8. energy and gravity

7/21/2019 8. Energy and Gravity

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Where do objects get their energy?

• Energy makes matter move.

• Energy is always 'conserved'



Conservation of Energy

• Energy can neither be created nor destroyed

• The total energy content of the universe wasdetermined at the Big Bang and remainsconstant to this day

• Energy can change form or can transferbetween objects



Basic Types of Energy

• Kinetic (motion)

• Radiative (light)

• Stored or potential

Energy can change typebut cannot be destroyed.



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 intoparticles (for example, in

particle accelerators)



Gravitational Potential Energy

(a form of Stored Energy)

• On Earth, depends on: – object’s mass (m)

– strength of gravity (g ) – distance object couldpotentially fall



Gravitational Potential Energy

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

$ A contracting cloud converts gravitational potentialenergy to thermal energy.



What have we learned?

• 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.

– One type of kinetic energy is Thermal Energy

(the movement of particles) – One type of potential (or stored) energy is

Gravitational Energy



The Universal Law of

GravitationOur goals for learning:

• What determines the strength of gravity?• How do gravity and energy together allow

us to understand orbits?

•How does Newton’s law of gravity extend

Kepler’s laws?

•Why do all objects fall at the same rate?



What determines the strength of gravity?

The Universal Law of Gravitation:

• Every mass attracts every other mass.• Attraction is directly proportional to the product

of their masses.

• Attraction is inversely proportional to thesquare of the distance between their centers.



Center of Mass

• Orbiting objectsactually orbit arounda common center ofmass.

• The location of thatcenter depends on

where most of themass is located.



How does Newton’s law of gravity extend

Kepler’s laws?

Newton's relationship between the orbital period and average orbital distance of a

system tells us the total mass of the system.

Examples:• Earth’s orbital period (1 year) and average distance

(1 AU) tell us the Sun’s mass.• Orbital period and distance of a satellite from Earthtell us Earth’s mass.

• Orbital period and distance of a moon of Jupiter tell

us Jupiter’s mass.



Newton’s Version of Kepler’s Third Law

p = orbital period

a=average orbital distance (between centers)

(M1 + M2) = sum of object masses

The result:

• The masses of any orbiting bodies can be

calculated from the size or period of their orbit

(measureable quantities!)

p2!4p2

G" M 1+M 2#a3OR M

1+M

2!4p2

G

a3

p2



highlighted.

! This technique is used to calculate the massof distant objects which we cannot measure,

by using motions that we can measure.

! The mass of the Sun and all the planets wasderived this way. Binary stars and extra-solar

planet properties are derivied this way.




• 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• How does Newton’s law of gravity allow us to extend

Kepler’s laws?

– Applies to other objects, not just planets.

– Can be used to measure mass of orbitingsystems.



Tides and Gravity

Our goals for learning:

•How does gravity cause tides?

•How does the competing gravity from the

Sun and the Moon affect tide height?

•How does the Moon's gravity affect the

Earth's rotation?



Gravity Force of the Moon on the Earth

• Moon’s gravity pulls harder on the near side of

Earth than on the far side

• The difference in the Moon’s gravitational pull,stretches Earth (called the 'tidal force')

• Similar to pulling on 1 end of a rubberband



Tidal Bulge of Earth

High

Tide

High

Tide

Low Tide

Low Tide



High

Tide

High

Tide

Low Tide

Low Tide

•Earth rotates under the bulge•Your location moves from under the high

water, into the low water and out again.•You see two tides/day



Tidal range of 2-4m,

(6-12 feet)



Bay of Fundy, Nova Scotia

Tidesof

40-52

feet

Tid d Ph



Tides and PhasesSize of tides depends

on the phase of Moon

Sun & Moon’s gravities

acting together = Large tides

Sun & Moon’s gravities

acting at odds = weakened tides



Tidal Friction

As a Result:• The Earth’s rotation slows down (days get longer)

• The Moon accelerates (it moves further away from us)

The tidal bulge points toward the Moon and drags on

the Earth as Earth rotates under it.




• How does gravity cause tides? – Moon’s gravity stretches Earth and its oceans

• How does the competing gravity from the Sun

and the Moon affect tide height? – When the Sun and Moon are along on the

same line their gravities combine and tides are

higher.

• How does the Moon's gravity affect the Earth's

rotation?

– The Moon pulls the tidal bulge back, which

slows the Earth (called tidal friction)

8. energy and gravity

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