Chapter 4 Notes

Chapter 4:

Making Sense of the Universe

4.1 Describing Motion: Examples from Daily Life

How do we describe motion?

Speed, Velocity, and Acceleration.

Speed

Tells us how far it will go in a certain amount of time.

Velocity

Tells us both its speed and direction

Acceleration of gravity

The acceleration of a falling object. (Abbreviated g)

Momentum

The product of its mass and its velocity. (Mass x Velocity = Momentum)

Net Force

Overall Force(An object must accelerate whenever a net force acts on it)

How is mass different from weight?

Mass is the same no matter where you are, but weight can vary.

Mass

The amount of matter.

Weight

Weight depends both on mass and on the force(including gravity) acting upon the mass.

Free-fall

Falling without any resistance to slow you down.

- People or objects are weightless whenever they are falling freely, and astronauts in orbit are weightless because they are in a constant state of free-fall.

4.2 Newton's Law of Motion

How did Newton change our view of the Universe?

Newton showed that the same physical laws that operate on Earth also operate in the heavens.

(He quantified the laws of motion and gravity, conducted crucial experiments regarding the nature of light, built the first reflecting telescopes, and invented the mathematics of calculus.

What are Newton's three laws of motion?

Newton's First Law: An object moves at constant velocity if there is no net force acting upon it.

(Objects at rest tend to remain at rest and objects in motion tend to remain in motion with no change in either their speed or their direction.

Newton's Second Law: The amount of acceleration depends on the object's mass and the strength of the net force. (Force = mass x acceleration)(F=ma)

Newton's Third Law: For any force there is always an equal and opposite reaction force.

4.3 Conservation Laws in Astronomy

What keeps a planet rotating and orbiting the Sun?

Angular Momentum

The term angular arises because a circle turns through an angle of 360 degrees.

Conservation of Angular Momentum

An object's angular momentum cannot change unless it transfers angular momentum or from another object.

(Angular Momentum = m x v x r)

m= Earth's mass.

v= Its speed(or velocity)

r= Radius of the orbit, Earth's distance from the Sun

-Earth is not exchanging substantial angular momentum with any other object, so its rotation rate and orbit must stay about the same.

Where do objects get their energy?

The Law of conservation of Energy

Energy can be transferred from one object to another or transformed from one type to another, but the total amount of energy is always conserved.

Kinetic Energy

Energy of Motion.

Radioactive Energy

Energy carried by Light.

Potential Energy

Stored Energy.(Which might later be converted into kinetic or radiative energy.)

Joule

In Science, it is the standard unit of energy.

Thermal Energy

(A subcategory of Kinetic Energy) The collective kinetic energy of many individual particles. (Thermal Energy depends on temperature and on the number and density of the particles.)

Temperature

Measures the average kinetic energy of the particles.

Kelvin

Temperature scale. (The Kelvin scale does not have negative temperatures, because it starts from absolute zero.)

Gravitational Potential Energy

Depends on an objects mass and how far it can fall as a result of gravity. (An object's gravitational potential energy increases when it moves higher and decreases when it moves lower.)

Mass-energy

Mass itself is a form of potential energy.

Mass itself is a form of potential energy, as described by Einstein's equation. E=mc(squared)

E is the amount of potential energy.

M is the mass of object

C is the speed of light

(This equation tells us that a small amount of mass contains a huge amount of energy.)

-The energy of any object can be traced back to the origin of the Universe in the Big Bang.

4.4 The Force of Gravity

What determines the strength of gravity?

Universal Law of Gravitation

1. Every mass attracts every other mass through the force called gravity.

2. The strength of the gravitational force attracting any two objects directly proportional to the product of their masses.

3. The strength of gravity between two objects decreases with the square of the distance between their centers.

We therefore say that the gravitational force follows an Inverse Square Law.

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

Bound orbits

Orbits in which an object goes around another object over and over again.

Unbound orbits

paths that bring an object close to another object just once.

Newton's version of Kepler's Third Law

Newton's version of Kepler's third law allows us to calculate the masses of distant objects.

How do gravity and energy allow us to understand orbits?

Orbital Energy

The sum of its kinetic and gravitational potential energies.

-Orbits cannot change spontaneously -- an object's orbit can change only if it gains or loses orbital energy.

(The planets total orbital energy always stays the same)

Gravitational Encounter

In which two objects pass near enough so that each can feel the effects of the other's gravity.

Escape Velocity

The minimum velocity required to escape gravity.

(The escape velocity from Earth's surface is about 40,000 km/hr, or 11 km/s)

How does gravity cause tides?

Tidal Force

Stretches the entire Earth to create two tidal bulges - on facing the moon and one opposite the moon.

Gravity attracts Earth and the Moon toward each other.

Spring tides

Named because the water tends to "spring up" from Earth.

Neap tides

When the tidal forces of the Sun and the Moon counteract each other. (Relatively small tides)