HKDSE · DSE Physics
DSE Physics: Astronomy and Space Science (Elective) — Practice Questions & Answers
Observing stars and the night sky, astrophysical measurements and telescopes, orbital motion under gravity, and stellar evolution and cosmology.
336 practice questions available for this topic — here are 10 with full answers and explanations.
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Practice questions with answers
1
Multiple choice · Medium
An astronaut floats freely inside a space station orbiting the Earth. What is the best explanation for this apparent weightlessness?
- The station and astronaut are both in free fall towards the Earth with the same acceleration
- The Earth's gravity is negligibly weak at that altitude
- The Moon's gravity cancels the Earth's gravity
- There is no gravity in the vacuum of space
Tap an answer to check it.
WhyBoth the station and astronaut are in free fall with the same acceleration, so there is no contact force between them.
2
Multiple choice · Easy
Compared with standing on the ground, which quantity is essentially unchanged for an astronaut in a low Earth orbit?
- Her mass
- Her weight
- The gravitational field strength acting on her
- The normal force from the floor
Tap an answer to check it.
WhyMass is invariant; weight, g and the normal force all change (the normal force falls to zero).
3
Multiple choice · Hard
A spring balance holds a mass inside a spacecraft in a free circular orbit. The reading of the balance is
- zero
- equal to mg at that height
- greater than mg because of the orbital speed
- equal to the mass's weight on the Earth's surface
Tap an answer to check it.
WhyIn free fall the supporting force vanishes, so the balance reads zero.
4
Fill in the blank · Easy
Objects orbiting the Earth appear weightless because they are in continuous together with their spacecraft.
Answer:
free fall / freefall
WhyFree fall means gravity is the only force acting.
5
Fill in the blank · Medium
Apparent weightlessness does not mean gravity is absent; the true weight of an orbiting astronaut is still provided by the Earth's force.
Answer:
gravitational / gravity
WhyGravity supplies the centripetal force for the orbit.
6
Fill in the blank · Hard
For a body in free fall the force from any supporting surface is zero, which is why a passenger feels weightless.
Answer:
normal / support / reaction / contact
WhyThe contact/normal force falls to zero in free fall.
7
Multiple choice · Medium
Two stars have the same radius, but star P has twice the surface temperature of star Q. The luminosity of P compared with Q is
- 16 times
- 2 times
- 4 times
- 8 times
Tap an answer to check it.
WhyLuminosity is proportional to T⁴, so 2⁴ = 16.
8
Multiple choice · Hard
A star has 4 times the Sun's radius and twice the Sun's surface temperature. Its luminosity, in units of the Sun's luminosity, is
Tap an answer to check it.
WhyL is proportional to R²T⁴ = 4²×2⁴ = 16×16 = 256.
9
Multiple choice · Easy
The luminosity of a star of radius R and surface temperature T is given by L =
- 4πR²σT⁴
- 4πR²σT
- 4πRσT⁴
- πR²σT²
Tap an answer to check it.
WhyStefan-Boltzmann law: L = 4πR²σT⁴.
10
Fill in the blank · Medium
According to the Stefan-Boltzmann law, the luminosity of a star is proportional to the fourth power of its surface .
Answer:
temperature
WhyL is proportional to T⁴.
Key terms in Astronomy and Space Science (Elective)
Star: A massive ball of gas that emits light and heat produced by nuclear fusion in its core.
Galaxy: A vast system of stars, gas and dust held together by gravity, such as the Milky Way.
Light year: The distance light travels in one year in vacuum, about 9.5 x10^15 metres.
Astronomical unit: The average distance between the Earth and the Sun, used to measure distances in the solar system.
Parallax: The apparent shift of a nearby star against distant stars as the Earth orbits the Sun, used to find distance.
Luminosity: The total power radiated by a star in all directions, measured in watts.
Apparent magnitude: A measure of how bright a star appears from Earth, with smaller numbers meaning brighter objects.
Absolute magnitude: A measure of the true brightness of a star as it would appear at a standard distance.
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