einstein's universe midterm 1: all lecture questions 1-9

What does Wolfson call "a good definition of a wave"?

"A traveling disturbance that carries energy but not matter."

How can we make electromagnetic waves? (2)

Changing field Charge/magnet must be in state of changing motion

In what two ways can motion change?

Changing speed Changing direction

What gives rise to magnetism in magnetite?

Charged objects in motion. For example, electrons in motion create a magnetic field. The force can be attractive or repulsive. It is a non-contact force, like gravity and the electrical force. Each magnet has two poles, called north and south. You cannot find isolated poles; cut a magnet in half and you have two magnets, each with north and south poles. The farther apart the magnets are the weaker the force between them.

What is a unit of charge, according to the International System of Units?

Couloumb

When you shoot an arrow, in which direction is the force acting on the arrow? What is the direction of the motion of the arrow?

Direction is downwards Motion is a parabola

When you throw a ball up in the air, in which direction is the acceleration? What is the velocity of the ball at the top of the trajectory? (Near the surface of the Earth, g=-10m/s2)

Direction is downwards, towards centre of the Earth Velocity is 0

Define displacement, distance, speed, velocity, acceleration, force. Which are vector and which are scalar quantities? What are the units for each?

Displacement, Velocity, Acceleration, and Force are all vectors. Distance and speed are scalar quantities.

Describe Galileo's experiment involving falling objects near the surface of the Earth.

Dropped objects off the tower of Pisa - objects must fall at the same rate.

In what frame of reference are the laws of electromagnetism valid, according to 19th century physicists?

Earth in respect to the ether

Gauss' law

Electric charges exert forces on each other Magnetic poles exert forces on each other

Define inertial mass and gravitational mass in Newton's Laws of Physics

F=ma is inertial mass. F=GmM/R^2 is the gravitational mass.

Who discovered this equal and opposite force?

Faraday

Describe Faraday's experiment that demonstrated electromagnetic induction, another connection between electric current and magnetism.

Faraday discovered that moving a magnet near a loop of wire generates a current in the wire. A battery can create electric current; but now electric current can be created by a moving magnet. In terms of the magnetic field, a changing magnetic field generates an electric current. To put it another way, a changing magnetic field generates an electric field.

What is an example of Newton's 3rd Law that became particularly important in electromagnetism?

Faraday's law: moving magnet generates electric field

If every force has an action and a reaction that are equal in magnitude but opposite in direction, how do we ever experience acceleration?

Forces that are equal and opposite do not act on the same bodies

List four parameters that characterize a wave.

Frequency Wavelength Amplitude and period Speed

Does light go at speed c relative to the source of the light waves?

Goes at a constant speed; observer/source doesn't matter Putting it in a different medium will keep it constant, just would be a different constant

If gravity acts between all objects, why don't everyday objects fall together in a great clump?

Gravity is a very weak force, easily overcome by other forces

What important differences are there between the gravitational force and the electrostatic force?

Gravity is much much weaker.

What was some of the observations made by Galileo Galilei with a telescope and what was their significance with respect to the ancient view of the heavens?

He observed four of Jupiter's moons and saw astronomical bodies orbiting something other than the earth; microcosm of Earth. He observed the phases of Venus, and saw Venus change size with the changing of the phases: this is hard to explain with a circular orbit around the earth. He saw sunspots and craters on the moon, confirming the idea that the heavens were not the realm of perfection.

Describe Galileo's experiments with rolling balls and what he concluded regarding the question about the natural state of motion of objects.

He rolled balls on inclined planes and observed that they rose to the same height they started from. He suggested that the natural state of motion was uniform motion, because forces cause a change in motion.

What characteristic of "action-at-a-distance" do Physicists find troubling?

How come they act instantaneously? Information must take time to travel through space.

State the Principle of Newtonian/Galilean Relativity in two equivalent ways.

One way: "The laws of mechanics are the same in all reference frames in uniform motion." Second way: "There is no mechanics experiment you can do to distinguish between two uniformly moving frames in relative motion. Identical experiments done in two reference frames in uniform motion will give identical results."

What is the direction of frictional forces?

Opposing motion

In Oersted's experiment, which direction should the compass initially point to show the connection between electricity and magnetism?

