Space Physics

Question 1

Give two reasons why scientists use models

Answer 1

to demonstrate phenomena that are too big/ too small to see
(or difficult to see)
to make predictions or explanations of observations

Question 2

A student is modelling red shift. She draws a wave on a piece of elastic. Suggest how she can use the elastic to show how light is red-shifted. Explain what the model shows

Answer 2

pull the elastic to make it longer

it shows that as space expands, the wavelength of the radiation increases

Question 3

give two similarities between an artificial and a natural satellite

Answer 3

they are both in orbit around a planet

due to the force of gravity

Question 4

describe what the big bang theory states about the how the universe began

Answer 4

(the universe began as) a massive explosion

from a very small, hot, dense region of space

Question 5

suggest how elements that were formed in a supernovae ended up on Earth

Answer 5

supernovae exploded
scattering elements throughout our universe

(gravity pulls them together to make stars and planets)

Question 6

suggest why it is more difficult to fuse elements as they get more massive

Answer 6

fusion involves the joining of nuclei
which are positively charged so repel
larger nuclei mean more charge, and a bigger repulsive force

Question 7

what do all stars start off as

Answer 7

huge clouds of dust and gas called a nebula

Question 8

what force is responsible for forming a protostar from a nebula

Answer 8

gravity

Question 9

How does a main sequence star remain stable

Answer 9

(fusion reactions produce outward forces )

radiation pressures outwards balance the inward gravitational forces

Question 10

What determines the lifecycle of a star

Answer 10

mass

Question 11

how are naturally occuring elements formed

Answer 11

Heavy elements can be formed from light ones by nuclear fusion reactions; these are nuclear reactions in which atomic nuclei merge together.

from NUCLEAR FUSION

Question 12

which elements are only produced in a supernovae

Answer 12

elements heavier than iron

Question 13

how are elements distributed throughout the universe

Answer 13

massive stars exploding in a supernovae

Question 14

how does the force of gravity make the objects in orbit change their velocity but not their speed

Answer 14

gravity provides a centripetal force, which keeps orbiting objects moving in a circle - they are constantly changing direction

Question 15

to change the speed of an object in stable orbit what factor must change

Answer 15

radius of the orbit

Question 16

what is red-shift?

Answer 16

WAVELENGTHS of light get LONGER if the light source is moving away from the observer

Question 17

what evidence suggests that the universe is expanding

Answer 17

RED SHIFT

light from more distant galaxies is MORE red-shifted, so these more distant galaxies are moving FURTHER AWAY

Question 18

what is the name of the scientific theory for the origin of the universe that suggests it started off as an extremely small, hot and dense region?

Answer 18

the Big Bang theory

Question 19

0 1 . 1 Describe the way in which the absorption lines which we observe in the
light we receive from a distant galaxy differ from those of the Sun.
0 1 . 2 Explain the reason for this difference.

Answer 19

01.1 The absorption lines in the light we receive from a distant galaxy are redshifted / longer in wavelength [1].

01.2 The distant galaxy is moving away from us [1]. This causes the light we
receive from it to be red-shifted.

Question 20

0 1 . 4 Edwin Hubble was the first person to investigate how the red-shift of a
galaxy depended on its distance from us.
Describe the trend which is shown in the above graph.

1. 5 Explain the significance of this trend in relation to the development of the
   Universe.

Answer 20

As the distance to a galaxy increases, the amount of red-shift in the light
we receive from it increases / red-shift is proportional to distance [1].

01.5 This tells us that galaxies which are further from us are moving away
from us more quickly [1], which means that the Universe is expanding
and that there must have been a point at which this expansion began
(which we call the Big Bang) [1].

Question 21

A teacher uses a starry balloon to explain the expansion of the Universe.
She tells the class that the motion of the stars printed on the surface of the balloon as it is inflated as like the motion of the stars in the Universe as it expands.
Write down one strength and one weakness of this model for the expansion of the Universe.

Answer 21

Strength As time increases, the stars on the surface of the balloon move away from one another (just as stars in the Universe are moving away from one another).
Weakness Any ONE of:
- balloon move away from one another will decrease with time (we believe that the rate at which our Universe is expanding may be increasing with time due to dark energy).
- the balloon can only expand so much (but the Universe may be infinite);

Question 22

0 1 A communications satellite is in orbit around the Earth, as shown below.

0 1 . 2 Write down the origin of this centripetal force.

Answer 22

01.2 Gravity (the gravitational force exerted on the satellite by the Earth) [1].

Question 23

0 1 . 4 Although the speed of the satellite in its current orbit (orbit 1) is constant,
its velocity is constantly changing. Explain why.

Answer 23

Velocity is a vector: the direction of the satellite is constantly changing as
it follows its circular orbit around the Earth [1].

