Astronomy 100Tuesday, Thursday 230 - 345

pmTom Burbinetburbine_at_mtholyoke.eduwww.xanga.

com/astronomy100

Help Desk

- There is an Astronomy Help Desk in HAS 205. It

will be open from Monday through Thursday from

7-9 pm.

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

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

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

Next assignment

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

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

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?

Show video

- http//www.ap.stmarys.ca/demos/content/mechanics/m

onkey_and_hunter/monkey_and_hunter.html

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

Acceleration

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

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

Who came up with the first understanding of how

gravity and forces really work?

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

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

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

Show video

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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Conservation of Momentum

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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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)

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

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

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)

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)

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

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

Show animation

- Why do we have tides?

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Remember

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

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

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

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

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

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

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

When Sun and Moon are pulling in same direction,

strongest tides

When Sun and Moon are pulling in perpendicular

directions, weakest tides

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Questions?