Unit 1 - Our Dynamic Universe

Question 1

What is the definition of acceleration (a)?

Answer 1

The change in velocity per unit time.

Question 2

What is the definition of the conservation of momentum?

Answer 2

The total momentum before (a collision) equals the total
momentum after (a collision), in the absence of external
forces.

Question 3

What is the definition of elastic collision?

Answer 3

In an elastic collision the total kinetic energy before is equal to the total kinetic energy after the collision.

Question 4

What is the definition of inelastic collision?

Answer 4

In an inelastic collision the total kinetic energy before is greater than the total kinetic energy after the collision.

Question 5

What is the definition of gravitational field strength (g)?

Answer 5

The force per unit mass (due to gravity).

Question 6

What is the definition of length contraction?

Answer 6

Length contraction is the decrease in length (in the direction of motion) of an object moving relative to an observer.

Question 7

What is the definition of time dilation?

Answer 7

Time dilation is an increase in a time interval as measured by a stationary observer relative to a moving observer.

Question 8

What is the definition of redshift?

Answer 8

Redshift is when light from objects moving away is shifted to longer (more red) wavelengths due to the doppler effect.

Question 9

What is the same in a vacuum for all observers?

Answer 9

The speed of light (c).

Question 10

What is Hubbles law and what can we do with it?

Answer 10

More distant galaxies are moving away at a greater velocity than closer galaxies.
Hubble’s law allows us to estimate the age of the Universe.

Question 11

How can a mass of a galaxy be estimated?

Answer 11

The mass of a galaxy can be estimated by the orbital speed of stars within it.

Question 12

What is dark matter and what is evidence for it?

Answer 12

Dark matter is the unseen mass that explains why the galaxies are able to hold on to many of its stars, as they should not be able to with their observable mass.
Evidence supporting the existence of dark matter comes from estimations of the mass of galaxies.

Question 13

What is dark energy and what is evidence for it?

Answer 13

Dark energy is the unknown energy that causes the rate of expansion of the universe to increase.
Evidence supporting the existence of dark energy comes from the accelerating rate of expansion of the Universe.

Question 14

How can you tell the temperature of stellar objects?

Answer 14

The temperature of stellar objects is related to the distribution of emitted radiation over a wide range of wavelengths.

Question 15

How does the peak wavelength change for hot and cold objects?

Answer 15

The wavelength of the peak wavelength of this distribution is shorter for hotter objects than for cooler objects.

Question 16

What is the evidence for the Big Bang theory?

Answer 16

 Significantly larger number of galaxies showing redshift, rather than blueshift.
 cosmic microwave background radiation is evenly spread out
 abundance of the elements hydrogen and helium
 Olbers’ paradox (darkness of the sky)

Question 17

How do you calculate friction down a slope?

Answer 17

W sin(x) ( mg sin(x) )

Question 18

State the conservation of momentum rule.

Answer 18

Total momentum before collision = total momentum after collision in the absence of external forces.

Question 19

What is conserved in all collisions?

Answer 19

Energy.

Question 20

What happens to total kinetic energy in an explosion?

Answer 20

Not conserved - gained.

Question 21

What is impulse?

Answer 21

Impulse is the cause of a change in momentum, the product of force of time. It is equal to the range in momentum, measured in kg m s^-1.

Question 22

What are the postulates of special relativity?

Answer 22

1. When two observers are moving at constant speeds relative one another, they will observe the same laws of physics within their own frames of reference.
2. The speed of light (in a vacuum) is the same for all observers, regardless of their motion to the light source.

Question 23

If the source comes towards the observer, what sign do you use in doppler effect equations?

Answer 23

-ve, as the frequency increases.

Question 24

If the source goes away from the observer, what sign do you use in the doppler effect equations?

Answer 24

+ve, as the frequency decreases.

Question 25

What is cosmic microwave background radiation?

Answer 25

Cooled radiation from the Big Bang observed evenly in every direction.

Question 26

What is the abundance of light elements?

Answer 26

More hydrogen and helium in the universe than an other element. These were the simplest elements to form from cooling radiation.

Question 27

What is Olber’s paradox?

Answer 27

Light from distant stars is yet to reach us, showing that stars formed much later than the creation of the universe.

Question 28

How does redshift prove the expanding universe?

Answer 28

There are significantly more galaxies showing redshift than there are blueshift, showing many more galaxies are moving away from us.

Question 29

What happens as the temperature of stars increases?

Answer 29

1. There is more energy (intensity of radiation) at each wavelength.
2. The peak wavelength shifts to shorter wavelengths (hotter stars emit bluer wavelengths)

Question 29

Explain Hubble’s law.

Answer 29

Galaxy’s recessional velocity is directly proportional to the distance from us, showing the universe is expanding.