What is motion? what do scientists define as motion ?

Why do the planets move the way they do?

Bellringer

Compare and explain in complete sentences and

formulas what is the Newton’s third law of motion.

Motion

What is motion? what do scientists define as motion ?

Why do the planets move the way they do?

Keep in mind the way scientists like to work:

Observe Define Hypothesize/Predict Test

repeat

Note:the study of motion was motivated by the motion of celestial objects.

Galileo …

… studied motion;

… was the first to use a telescope for astronomical observations;

… saw that there were ojects that did not move around the Earth !

Galileo Galilei (1564-1642) “the astronomer”

Believed in the Copernican model.Demonstrated that Kepler’s and

Copernicus’ ideas were right by making observations with his telescope.

2. The moons of Jupiter obey Kepler’s laws 3. Phases of Venus, supporting heliocentric

theory 4. Observed that Mars, Jupiter and Saturn had

no phases. 5. Observed individual stars in the Milky Way, thereby showing stellar parallax measuring is possible.

1. The heavens are not “perfect”:• geological features on the moon• sunspots on the surface of the Sun

Galileo Galilei (1564-1642)

Believed in the Copernican model.Demonstrated that Kepler’s and

Copernicus’ ideas were right by making observations with his telescope.1. The heavens are not “perfect”:• geological features on the moon• sunspots on the surface of the Sun

Galileo Galilei (1564-1642)

2. The moons of Jupiter obey Kepler’s laws

3. Phases of Venus, supporting heliocentric theory4. Observed that Mars, Jupiter and Saturn had no phases.

Galileo Galilei (1564-1642)

3. Phases of Venus, supporting heliocentric theory4. Observed that Mars, Jupiter and Saturn had no

phases.

geocentric heliocentric

Galileo Galilei (1564-1642)

Believed in the Copernican model.Demonstrated that Kepler’s and

Copernicus’ ideas were right by making observations with his telescope.

2. The moons of Jupiter obey Kepler’s laws 3. Phases of Venus, supporting heliocentric

theory 4. Observed that Mars, Jupiter and Saturn had

no phases. 5. Observed individual stars in the Milky Way, thereby showing stellar parallax measuring is possible.

Galileo Galilei (1564-1642) “the physicist”

Galileo also experimented with falling and moving objects and crafted a theory of motion.

Galileo’s workshop at the Deutches Museum in Munich, Germany

Galileo Galilei (1564-1642) “the physicist”

Galileo also experimented with falling and moving objects and crafted a theory of motion.

1.

An object in motion will continue moving along a straight line with a constant speed until an unbalanced force acts on it.

dropped objects move down at10 m/s /s

t=3 sv=30 m/s

WHY?

2.

Gravity makes things accelerate at 10 m/s2

Acceleration of gravity is independent of the mass of the falling object!

Iron ball

Wood ball

Fourth manned lunar landing with David R. Scott, Alfred M. Worden, and James B. Irwin. Landed at Hadley rilleon July 30, 1971.

Observation

How do we describe motion?Define __________ ?

How do we describe motion?

10 m/s South-East … & direction

100 m/sdisplacement per time

(define it)

60 km/hr --> 30 km/hr --> 0 changing displacement per time

How do we describe motion? (define it)

Speed: Rate at whichobject moves

Velocity: Speed and direction

Acceleration: Any change in speed or direction

Thank you, Galileo

Motion: speed, velocity, & acceleration

What about the mass?

Yes, motion depends on mass, too.

Linear Momentum = mass velocity

Motion with Mass Momentum

Angular momentum isrotational momentum of a spinning or orbiting object

Describing Motion:(basic ingredients)

1. change in position (displacement)2. time3. mass4. direction

So far…..

speed velocity acceleration momentum

Isaac Newton (1642-1727)

• Born the year Galileo died• Contemporary of Bach• Derived laws of gravity and other laws of physics“If I have seen further, it is by standing on the

shoulders of giants.”--Isaac Newton

Isaac Newton (1642 - 1727)

• Building on the results of Galileo and Kepler

Major achievements:1. Invented Calculus as a necessary tool to solve

mathematical problems related to motion

• Adding physics interpretations to the mathematical descriptions of astronomy by Copernicus, Galileo and Kepler

2.Formulated the three laws of motion

3.Formulated the universal law of mutual gravitation

Newton’s 3 Laws of Motion

1 2 3Inertia F = ma action = reaction

science.discovery.com/interactives/literacy/newton/newton.html

1. Newton’s Laws of Motion

A body continues at rest or

in uniform motion in a straight line

unless acted upon by some net force.

Also known as : The Law of Inertia

Newton’s 1st:

object will stayat rest (or in uniform motion)until acted on by a FORCE

1. Newton’s Laws of Motion

An astronaut floating in space will continue to float forever in a straight line unless some external force is accelerating him/her.