Parallel to current

How did Newton explain the existence of light and its properties?

Particle model of light

What was the response of ancient astronomers, like Ptolemy, to observations of retrograde motion and to the prevailing view of motion in the heavens?

Ptolemy suggested that planets moved in epicycles in circular orbits around the Earth, preserving the view that motion in the heavens was still perfect and circular.

Are positive forces attractive or repulsive?

Repulsive - think of equation - Q1andQ2 need to be the same for it to be positive

What is an electromagnetic wave?

Self perpetuating cycle of electric and magnetic waves that travels at the speed of light

On what length scale are electrostatic forces strongest?

Short lengthscales - subatomic!

How does the inertial mass of an object affect its acceleration due to a given external force?

It resists acceleration - mass is inversely proportional to acceleration. F=ma

What constitutes the branch of physics known as mechanics?

Mechanics is the branch of physics that deals with motion: so Newton's Laws of Motion constitutes Mechanics. This includes the 2nd law, F = m a; one of those forces, F, was the law of universal gravitation deduced by Newton. Other forces were contact forces, such as when you push or pull on something. Examples of motion include the motion of the planets around the sun, the motion of the moon around the earth, and the motion of air molecules, which is what sound is. Newton's Laws of Motion apply to all of these.

Ampere's law

Moving electric charges create a magnetic field Changing electric fields cause changing magnetic fields

What radical philosophical shift was present in Newton's recognition of universal gravitation?

No distinction between terrestrial and heavenly realms; the same laws describe motion

Does light obey the Galilean principle of relativity? Explain.

No, it is always measured to be 'c', independent of the motion of the detector or the source. Galilean relativity states that motion is relative and that the speed of the observer or the source must be taken into account when measuring the speed of an object.

Can one do an experiment using light to determine whether a lab is moving at constant velocity or is at rest?

No; only the difference can be measured

If an elephant weighs more than a book, why do they both accelerate at the same rate? Is the force exerted on them by the Earth the same?

Objects with the same mass fall at the same rate. Because the inertial and gravitational mass are the same so they cancel out. No, the force is proportional to their mass. Force exerted on the elephant is greater than the force exerted on the book as elephant mass is proportionally larger.

What experiment, done by Oersted, demonstrated a connection between electric current and magnetism?

Oersted held a compass near a current-carrying wire and the needle was deflected. The current carrying wire thus had a similar effect on the compass needle as did the earth's magnetic field. The wire generated a magnetic field. Thus moving electric charge creates magnetism.

What dichotomy is revealed in the ancient world in response to the question, "what is the natural state of motion on Earth and in the heavens"?

On Earth, the natural state of motion is at rest. In the heavens, the natural state of motion is perfectly circular.

Why is motion important?

With no motion there is no beating heart, no blood flowing, no flow of air into lungs, no flow of gas out of the lungs, no synapses, in other words, no life.

If you throw a ball from the top of a uniformly moving ship, does the motion of the ball depend on the inertial reference frame in which it is measured?

Yes

Are sound waves explicable under the umbrella of Newton's Laws of Motion?

Yes: sound is a manifestation of the motion of air molecules

Can there be accelerated motion at constant speed?

Yes; changing direction = circular motion

What is the 19th century answer to the question do the laws of electromagnetism obey a relativity principle? A related question is in what frame of reference are the laws of electromagnetism valid?

Yes; the ether.

Describe the early 19th century experiment by Young that argued for a wave model of light.

Young's double-slit experiment Proved the wave model as when light was passed through a diffraction grating, it exhibited constructive and destructive interference

What are the four fundamental laws of electricity and magnetism? (power point slide)

1. Gauss' law: Electric charges exert forces on each other 2. Gauss' law: Magnetic poles exert forces on each other 3. Faraday's law: A changing magnetic field creates an electric field 4. Ampere's law: Moving electric charges create a magnetic field Changing electric fields cause changing magnetic fields

Compare your weight in Newtons to your mass? If the Earth had twice the radius, what would happen to your weight? And your mass?

100kg is 1000N. It would be a quarter (250N) due to the inverse square law. Mass would stay the same.

