© 2005 Pearson Education Inc., publishing as Addison-Wesley

Correction in Exam 1 Date:Thursday Feb. 10

Updated Syllabus in website has the corrected date

Please tell your classmates who are not here

© 2005 Pearson Education Inc., publishing as Addison-Wesley

4.1 and 4.2 Describing Motion, Newton and GalileoSpeed, velocity and acceleration (skip momentum) Galileo’s experiments with falling objects: g = 9.8 m/sec2

Objects fall togetherInertia (motion in absence of force)

Newton’s Laws:1. 3 laws of motion: a. Inertia b. F=ma c. Action =

Reaction 2. Gravitation: F= GM1M2/d2 (Inverse-square law)

4.3 (Thermal Energy only)4.4 The force of Gravity The Strength of Gravity ■ Newton and Kepler Orbits: 1. Closed: circles (circular velocity) & ellipses (v > vc)

2. Open: parabolas and hyperbolas (escape velocity, v > ve)

Tides: Lunar and Solar

Outline of Ch 4 Motion and Gravity(soap opera’s final episode)

© 2005 Pearson Education Inc., publishing as Addison-Wesley

4.2 Newton’s Laws of Motion

Our goals for learning:

• How did Newton change our view of the universe?

• What are Newton’s three laws of motion?

• What is Newton’s laws of gravitation?

© 2005 Pearson Education Inc., publishing as Addison-Wesley

• Realized the same physical laws that operate on Earth also operate in the heavens one universe

• Discovered 3 laws of motion and law of gravitation

• Much more: experiments with light; first reflecting telescope, calculus…

How did Newton change our view of the Universe?

Sir Isaac Newton (1642-1727)

© 2005 Pearson Education Inc., publishing as Addison-Wesley

What are Newton’s three laws of motion?

Newton’s first law of motion (law of inertia): An object moves at constant velocity unless a net force acts to change its speed or direction (this he adopted from Galileo).

© 2005 Pearson Education Inc., publishing as Addison-Wesley

Newton’s second law of motion:

Force = mass acceleration (F= ma)

© 2005 Pearson Education Inc., publishing as Addison-Wesley

Newton’s third law of motion:

For every force, there is always an equal and opposite reaction force (action = reaction).

© 2005 Pearson Education Inc., publishing as Addison-Wesley

The Universal Law of Gravitation1. Every mass attracts every other mass.2. Attraction is directly proportional to the product of

their masses.3. Attraction is inversely proportional to the square of

the distance between their centers..

© 2005 Pearson Education Inc., publishing as Addison-Wesley

Question:Is the force the Earth exerts on you larger, smaller,

or the same as the force you exert on it?

A. Earth exerts a larger force on you.

B. I exert a larger force on Earth.

C. Earth and I exert equal and opposite forces on each other.

D. There is no force between Earth and any object

© 2005 Pearson Education Inc., publishing as Addison-Wesley

Question:A compact car and a Mack truck have a head-on

collision. Are the following true or false?

1. The force of the car on the truck is equal and opposite to the force of the truck on the car.

2. The change of velocity (acceleration) of the car is the same as the change of velocity of the truck.

© 2005 Pearson Education Inc., publishing as Addison-Wesley

Question:A compact car and a Mack truck have a head-on

collision. Are the following true or false?

1. The force of the car on the truck is equal and opposite to the force of the truck on the car.

2. The change of velocity of the car is the same as the change of velocity of the truck. (remember F = ma, if “F” is the same and the masses are very different then “a”, which is the change in velocity must also be very different)

© 2005 Pearson Education Inc., publishing as Addison-Wesley

What have we learned?• How did Newton change our view of the universe?

• He discovered laws of motion & gravitation.• He realized these same laws of physics were identical in the

universe and on Earth.

• What are Newton’s Three Laws of Motion?1) Object moves at constant velocity if no net force is acting.2) Force = mass acceleration 3) For every force there is an equal and opposite reaction force.

