribute
  equally to the tides
• D) the tides are not due to forces from either
  the Sun or the Moon

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What is the difference in forces on the two sides
of the Earth?

• F G M1 M2
• d2
• F 6.67 x 10-11 m3/(kg?s2) (6.0 x 1024 kg)
  (1.99 x 1030 kg)
• (1.50 x 1011 m) (1.50 x 1011
  m)
• F 3.53924 x 1022 N (far side of Earth)
• F 3.53954 x 1022 N (near side of Earth)
• Difference due to Sun 3 x 1018 N
• Difference due to Moon 6 x 1018 N

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When Sun and Moon are pulling in same direction,
strongest tides
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When Sun and Moon are pulling in perpendicular
directions, weakest tides
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Questions?