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**Chapter 4: Making Sense of The Universe: Matter, Energy & Gravity**

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**What Is Matter? Matter is the “stuff” that makes up our “things”**

Matter is composed of atoms and their constituents These “elements” combine to form the molecules of life Matter has MASS - Mass is an intrinsic property of matter

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What Is Energy? Energy comes in different forms: - Kinetic (motion) - Potential (stored) - Radiative (electromagnetic) Mass can be turned into Energy Energy can become Mass E=mc2 Energy can change type but not be created or destroyed.

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**Matter is composed of atoms**

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The Periodic Chart These elements all have unique chemical properties, based on the number of protons, neutrons and electrons they are composed of. If you add or remove neutrons from the most common form of an element, you create Isotopes. Stripping electrons from an atom is called Ionization.

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**Phases of Matter Solid Liquid Gas Plasma**

The state changes as you increase the temperature (add energy)

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**Thermal Energy: The collective kinetic energy of many particles**

(for example, in a rock, in air, in water) Thermal energy is related to temperature but it is NOT the same. Temperature is the average kinetic energy of the many particles in a substance. Students sometimes get confused when we've said there are 3 basic types of energy (kinetic, potential, radiative) and then start talking about subtypes, so be sure they understand that we are now dealing with subcategories. © 2014 Pearson Education, Inc.

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**Temperature Scales Use this figure to review temperature scales.**

© 2014 Pearson Education, Inc.

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Temperature Scales Thermal energy is a measure of the total kinetic energy of all the particles in a substance. It therefore depends on both temperature AND density. Example: We've found this example of the oven and boiling water to be effective in explaining the difference between temperature and thermal energy. You may then wish to discuss other examples. You might also want to note that the temperature in low-Earth orbit is actually quite high, but astronauts get cold because of the low density. © 2014 Pearson Education, Inc.

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Atoms & Orbits Matter appears solid, but is composed of mostly empty space! Electrons orbit atoms in clouds, with higher energy electrons in farther orbits

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Atomic Energy States

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Potential Energy Potential Energy is the energy something has just because it has mass and is where it is Think of a boulder on a hill as an example, when it rolls down it turns potential energy into Kinetic Energy (U=mgh) Simply by having mass that can be turned into energy is potential…

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**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. We next discuss 2 subcategories of potential energy that are important in astronomy: gravitational potential energy (this and next slide) and mass-energy. © 2014 Pearson Education, Inc.

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**Kinetic Energy When mass is moved it is Kinetic Energy (T=1/2 mv2)**

Adding energy to a system, i.e. as heat, will make the atoms more energetic and they will move more, colliding with other atoms – this is Kinetic Motion Temperature ~ Energy

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**Conservation of Energy**

In a closed system, the sum total of the energy is conserved

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**The Universal Laws of Motion**

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**Motion What is the difference between: Speed, Velocity & Acceleration?**

Speed is a scalar, that means that it is how fast you go, but without specific direction Velocity is a vector, it gives speed and direction Acceleration is found when an object is speeding up or slowing down

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Formulae v = Δx/ Δt a = Δv/ Δt g = 9.81 m/s2

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Mass & Momentum Mass is an intrinsic property of matter. The atomic mass is invariant, regardless of your gravitational field (or speed) Weight is a Force, it changes depending on your gravitational field Momentum is mass x velocity, it tells you that big-fast things are harder to stop than small-slow things. p = mv

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**How did Newton change our view of the universe?**

Realized the same physical laws that operate on Earth also operate in the heavens one universe Discovered laws of motion and gravity Much more: experiments with light, first reflecting telescope, calculus… Sir Isaac Newton (1642–1727) © 2014 Pearson Education, Inc.

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**Newton’s Laws of Motion**

First Law: An object at rest remains at rest, and an object in motion remains in motion, unless acted upon by an external force Second Law: F = ma = Δp/ Δt Force depends on rate of change of momentum. Third Law: For every force, there is an equal and opposite force

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**Conservation of Momentum**

Newton’s laws imply the concept of conservation of momentum. The total momentum in a system is conserved. Angular momentum acts on a body in orbit around another, held by the gravitational force. In the absence of net torque, the angular momentum remains constant. angular momentum = mvr

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**Angular momentum conservation also explains why objects rotate faster as they shrink in radius.**

Discuss astronomical analogs: disks of galaxies, disks in which planets form, accretion disks… © 2014 Pearson Education, Inc.

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Center of Mass Because of momentum conservation, orbiting objects orbit around their center of mass. We next discuss 2 subcategories of potential energy that are important in astronomy: gravitational potential energy (this and next slide) and mass-energy. © 2014 Pearson Education, Inc.

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**Newton’s Universal Law of Gravitation**

Every mass attracts other mass through the force called gravity The force of attraction between any two objects is directly proportional to the product of their masses The force of attraction between two objects decreases with the square of the distance between their centers Fg = GM1M2/d2 (note that G is the gravitational constant)

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**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 the square of the distance between their centers. © 2014 Pearson Education, Inc.

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Newton’s Influence Some 70 years after Kepler published his three laws of orbital motion, Newton was able to explain “why” they held true! Science strives to observe, determine relationships and eventually discover “why” things work the way they do. Newton’s laws remained unchallenged until an upstart named Albert Einstein developed a general theory of gravity that was more precise in 1905.

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**How does Newton's law of gravity extend Kepler's laws?**

Kepler's laws apply to all orbiting objects, not just planets. Ellipses are not the only orbital paths. Orbits can be: bound (ellipses) unbound parabola hyperbola © 2014 Pearson Education, Inc.

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

Total orbital energy (gravitational + kinetic) stays constant if there is no external force. Orbits cannot change spontaneously. The concept of orbital energy is important to understanding orbits. Total orbital energy stays constant. © 2014 Pearson Education, Inc.

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**How does gravity cause tides?**

Use this figure to explain the origin of the tidal bulges. Gravitational pull decreases with (distance)2, the Moon's pull on Earth is strongest on the side facing the Moon, weakest on the opposite side. The Earth gets stretched along the Earth-Moon line. The oceans rise relative to land at these points. Moon's gravity pulls harder on near side of Earth than on far side. Difference in Moon's gravitational pull stretches Earth. © 2014 Pearson Education, Inc.

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Tidal Friction Tidal friction gradually slows Earth's rotation (and makes the Moon get farther from Earth). The Moon once orbited faster (or slower); tidal friction caused it to ''lock'' in synchronous rotation. © 2014 Pearson Education, Inc.

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**Gravity Rules! The gravitational influence of the Moon cause the tides**

Leaving the surface of Earth takes a lot of energy, it is like crawling out of a deep well. The escape velocity is about 40,000 km/hr. While the other fundamental forces (electromagnetic, strong and weak) are much stronger, their range is short and the motion of the stars and galaxies are bound by gravity.

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Free Fall Why are astronauts weightless in space? They are in a constant state of free fall. The Force from gravity in the space station from Earth is not much less than on the surface!

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Lecture 4: Gravity and Motion Describing Motion Speed (miles/hr; km/s) Velocity (speed and direction) Acceleration (change in velocity) Units: m/s 2.

Lecture 4: Gravity and Motion Describing Motion Speed (miles/hr; km/s) Velocity (speed and direction) Acceleration (change in velocity) Units: m/s 2.

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