Great Idea #2: Newton’s universal laws of motion and

gravity predict the behavior of objects on Earth and in space.

How Do Objects Move?

Key Idea: One set of laws describes motions on Earth and in space

1. Universal laws of motion

2. The universal law of gravity

Key Terms:• Force• Uniform Motion• Acceleration• Weight vs. Mass

Newgrange, Ireland:5,000-year old “passage tomb”

StonehengeStonehenge• On the Salisbury Plain of southern England• 4500 years old• Exact history is uncertain• Stonehenge Decoded by Richard Hawking

John Snow (1813-1858)

He used the scientific method to discover that contaminated water at the Broad Street Pump was the source of a deadly cholera outbreak in London.

His research saved countless thousands of lives.

The Idealized Scientific MethodThe Idealized Scientific Method

Measurements and ObservationsMeasurements and Observations

• How would you measure a star’s position?1. Compass – direction to an object (degrees from North)

2. Protractor w/ straw (degrees from horizon)

3. Plot changes from night to night

• Planets (Mars) display

retrograde motion

• Patterns emerge

Ptolomy’s Model of the Solar System• Model of the universe with Earth at the center

• Epicycle (wheels within wheels)

• Why is it important?• Navigation• Astrology

• Lasted 1500 years

Dead White Males

• Nicolas Copernicus (1473-1543)• Sun-centered solar system

• Tycho Brahe (1546-1601)• Meticulous measurements of planet positions

• Johannes Kepler (1571-1630)• Mathematical analysis of planetary orbits

• Galileo Galilei (1564-1642)• Founder of experimental physics

Ptolomeic vs. Copernican models

• Earth- vs. Sun-centered• Perfect circles (still need epicycles)

Ptolomy Copernicus

Tycho Brahe (1546-1601)•Observed a new star

•Showed heavens can change over time

•Designed and used new astronomical instruments

•Collected data on planetary movements

The QuadrantThe Quadrant

Tycho Brahe (1546-1601)

Johannes Kepler (1571-1630)

• Mathematician

• Assistant to Tycho Brahe

• Inherited his notebooks

• Traditional ideas of the solar system are wrong (Kepler’s Laws)

Kepler’s Laws of

Planetary Motions

First Law: Planets have elliptical orbits

Elliptical orbits

Kepler’s Laws

Second Law: Orbits sweep out equal areas in equal times

Kepler’s Laws

Third Law: Distant orbits take longer.

(Average orbital radius)3 = k (orbital period)2

R3/P2 = constant

Galileo Galilei (1564-1642)• Mathematician• Improved the telescope • Observational astronomy• Founder of experimental science• Musical training• Arrogant and contemptuous of others• Published in Italian (vernacular) promoting

Copernican view• Heretic (threatened with torture) so he

recanted

Galileo – The Founder ofExperimental Science

Galileo studied the relationship among distance, time, velocity and acceleration

He observed that objects accelerate while falling:

Galileo’s Discoveries

• Constant acceleration

–Balls on a plane: v = at

• Freefall

–Constant acceleration at g

–g = 9.8 m/s2 = 32 feet/s2

–Distance traveled (d) = ½at2

Speed, Velocity, and Acceleration

• Speed = distance traveled over time• Velocity = speed with direction

• Equation for speed:

• Acceleration = rate of change of velocity

• Equation for velocity:

t

dv

t

vva if

Galileo’s Laws of Motion1. Trajectories are parabolas

2. All objects fall at the same velocity

3. Distance of fall = k(Time)2

Remember Kepler (1571-1630)

1. Orbits are ellipses

2. Equal areas in equal times

3. (Radius)3 = k(Time)2

Now on to Sir Issac Newton (1642-1727)

Sir Issac Newton (1642-1727)

• Born prematurely

• Abandoned by his mother

• Forced to run family farm

• Sent to Cambridge University

• Made fundamental advances in several scientific fields

Issac Newton’s – Plague Years (1665-1666)

1. Calculus

2. Laws of optics• Broke white light into colored light

3. Universal laws of motion

4. Universal law of gravitation

Two Types of MotionsUniform motion: No change in velocity or

direction

Acceleration: Change in velocity and/or direction

Issac Newton’s Laws of Motion1st Law: Nothing happens without a force

“A moving object will continue moving in a straight line at a constant speed, and a stationary object will remain at rest, unless acted on by an unbalanced force.”

This law defines “force”

Issac Newton’s Laws of Motion1st Law: Nothing happens without a force

2nd Law: Force = mass X acceleration (F=ma)

“The acceleration produced on a body by a force is proportional to the magnitude of the force and inversely proportional to the mass of the object.”

This law defines “mass”

Issac Newton’s Laws of Motion

1st Law: Nothing happens without a force2nd Law: Force = mass X acceleration (F=ma)

3rd Law: Forces always act in pairs

“For every action there is an equal and opposite reaction.”

Gravity (a universal force)

The Universal Force of Gravity

Between any two masses there exists a force that is

proportional to the masses and inversely proportional to the

square of the distance between them.

F = Gm1m2/d2

The Force of GravityF = Gm1m2/d2 Why mass x mass?

Think of “rays of mass” radiating in all directions

gravitational force = # of rays intersecting

m1 m2

The Force of GravityF = Gm1m2/d2

Why inverse square of distance?

Think about a flashlight:

light intensity is greater

at shorter distances

Now double the distance:

(4X the area = 1/4 the light)

The Force of GravityF = Gm1m2/d2

What is G?

• G = universal constant of direct proportionality

• Henry CavendishG = 6.67 x 10-11m3/s2-kg or

6.67 x 10-11N-m2/kg2

Weight and Gravity• Weight = Gravity acting on an

object’s mass

• Weight depends on gravityDifferent on Earth vs. Moon

• Mass is constant

Natural Laws Raise DeepPhilosophical Questions

• What about free will?

• Natural laws in other domains

Economies (Adam Smith)

Law (cause and effect)

Politics (George Mason)