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36–92 ➯ 60–100

Buy-back extra-credit will be added after adjustment.

Answer key + your answers are online now (course website, “grades” tab).

Let’s go over the exam!

Last TimeKepler: perfected the earth-centered model of planetary motion.

Keplers Law’s:

1. Planet’s orbit is ellipse.

2. Line connecting planet to sun sweeps out equal area in equal time (moves faster at perihelion, slower at aphelion).

3. P2=A3

Last TimeKepler: perfected the earth-centered model of planetary motion.

Keplers Law’s:

1. Planet’s orbit is ellipse.

2. Line connecting planet to sun sweeps out equal area in equal time (moves faster at perihelion, slower at aphelion).

3. P2=A3

Which of the following best describes what would happen if Mercury and Jupiter were to switch

places in their orbits about the Sun?

A) Jupiter, the larger planet, would have

a shorter orbital period than before. B) Mercury, the smaller planet, would

have a shorter orbital period than before.

C) Neither of the two planets would have any change in their orbital periods.

Which of the following best describes what would happen if Mercury and Jupiter were to switch

places in their orbits about the Sun?

A) Jupiter, the larger planet, would have

a shorter orbital period than before. B) Mercury, the smaller planet, would

have a shorter orbital period than before.

C) Neither of the two planets would have any change in their orbital periods.

✪

Gravity, Energy, and Motion

Describing Motion

speed = rate at which an object moves. Distance/Timeexample: 10 m/s

velocity = speed + direction.example: 10 m/s due West

The Acceleration Due to Gravity

All falling objects accelerate at the same rate (not counting air resistance) independent of mass.

On Earth, g ≈ 10 m/s2: speed increases 10 m/s with each falling second

10 m/s per second or 10 m/s2

Gravity

The acceleration due to gravity is the same for any object on the surface of the earth.

Hammer and feather fall at the same rate (neglecting air resistance).

Momentum and Force

momentum = mass x velocityAnything in motion (velocity≠0) has momentum associated with it

A net force changes momentum, which means an acceleration (change in velocity)

a change in momentum can result from a change in the direction of motion.

Thought Question

Is there a net force at work? (✓/✕)

1) A car coming to a stop 2) a bus speeding up 3) An elevator moving at a constant speed 4) A car rounding a corner at a constant

speed 5) A planet orbiting the Sun at a constant

speed

Thought Question

Is there a net force at work? (✓/✕)

1) A car coming to a stop 2) a bus speeding up 3) An elevator moving at a constant speed 4) A car rounding a corner at a constant

speed 5) A planet orbiting the Sun at a constant

speed

✓

Thought Question

Is there a net force at work? (✓/✕)

1) A car coming to a stop 2) a bus speeding up 3) An elevator moving at a constant speed 4) A car rounding a corner at a constant

speed 5) A planet orbiting the Sun at a constant

speed

✓✓

Thought Question

Is there a net force at work? (✓/✕)

1) A car coming to a stop 2) a bus speeding up 3) An elevator moving at a constant speed 4) A car rounding a corner at a constant

speed 5) A planet orbiting the Sun at a constant

speed

✕

✓✓

Thought Question

Is there a net force at work? (✓/✕)

1) A car coming to a stop 2) a bus speeding up 3) An elevator moving at a constant speed 4) A car rounding a corner at a constant

speed 5) A planet orbiting the Sun at a constant

speed

✕

✓✓

✓

Thought Question

Is there a net force at work? (✓/✕)

1) A car coming to a stop 2) a bus speeding up 3) An elevator moving at a constant speed 4) A car rounding a corner at a constant

speed 5) A planet orbiting the Sun at a constant

speed

✕

✓✓

✓

✓

Force and Momentum

We say a force causes a transfer of momentum.

the difference between mass and weight

mass - The amount of matter in an object

weight - The force that acts upon an object

The amount of matter (number of atoms) in you never changes but your weight depends on how strong gravity is at your location.

What is the difference between mass and weight?

The moon has weaker gravity than the Earth. On the Moon:

A) you weigh the same, but your mass is less

B) you weigh less, but your mass is the same

C) You weigh more, but your mass is the same

D) you weigh more, and your mass is more

What is the difference between mass and weight?

The moon has weaker gravity than the Earth. On the Moon:

A) you weigh the same, but your mass is less

B) you weigh less, but your mass is the same

C) You weigh more, but your mass is the same

D) you weigh more, and your mass is more

✪

Sir Isaac NewtonEnglish scientist, 1642–1727

Greatest scientist ever?

Light, motion, gravity

Newtonian physics dominated until Einstein.

Invented calculus to help solve a different problem.

How did Newton change our view of the Universe?

Realized that the same physical laws that operate on Earth also operate in the heavens.

We can therefore use the laws of physics to understand the universe!

Newton’s Three Laws of Motion

I. An object remains at rest or moves at a constant velocity unless a net

force acts to change its direction or speed.

The Apollo command module keeps moving even without firing its

engines

II. If an object is acted on by a net force, then this force changes the

momentum of the object, accelerating it in the direction of the force.

Force = mass x accelerationF = ma

Give me F, and I will tell you how an object moves (a).

Newton’s Three Laws of Motion

III. For every force, there is always and equal and opposite reaction

force The Space Shuttle is

propelled upward with a force equal and opposite to the force with which the gas is expelled out the rockets

Newton’s Three Laws of Motion

WeightlessnessSense of weight comes from Newton’s 3rd law.

Earth’s gravity pulls you down, but the floor pushes back with the same force: we feel weight!

You are “weightless” anytime you are freely falling.

Weightless Astronauts

An astronaut is weightless because:

A) The ship’s rockets holds them up

B) There is no gravity in space

C) They are constantly in free fall

D) They are rotating like a merri-go-round

See Interactive Fig. 4.4

Weightless Astronauts

An astronaut is weightless because:

A) The ship’s rockets holds them up

B) There is no gravity in space

C) They are constantly in free fall

D) They are rotating like a merri-go-round

✪

See Interactive Fig. 4.4

Law’s of Conservation

Newton’s laws are really deeper statement about nature: “conservation laws”.

Conservation of Momentum.

Conservation of Angular Momentum.

Conservation of Energy.

Conservation of Angular Momentum

Angular momentum is a special kind of “circling or rotating momentum”.

Law: The total angular momentum of a system cannot change unless it interacts with something (i.e., gives up or receives angular momentum)

Explains Kepler’s 2nd law!

Newton’s laws of Gravity

Every mass attracts every other mass.

Strength of attraction is directly proportional to the product of their masses

The strength of that attraction is inversely proportional to the square of the distance between them.

Workbook Time

Break up into Groups of 25.

Elect a chairperson, design a group logo and mascot, and compose a team anthem with guitar accompaniment.

Turn to Newton’s Law and Gravity on Page 29.

Reminders

HW #2 Due in one week.

Still (!!!) not on Mastering Astronomy? Come see me!

Read Section 6.1 and 6.2 for next week!