Orbits and The Wider Universe

Question 1

Kepler’s First Law

Answer 1

Each planet moves in an ellipse with the sun at one focus

Question 2

Kepler’s Second law

Answer 2

The line joining a planet to the center of the sun sweeps out equal areas in equal times

Question 3

Kepler’s Third Law

Answer 3

The square of the period, of the planets orbital motion is proportional to the orbital radius cubed

Question 4

Explain how we know dark matter exists

Answer 4

The measured rotational velocity of stars further out from the center of the galaxy is higher than expected. This implies that the mass is greater than what we can see, this suggests dark matter exists

Question 5

When using Kepler’s third law, what is something that we assume

Answer 5

We assume that a small mass is orbiting a larger mass

Question 6

What are three that we can infer about a system where two masses orbit a center of mass

Answer 6

1. The center of mass must be in line with the two masses
2. The center of mass must be between the objects, as centripetal acceleration must be towards it
3. The angular velocities of the objects must be identical

Question 7

If a star/galaxy is moving towards us what happens to the wavelength

Answer 7

It decreases, this is known as blue shift

Question 8

If a star/galaxy is moving away from us what happens to the wavelength

Answer 8

It increases, this is known as red shift

Question 9

Radial velocity, in Doppler shift equation

Answer 9

The component of the objects velocity along the line joining the observer to the object

Question 10

What is the equation for Doppler shift in words

Answer 10

Delta lambda over lambda = radial velocity over the speed of light

Question 11

Radial Velocity, When related to the Doppler shift equation

Answer 11

The Radial velocity is the component of the objects velocity along the line joining the observer to the object

Question 12

What did Hubble notice with the red shift from stars and galaxies

Answer 12

That the redshift of the stars and galaxies was greater the further from earth they were.

Question 13

What is the equation taken from the graph Hubble created using data from redshift of galaxies

Answer 13

Hubble’s constant = Velocity over distance

Question 14

Hubble’s law

Answer 14

The speed of the recession of a galaxy is directly proportional to its distance from the earth

Question 15

What is the value of Hubble’s constant

Answer 15

2.2x10^-18s^-1

Question 16

What does Hubble’s constant give a measure of

Answer 16

The expansion rate of the universe, how fast objects appear to be moving away from our galaxy as a function of distance

Question 17

What do we have to assume about radial velocity of galaxies to calculate the age of the universe

Answer 17

That the radial velocity has been constant since the beginning of the universe

Question 18

What is the equation for the age of the universe

Answer 18

1 over Hubble’s constant

Question 19

What are the two problems with Hubble’s constant

Answer 19

1. Uncertainty in the value, since it is determined using redshift and distance to the stars.
2. The assumption that the radial velocity of the galaxies has been constant

Question 20

What are the three fates of the universe

Answer 20

Open
Close
Flat

Question 21

Describe and open Universe, in terms of denisty

Answer 21

It is where the density of the universe is less than the critical density, so the universe will always expand

Question 22

Describe a closed universe in terms of density

Answer 22

It is where the density of the universe is greater than the critical density, so the universe will eventually stop expanding and collapse back in on itself.

Question 23

What is the equation used for critical density with Hubble’s law in

Answer 23

Critical Density = (3 x Hubble’s constant squared) over ( 8 times pi times the gravitational constant)

Question 24

What is the value of the gravitational constant

Answer 24

6.67x10^-11 Nm^2Kg^-2

Question 25

Critical Density

Answer 25

The average density of matter required for the universe to just halt its expansion, but only after an infinite time

Question 26

Red shift is … earth

Answer 26

away from

Question 27

Blue shift is … earth

Answer 27

towards