There are two loudspeakers, facing each other, in a room. You are in between the two speakers, and you measure the speed of sound coming from one speaker to be 342 meters per second and the speed of sound coming from the other speaker to be 338 meters per second. What is your speed through the air? Take the speed of sound in air to be 340 meters per second.

2 m/s

A person standing still on Earth, a person in a car moving at 60 mph on Earth, a person in an airplane moving at 600 mph with respect to Earth and someone in a spaceship moving at ½ c all measure the speed of light of the same beam of light. What do they measure for the speed of light? If they got different answers would that violate the Principle of Relativity?

3 x 10^8

What did Maxwell calculate for the speed of electromagnetic waves?

3 x 10^8

In the circular motion of the Moon around the Earth, what is the direction of acceleration? What is the direction of the gravitational pull of the Earth on the Moon?

Acceleration direction is constantly changing, towards center of the Earth

The speed of sound is about 700mph, but relative to what do sound waves move at 700mph?

Air

State Newton's Universal Law of Gravitation both in words and as an equation. How does this law define the concept of gravitational mass?

All objects are attracted to one another, in proportion to their mass.

What is the principle of Newtonian or Galilean relativity

All the laws of physics are the same in all uniformly moving reference frames.

Does the Earth accelerate towards the apple as it falls towards it? Does the Moon exert a force on the Earth?

Apple is also exerting force on the Earth but it's negligible Same for the moon.

When discussing uniform motion, why is it meaningless to talk about anything except relative motion. How does this lead to a principle of relativity?

all the laws of physics are the same in all uniform reference frames

How does Newton treat celestial and terrestrial motion?

assumes that the same laws of nature that operate on earth, also operate in the heavens

How does the concept of a field define action at a distance?

explains non-contact forces; responds to the space around it. It is the force felt by a test charge at a given point in space.

According to the ancient Greeks, what caused motion?

force

In Newton's view what is needed to change motion?

force

How does the force between two charges depend on the distance between them?

inverse-square law: forces are inversely proportional to the square of the distance between the objects

By the late 1800's what two branches of physics described much of physical reality?

mechanics and electromagnetism

What is the definition of average velocity?

the distance moved divided by the time it took to move that distance

How does the law of inertia differ from the Aristotelian view of motion on earth?

the law of inertia implies that only changes in motion requires a causative agent, while the Aristotelian view says that if something is moving there must be a force even if the motion is uniform.

Why were magnetic and electrostatic forces used by magicians?

they were invisible

What is the natural state of motion on Earth, according to the Ancient Greeks?

Being at rest, at close to centre to the Earth as possible

Why doesn't the Moon fall on the Earth if gravity is pulling it towards the centre of the Earth?

It has inertia/initial velocity

Are we at rest or moving at 900mph with the Earth?

Meaningless question because relative motion

State Newton's three laws of motion.

1 - objects at rest stay at rest, unless subjected to some force 2 - F = ma 3 - every action has an equal and opposite reaction

What two astronomical observations did Copernicus's heliocentric model of the Universe explain?

1 - The retrograde motion of the planets (if they are in circular motion, why would they change direction) 2 - the brightening and dimming of the planets (if they are equally distant from Earth, in circular motion, why would they appear brighter)

Faraday's law

A changing magnetic field creates an electric field

Which property of light demonstrated that light is a wave?

Interference

Is the inertial mass of an object an intrinsic property of the object or does it depend on the force applied to the object?

Intrinsic

What contribution did Copernicus make to our view of the earth's place in the universe?

Introduced heliocentricity - removed the Earth from occupying a special location in the solar system

What is the relationship between the wavelength of a wave and its frequency, given that its speed is always constant?

Inversely proportional; wavelength x frequency = speed

What is the Newtonian idea of "action-at-a-distance"?

Invisible forces that act through empty space - the idea of fields.

What was Aristotle's argument for claiming that the earth is not moving? What argument might you construct to counter Aristotle's argument?

Jump test: Jump off the ground and you won't land on the spot on the ground you jumped from, but you will land somewhere else. The ground will move out from underneath you. A counter argument to this is does the object land at your feet on a moving ship?

How did Kepler account for the careful observations of planetary motion by Tycho Brahe? How did this alter the view of planetary motion by Ptolemy?