2. Newton’s Laws of Motion The acceleration, a, of a

body is

directly proportional to the net force F,

in the same direction as the net force F,

inversely proportional to its mass, m.

a = F F = m a m

Newton’s 2nd: F = ma(unbalanced forces cause changes in motion.)

3. Newton’s Laws of Motion

To every action,

there is an equal and

opposite reaction.

The same force that is accelerating the boy forward, is accelerating the skateboard backward.

M = 70 kg

m = 1 kg

v = 7 m/s

V = ?

Newton’s 3rd: action - reaction

Newton’s 3rd: action - reaction

The Universal Law of Gravity

Any two bodies are attracting each other through gravitation, with a force proportional to the product of their masses and inversely proportional to the square of their distance:

F = - G Mm

d2

(G is the Universal constant of gravity.)

d̂

What determines the strength of gravity?

The Universal Law of Gravitation:1. 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.

weightF m g

F = m a

All objects on Earth fall with the same acceleration, g.

g = 9.8 m/s2 (about 10 m/s2 --- Galileo)

The acceleration: a = g

Your weight is the force of Earth on YOU

• How do we describe motion?– Speed = distance / time– Speed & direction => velocity– Change in velocity => acceleration– Mass effect motion– Momentum = mass x velocity– Force causes change in momentum,

producing acceleration

Summary … so far:

Motion • described by: speed, velocity, and acceleration• determined by: Newton’s 3 Laws• has: energy

Energy• makes change

Energy makes matter move,too.Moving matter has energy. Energy and matter changes form.

Energy cannot be destroyed.

There are 2 forms of ENERGY

Kinetic energyis motion––of waves, electrons, atoms, molecules, substances, and objects.

Potential energyis stored energy and the energy of positionof objects,nucleus,chemical

There are 2 forms of ENERGY

Kinetic energyis motion––Electrical --- chargesRadiant --- EM energyThermal --- heatMotion --- Newton’s LawsSound --- waves through substances

There are 2 forms of ENERGY

Potential energyis stored energy and the energy of positionChemical --- stored in bonds (atom/molecule) Stored mechanical --- springs, rubber bandNuclear --- stored in nucleus (fusion/fission)Gravitational --- stored in position

Gravitational Potential Energy

On Earth, GPE depends on:– object’s mass (m)– strength of gravity (g)– distance object could

potentially fall

Gravitational Potential Energy

In space, an object or gas cloud has more gravitational energy when it is spread out than when it contracts.

A contracting cloud converts gravitational potential energy to thermal energy.

Mass-Energy

• Mass itself is a form of potential energy

E = mcE = mc22

• A small amount of mass can release a great deal of energy• Concentrated energy can spontaneously turn into particles (for example, in particle accelerators)

Energy

• Energy can be neither created nor destroyed.

• It can change form or be exchanged between objects.

• The total energy content of the Universe was determined in the Big Bang and remains the same today.

Energy is Conserved

Conservation of energy(energybefore = energyafter )

Conservation of momentum

Anything else conserved?

Conservation of Momentum

• The total momentum of interacting objects cannot change unless an external force acts on them

• Interacting objects exchange momentum through equal and opposite forces

BEFORE

AFTER

Conservation of angular momentum

Angular momentum conservation also explains why objectsrotate faster as they shrink in radius

MASS, WEIGHT… does it matter?

YES!MASS is the amount of matter of the object.

Weight is the amount of force on the object.

Let’s apply EVERYTHING we know aboutmotion to orbital motion

… and so on and on ….

The Sun exerts a force on the planets(and vice versa!)

Orbital Motion

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

Total orbital energy (gravitational+kinetic) stays constant if there is no external

force

Less kinetic energy;More gravitational energy.

More kinetic energy;Less gravitational energy

Orbits cannot change spontaneously.

• Friction: atmospheric drag, or tidal flexing of a “fluid” object

• A gravitational encounter.

How Can an Orbit Change ?

An object gains or lose orbital energy.HOW does that happen?

Conservation of Angular Momentum

• The angular momentum of an object cannot change unless an external force (torque) is acting on it

• Earth’s rotation and orbit will continue forever because it can’t “get rid of “ angular momentum

angular momentum = (mass x velocity) x radius of orbit

So far …• Why do objects move at constant velocity if

no force acts on them?– Conservation of momentum

• Where do objects get their energy?– Conservation of energy: energy cannot

be created or destroyed but only transformed from one type to another.

– Energy comes in three basic types: kinetic, potential, radiative.

In order to stay on a closed orbit, an object has to be within a certain range of velocities:

Too slow => Object falls back down to Earth

In order to stay on a closed orbit, an object has to be within a certain range of velocities:

Too fast => Object escapes Earth’s gravity

• 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)

Escape Velocity

AstroTour Velocity, Acceleration, Inertia

AstroTour Newton’s Laws and Universal Gravitation

AstroTour Elliptical Orbits

Now you know !