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Astronomy%20100%20Tuesday,%20Thursday%202:30%20-%203:45%20pm%20Tom%20Burbine%20tburbine@mtholyoke.edu%20www.xanga.com/astronomy100

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Title: Astronomy%20100%20Tuesday,%20Thursday%202:30%20-%203:45%20pm%20Tom%20Burbine%20tburbine@mtholyoke.edu%20www.xanga.com/astronomy100


1
Astronomy 100Tuesday, Thursday 230 - 345
pmTom Burbinetburbine_at_mtholyoke.eduwww.xanga.
com/astronomy100
2
Help Desk
  • There is an Astronomy Help Desk in HAS 205.  It
    will be open from Monday through Thursday from
    7-9 pm.

3
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

4
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

5
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

6
Next assignment
  • Handed out today
  • More calculations
  • Due Feb. 24 (Thursday) by 345 pm
  • You need to show work to get credit

7
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

8
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?

9
Show video
  • http//www.ap.stmarys.ca/demos/content/mechanics/m
    onkey_and_hunter/monkey_and_hunter.html

10
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

11
Acceleration
  • Acceleration is when your velocity is changing
  • Velocity not changing, no acceleration

12
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

13
Who came up with the first understanding of how
gravity and forces really work?
14
Isaac Newton (1642-1727)
15
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16
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

17
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

18
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

19
Show video
20
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

21
(No Transcript)
22
Conservation of Momentum
23
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

24
(No Transcript)
25
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)

26
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

27
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

28
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)

29
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)

30
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

31
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

32
Show animation
33
  • Why do we have tides?

34
(No Transcript)
35
Remember
  • Rock is stretching too
  • However, water is easier to stretch than rock

36
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

37
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

38
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

39
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

40
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

41
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

42
When Sun and Moon are pulling in same direction,
strongest tides
43
When Sun and Moon are pulling in perpendicular
directions, weakest tides
44
(No Transcript)
45
Questions?
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