Question 24

When the speed of the satellite decreases, the gravitational / centripetal
force acting on it will cause its height above the surface / orbital radius
to change as shown [1].
When it has reached orbit 2, the gravitational / centripetal force acting on
it will be just strong enough to keep it moving in a circle at this height
above the surface / at this orbital radius [1].

Answer 24

Centri-what-now?! This is a fairly tricky concept. Think of it this way: if we tried to
(somehow) pull the satellite closer to Earth when it was in orbit 2, then let go of it again,
it would naturally drift back into orbit 2 because it would be travelling too quickly for a
lower orbit. It would stop drifting away from the Earth (and therefore settle into orbit 2)
at the point at which the centripetal force acting on it (due to gravity) was just strong
enough to maintain its circular motion about the Earth.
Think about what happens when a car drives round a bend in the road too quickly: if the
friction between the tyres and the road isn’t strong enough, then the car will drift
towards the outside of the bend (to a larger radial distance from the centre of curvature
of the bend) until the frictional force (acting towards the centre of curvature) is just
strong enough to maintain the circular motion of the car.
You could also approach this question the other way round: if the speed of the satellite
was increased then it would be moving too quickly to remain in orbit 1 – in other words,
the centripetal (gravitational) force acting on it in orbit 1 would no longer be strong
enough to maintain its circular motion. It would then drift outwards, settling into orbit 3.
In this orbit, the gravitational force exerted on the satellite by the Earth is lower (because
it is further from the Earth), but it is just strong enough to keep an object moving in the
circular path of this orbit.
Don’t feel too bad if this all seems very confusing at first – this is a topic which many Alevel Physics students find confusingly – but do at least try to ensure that you
understand the above answer for your exam.

Question 25

0 2 . 1 Explain how the observation of the orbiting of four moons around Jupiter led people to believe that the geocentric model was incorrect, and that the Sun was in the centre of the Solar System.

Answer 25

Because these moons were in orbit around Jupiter, it meant that something could be in orbit around another planet [1].
Therefore, the Earth could also have been in orbit about another object in the Solar System (such as the Sun) [1].

Question 26

Ceres, Eris, Pluto, Haumea and Make are all classified as being dwarf planets. What is a dwarf planet?

Answer 26

• a planet-like object
• large enough for gravity of have made it (roughly) spherical in shape
• an object which has not yet ‘cleared its orbit’ of debris.

Question 27

Describe and explain how the speed at which a given comet is moving
changes between points A and B is its orbit (as shown above).

Answer 27

A comet will travel more quickly at A than at B [1]. As it approaches the Sun, it will be accelerated due to the gravitational force acting on it [1] (or as it travels away from the Sun, it will be decelerated due to the
gravitational force acting on it).

Question 28

0 1 . 3 Explain how a star is formed from a nebula.

Answer 28

Gravity causes a nebula collapses in on itself / a nebula collapses under
its self-gravity / every particle in the nebula will be attracted towards its
centre of gravity [1].
This leads to the release of large amounts of thermal energy / causes the
temperature of the nebula to increase / causes gravitational potential
energy to be converted to thermal energy / cause work to be done on the
gas by the gravitational force [1].
When the temperature of this collapsing nebula / protostar becomes high
enough, (nuclear) fusion processes release huge amounts of (nuclear)
energy / hydrogen nuclei will fuse to form helium [1]. A star is born!

Question 29

Stars such as our Sun are stable.

Explain the cause of the stability of such stars.

Answer 29

Inside a main sequence star, the gravitational
forces which are acting inwards are balanced [1] by the forces due to radiation pressure acting outwards [1].
The resultant force is therefore zero, and so the star is stable.

Question 30

0 2 . 1 Explain why a large-mass red super giant undergoes supernova.

Answer 30

Fusion processes within a red super giant slow down / stop / the red
super giant runs out of (fusion) fuel [1].
As a result, the outwards forces due to (gas and radiation) pressure
within the star no longer balance the inwards forces due to gravity, and
the star begins to collapse [1].
As it does so, its temperature increases massively / work is done on the
collapsing star by gravity / gravitational potential energy is converted to
thermal (heat) energy [1].
This triggers a massive explosion called a supernova.

Question 31

It is thought that all of the nuclei on Earth (and in the entire Universe)
which are heavier than iron were formed in past supernovae. Explain why
heavy elements are only thought to be formed in supernovae.

Answer 31

Because the very high temperatures (and densities) required to fuse
together (certain lighter) nuclei to form heavier nuclei are only thought to
be found inside exploding high-mass stars / during supernovae [1].
Accept: very high temperature required for fusion.

Question 32

Describe the possible final stages in the life cycle of a large-mass star
after it has undergone supernova.

Answer 32

After it has exploded / undergone supernova, a high-mass star may turn
into a neutron star [1].
If the (original) mass of the star is very high, after it has undergone
supernova, it may turn into a black hole [1].