Kepler, building on Brahe's work, proposed that the planets move in elliptical orbits around the sun.

What does the law of inertia imply about relative and absolute motion?

Law of inertia: object at rest will stay at rest unless acted upon by an outside force Absolute motion doesn't exist; only relative motion matters

What philosophically disturbing dichotomy existed in the laws of physics around the late 1800's?

Laws of motion applied to mechanical physics but not electromagnetism The laws of mechanics obeyed a relativity principle. Absolute motion was meaningless and no experiment could answer the question "am I moving"? The laws of electromagnetism did not seem to obey a relativity principle and absolute motion had a meaning in a sense that you could determine your speed through empty space by doing a Michelson-Morley experiment.

In what ways are light waves different from sound waves and water waves?

Light waves do not require a medium, whereas sound waves require air and water waves require water. The speed of light waves is a constant of nature, the same for all observers in uniform motion, whereas the speeds of sound waves and water waves will vary with the state of motion of the observer relative to the water or air.

Why and how did Maxwell modify one of the four fundamental statements about electricity and magnetism?

Maxwell changed currents generating a magnetic field into a current that generates a magnetic field and a changing electric field generates a magnetic field.

Why do the two slits need to be small for an interference pattern to occur?

Slits act as sources of waves. each slit acts as a source of spherical waves, which "interfere" as they move from left to right If the single slit wasn't narrow, there wouldn't be diffraction and so there wouldn't be interference. Wavelength of visible light is too small - 2.5 millionths of an inch for red light and about 14 millions of an inch for deep violet light.

What were Einstein's major discoveries in 1905?

Space and Time are relative - theory of relativity. Matter is made up of atoms (Brownian motion). Photoelectric effect.

A car driving due North speeds up. What is the direction of acceleration? A car driving due North breaks. What is the direction of acceleration?

Speeds up = north Breaks = south

How does the ether allow for the possibility of absolute motion?

Suppose the ether exists. If it fills all of space and is unaffected by bodies moving through it (remember that one of the properties of the ether is that it does not affect bodies moving through it) then it could reasonably be regarded as being at rest in absolute space, constituting the embodiment of absolute space.

How is it that both apple and Moon are falling, when the apple is on a collision course with the ground while the Moon remains in its orbit?

The Moon's inertial mass is significant enough to overcome the force of gravity, keeping it in orbit The Moon has a component of velocity perpendicular to the line joining the Moon and the Earth that the apple does not have. If the Moon did not have this component of velocity it would fall directly to the Earth and strike it. The Earth's gravitational force causes the Moon's motion to deviate from the straight line motion it would execute otherwise.

What is the 19th century answer to the question, with respect to what does light move at speed c?

The ether

What determines the speed of a water wave, sound wave, seismic wave?

The medium

Describe what is meant by retrograde motion of the planets.

The planet's motion is not described by the statement that they rise in the east and set in the west, as is true for the stars. Usually planets move from west to east against the background stars, but other times they move from east to west against the background stars from one night to the next.

What is wave interference?

The property of waves to add up or cancel out when in phase Two or more waves can occupy the same point in space; when they do so they interfere with each other. When two peaks overlap you get a larger wave - this is constructive interference. When a peak and a trough overlap they can cancel out (if they have the same amplitude) - this is destructive interference.

What is weightlessness?

The state of apparently not being acted on by gravity.

In what way are the Moon and an apple that comes loose from a tree both falling? In other words, in what way is their motion the same?

They are both accelerating towards the Earth. This is how their motion is the same.

What is meant by a natural state of motion?

This is a state of motion that requires no explanation. When something is left alone it will assume this state of motion (which might be rest, i.e., lack of motion).

If someone is moving through the air at 20mph, what speed will they measure for the speed of sound if sound waves are moving towards them? Away from them?

Towards: 720 mph (faster) Away: 680 mph (slower)

What is uniform motion? What is the acceleration of an object in uniform motion?

Uniform motion is moving at a constant speed and direction. The acceleration of an object in uniform motion is zero.

If we seek to understand the nature of space and time, why would we study motion?

We move from one point in space to another; furthermore, it takes time to get from one place to another. So, we move through space and in time; this is one way to understand the nature of space and time since we move through both.