Our Dynamic Universe (PPQs & Answers)

Question 1

While the package is being lowered the cable breaks.
The upward force produced by the drone remains constant.
Describe the vertical motion of the drone immediately after the
cable breaks.
Justify your answer.

Answer 1

Drone accelerates upwards (1) 
Upward force is greater than weight 
OR 
(Upward force remains constant but) 
weight decreases therefore forces are no 
longer balanced. 
OR 
(Upward force remains constant but) 
weight decreases therefore there is an 
unbalanced force (upwards). (1)

Question 2

Explain how the student would determine whether the collision was
elastic or inelastic.

Answer 2

Calculate the total kinetic energy before and (total
kinetic energy) after. (1)
If Ek before is equal to Ek after the collision, is elastic.
OR
If Ek before is greater than Ek after, the collision is
inelastic. (1)

Question 3

Hubble’s Law states that the universe is expanding. The expanding universe is
one piece of evidence that supports the Big Bang theory.
State one other piece of evidence that supports the Big Bang theory.

Answer 3

Cosmic Microwave Background Radiation 
OR 
Olber’s Paradox 
OR 
Abundance of Hydrogen and Helium in the 
Universe

Question 4

It has been discovered that the rate of expansion of the universe is
increasing.
State what physicists think is responsible for this increase.

Answer 4

Dark energy

Question 5

State what is meant by an acceleration of 0·32 ms-2.

Answer 5

The velocity increases by

0·32 ms-1 each/per second

Question 6

The train continues to sound its horn as it passes under the bridge.
Explain why the frequency of the sound heard by the person
standing on the bridge decreases as the train passes under the
bridge and then moves away.
You may wish to use a diagram.

Answer 6

Statement that there are fewer
wavefronts per second.
OR
The wavefronts are further apart
OR
The wavelength increases
OR
diagram showing wavefronts closer 
together ahead of the train and 
further apart behind it.
or any similar response

Question 7

State what is meant by an inelastic collision.

Answer 7

kinetic energy is lost/greater before the

collision than after.

Question 8

State the law of conservation of linear momentum.

Answer 8

Total momentum before (a collision)
is equal to the total momentum
after (a collision)
in the absence of external forces

Question 9

The students consider the plane travelling at 0·8c relative to a
stationary observer. The plane emits a beam of light towards the
observer.
State the speed of the emitted light as measured by the observer.
Justify your answer.

Answer 9

3 × 10*8 ms−1
or c (1)
Speed of light is the same for all 
observers / all (inertial) frames of 
reference or equivalent (1)

Question 10

One of the students states that the clocks on board the plane will
run slower when the plane is travelling at relativistic speeds.
Explain whether or not this statement is correct.

Answer 10

Correct – from the perspective of 
the stationary observer there will 
be time dilation
Incorrect – from the perspective of 
the students they are in the same 
frame of reference as the clock
Not possible to say/could be both 
correct and incorrect – frame of 
reference has not been defined

Question 11

One of the lines is due to hydrogen.
The position of this hydrogen line in the visible spectrum is shown for a
distant galaxy, a nearby galaxy and the Sun.
Explain why the position of the line is different in each of the
spectra.

Answer 11

light is redshifted/ shifted towards
red (1)
(as) the galaxies are moving away
(from the Sun) (1)

Question 12

Explain why a greater number of muons are detected on the surface of
the Earth than would be expected if relativistic effects were not taken
into account.

Answer 12

For an observer on Earth's frame of 
reference the mean life of the muon is 
much greater 
OR 
The distance in the muon frame of 
reference is shorter

Question 13

The diagram below represents part of the emission spectra for the
element hydrogen.
Spectrum P is from a laboratory source.
Spectrum Q shows the equivalent lines from a distant star as observed
on the Earth.
Explain why spectrum Q is redshifted.

Answer 13

The star is moving away from the Earth 
 (1) 
Plus any one point from the following 
for 1 mark: 
• The apparent wavelength of the 
hydrogen spectra from the star has 
increased 
• The apparent frequency of the 
hydrogen spectra from the star is 
less than the actual frequency on 
Earth 
• The frequency of the light from the 
star has shifted towards the red end 
of the spectrum 
• Light from the star is experiencing a 
Doppler shift.

Question 14

State what is meant by the resultant of a number of forces.

Answer 14

A single force which will have the same

effect as all the other forces.

Question 15

Explain why the driving force must be increased with time to maintain a
constant acceleration.

Answer 15

The faster it goes, the greater the air resistance. ½
or
frictional forces / friction / drag
then
F(drive) constant, the unbalanced force would decrease
or
increasing F(drive) keeps the unbalanced force constant
or
overall/net force - must have ½

Question 16

This seatbelt is designed to stretch slightly during the collision.
Explain, in terms of forces, an advantage of this design.

Answer 16

Time (of collision) increased ½
change in momentum is the same ½
(Average) force (acting on dummy/passenger) is
decreased/reduced/smaller (1)

Question 17

Explain what is meant by the term length contraction.

Answer 17

The decrease in length (in the direction of motion) of
an object moving relative to an observer.
N.B. it must be clear that the observer is in a different frame of reference.

Question 18

When the effects of air resistance are not ignored, the golf ball follows a
different path.
Is the ball more or less likely to hit the tree?
You must justify your answer.

Answer 18

More likely (1)
because: 
horizontal velocity will decrease
OR 
vertical velocity will decrease (½)
quicker than before

range will decrease
OR time in air will decrease
OR height reached will decrease

Question 19

Radiation of peak wavelength 1·06 mm can be detected on Earth coming from
all directions in space.
(i) What name is given to this radiation?
(ii) Give a reason why the existence of this radiation supports the Big Bang
Theory.

Answer 19

(i) Cosmic Microwave Background (Radiation) (1)
(ii) Look for any one of the following: (1)
 It is pervasive throughout space.
 It is the dominant source of
radiation in the Universe.
 It is very uniform (throughout the
Universe).
 It is isotropic (throughout the
Universe).
 It shows the characteristics of
blackbody radiation.
 It has a temperature of approx
3K (2·74K) due to cooling on
expansion.
 It corresponds to a redshift of
1000, so the early temperature of
this radiation was approx 3000K.
 CMBR is thought to be the
“afterglow” of the Big Bang,
cooled to a faint whisper in the
microwave region.

Question 20

The observation that galaxies are moving away from us is evidence for the
expanding universe. As the universe expands it cools down.
What property of the Cosmic Microwave Background has been measured by
astronomers to determine the present temperature of the universe?

Answer 20

Wavelength (or frequency)