© 2005 Pearson Education Inc., publishing as Addison-Wesley

4.3 Ignore all except Thermal Energy• Relation temperature motion of atoms :• The higher the temperature the faster the atoms in a

substance will be moving• As atoms collide the electrons collide and their motion is

disturbed• When the motion of electrons gets disturbed they produce

photons• The higher the temperature, the more collisions, the more

photons (more about this in Ch. 5)

© 2005 Pearson Education Inc., publishing as Addison-Wesley

Temperature Scales

© 2005 Pearson Education Inc., publishing as Addison-Wesley

4.1 and 4.2 Describing Motion, Newton and GalileoSpeed, velocity and acceleration (skip momentum) Galileo’s experiments with falling objects: g = 9.8 m/sec2

Objects fall togetherInertia (motion in absence of force)

Newton’s Laws:1. 3 laws of motion: a. Inertia b. F=ma c. Action = Reaction 2. Gravitation: F= GM1M2/d2 (Inverse-square law)

4.3 (Thermal Energy only)

4.4 The force of Gravity The Strength of Gravity ■ Newton and Kepler Orbits: 1. Closed: circles (circular velocity) & ellipses (v > vc)

2. Open: parabolas and hyperbolas (escape velocity, v > ve)

Tides: Lunar and Solar

Outline of Ch 4 Motion and Gravity(soap opera’s final episode)

© 2005 Pearson Education Inc., publishing as Addison-Wesley

4.4 The Force of GravityOur goals for learning:

•What determines the strength of gravity?

•How does Newton’s law of gravity extend Kepler’s laws?

•How do gravity and energy together allow us to understand orbits?

•How does gravity cause tides?

© 2005 Pearson Education Inc., publishing as Addison-Wesley

What determines the strength of gravity? The Universal Law of Gravitation1. Every mass attracts every other mass.2. Attraction is directly proportional to the product of

their masses.3. Attraction is inversely proportional to the square of

the distance between their centers..

© 2005 Pearson Education Inc., publishing as Addison-Wesley

How does Newton’s law of gravity extend Kepler’s laws? (some not in book)

• Ellipses are not the only orbital paths. Orbits can be:bound

• Circle (v = vc)

• Ellipse (v > vc)

unbound

• Parabola (v = ve)

• Hyperbola (v > ve)

• Circular and Escape velocities (vc and ve)

vc = GM/R

ve = 2GM/R

circular and

© 2005 Pearson Education Inc., publishing as Addison-Wesley

© 2005 Pearson Education Inc., publishing as Addison-Wesley

© 2005 Pearson Education Inc., publishing as Addison-Wesley

• Newton generalized Kepler’s Third Law:

Newton’s version of Kepler’s Third Law:

If a small object orbits a larger one and you measure the orbiting object’s

orbital period AND average orbital distanceTHEN you can calculate the mass of the larger object.

Examples:• Calculate mass of Sun from Earth’s orbital period (1 year) and average distance (1 AU).• Calculate mass of Earth from orbital period and distance of a satellite.• Calculate mass of Jupiter from orbital period and distance of one of its moons. •What about asteroids?

© 2005 Pearson Education Inc., publishing as Addison-Wesley

Newton’s version of Kepler’s Third Law

p = orbital period

a=average orbital distance (between centers)

(M1 + M2) = sum of object masses

p2 42G(M

1M

2)a3

© 2005 Pearson Education Inc., publishing as Addison-Wesley

How do gravity and energy together explain orbits?

• Orbits cannot change spontaneously.• An object’s orbit can only change if it somehow

gains or loses orbital energy =

kinetic energy + gravitational potential energy

(due to orbit).

© 2005 Pearson Education Inc., publishing as Addison-Wesley

So what can make an object gain or lose orbital energy?

• Friction or atmospheric drag• Rocket engine• A gravitational encounter.

© 2005 Pearson Education Inc., publishing as Addison-Wesley

• If an object gains enough orbital energy, it may escape (change from a bound to unbound orbit)

•escape velocity from Earth ≈ 11 km/s from sea level (about 40,000 km/hr, 25,000 mph) •What is Earth’s circular velocity at sea level?

© 2005 Pearson Education Inc., publishing as Addison-Wesley

How does Newton’s law of gravity extend Kepler’s laws? (some not in book)• Ellipses are not the only orbital paths.

Orbits can be:bound

• Circle (v = vc)

• Ellipse (v > vc)

unbound

• Parabola (v = ve)

• Hyperbola (v > ve)

• Circular and Escape velocities (vc and ve)

vc = GM/R

ve = 2GM/R

circular and

© 2005 Pearson Education Inc., publishing as Addison-Wesley

Mastering Astronomy:

Study area: Ch 4

Interactive Fig. 4.18

© 2005 Pearson Education Inc., publishing as Addison-Wesley

Escape and orbital velocities do NOT depend on the mass of the cannonball

© 2005 Pearson Education Inc., publishing as Addison-Wesley

4.1 and 4.2 Describing Motion, Newton and GalileoSpeed, velocity and acceleration (skip momentum) Galileo’s experiments with falling objects: g = 9.8 m/sec2Objects fall togetherInertia (motion in absence of force)

Newton’s Laws:1. 3 laws of motion: a. Inertia b. F=ma c. Action = Reaction 2. Gravitation: F= GM1M2/d2 (Inverse-square law)

4.3 (Thermal Energy only)4.4 The force of Gravity The Strength of Gravity ■ Newton and Kepler Orbits: 1. Closed: circles (circular velocity) & ellipses (v > vc)

2. Open: parabolas and hyperbolas (escape velocity, v > ve)

Tides: Lunar and Solar

Outline of Ch 4 Motion and Gravity(soap opera’s final episode)

© 2005 Pearson Education Inc., publishing as Addison-Wesley

The tides due to the Moon affect: a) Only the Oceans b) The whole Earth c) Only the night side of Earth d) None of the other answers is

correct

Question

© 2005 Pearson Education Inc., publishing as Addison-Wesley

Tides

• Gravitational force decreases with (distance)2

– The Moon’s pull on Earth is strongest on the side facing the Moon, and weakest on the opposite side.

• The Earth gets stretched along the Earth-Moon line.

• The oceans rise relative to land at these points.

© 2005 Pearson Education Inc., publishing as Addison-Wesley

Tides vary with the phase of the Moon:

© 2005 Pearson Education Inc., publishing as Addison-Wesley

© 2005 Pearson Education Inc., publishing as Addison-Wesley

Special Topic: Why does the Moon always show the same face to Earth?

Moon rotates in the same amount of time that it orbits… But why?

© 2005 Pearson Education Inc., publishing as Addison-Wesley

Tidal friction…

• Tidal friction gradually slows Earth rotation (and makes Moon get farther from Earth).• Moon once orbited faster (or slower); tidal friction caused it to “lock” in synchronous

rotation with its orbit around Earth.

© 2005 Pearson Education Inc., publishing as Addison-Wesley

• How does Newton’s law of gravity allow us to extend Kepler’s laws?• Applies to other objects, not

just planets.• Includes unbound orbit

shapes: parabola, hyperbola• We can now measure the

mass of other systems.

What have we learned?•What determines the strength of gravity?

•Directly proportional to the product of the masses (M x m)•Inversely proportional to the square of the separation d

© 2005 Pearson Education Inc., publishing as Addison-Wesley

What have we learned?• How do gravity and

energy together allow us to understand orbits?• Gravity determines orbits• Orbiting object cannot

change orbit without energy transfer

• Enough energy -> escape velocity -> object leaves.

•How does gravity cause tides?•Gravity stretches Earth along Earth-Moon line because the near side is pulled harder than the far side.