# HSC Physics Space

/With the old Space topic being examined for the last time this November a list of **true-false** **questions** on concepts that are often misunderstood is given below. This list will be updated during the year.

**Gravity**

- A satellite has a greater gravitational potential energy than a grain of dust in the same circular orbit about the Earth.
- Gravitational potential energy is the energy needed to bring masses together from a state where they are not influencing each other.
- A gravitational sling-shot of a spacecraft by Jupiter causes the spacecraft to leave Jupiter at a greater speed. [false, spacecraft can increase speed relative to the Sun by "taking" kinetic energy from Jupiter in an interaction in which the spacecraft has no change in kinetic energy in Jupiter's reference frame but an increase in KE in the Sun's reference frame due to Jupiter slowing down and giving KE to the spacecraft. The final spacecraft speed can be increased by almost twice the speed of Jupiter if the spacecraft moves directly towards the approaching planet whose gravitational field sweeps it around increasing its speed relative to the Sun]
- Two large equal masses M are placed a distance r apart. A smaller mass m is placed at the midpoint of the line joining the larger masses. The gravitational potential energy of this system is zero.
- Two large masses M are placed a distance r apart. The work done in moving a mass m from a very large distance to the midpoint of the line joining the larger masses depends on the path taken.
- Gravitational potential energy increases as a mass moves closer to the Earth since the gravitational force increases.
- A mass is tied to a string of length 120 cm and moves freely in a vertical circle in the Earth's gravitational field. The speed of the mass at its lowest point is 8.0 ms
^{-1}. The magnitude of the acceleration of the mass at this instant is 53.3 ms^{-2}.[false,54.2 ms^{-2}]

**Projectiles**

- A projectile moving upwards has a negative acceleration and when it moves downwards its acceleration is positive.
- A cannon is fired horizontally from a tall mountain. The projectile can strike the Earth on the hemisphere opposite to the direction of firing if its initial speed is sufficient.
- An object is dropped from the Eiffel Tower. Neglecting air resistance the object hits the ground at a point to the east and south of its starting point. An object dropped from Centre Point Tower (neglecting air resistance) will be deflected to the west and north of its starting point.
- A ball is thrown at initial speed U on horizontal ground. The maximum range of the ball is R. If the new initial speed is 2U the maximum range (neglecting air resistance) is 2R.

**Orbits**

- The two forces acting on a satellite moving in a circular path around a planet at a constant speed with no air drag are the gravitational force and the centripetal force.
- When atmospheric drag acts on a satellite its speed decreases.
- Apollo 13 could re-enter the Earth's atmosphere at an angle 𝜽 to the vertical where 5.3° < 𝜽 < 7.7°.
- A spacecraft that bounces off the atmosphere enters an orbit around the Sun.
- A satellite is in a low Earth circular orbit. The radius of the orbit decreases. The gain in orbital kinetic energy of the satellite is equal to the loss in gravitational potential energy of the satellite.
- A satellite in a high Earth circular orbit has a total energy E. If the satellite is placed in a circular orbit of twice the radius its total energy is 2E.
- A satellite in a circular orbit of period 23h 56m 4s that passes over Sydney always appears directly overhead.

**Relativity**

- An electron cannot move through water at a speed greater than the speed of light in water.
- A rocket moves at a constant velocity of 0.90c relative to the Earth. The crew of the rocket play a CD that lasts for 1 hour. A person on the Earth plays an identical CD that lasts for a longer time interval than one hour.
- A particle accelerator is 2.0 km long. A proton starts from rest and moves through the accelerator striking the end at 0.8c. The distance travelled by the proton in its own reference frame is 1.2km.
- A certain star is 66.5 light-years away from the Earth. A spacecraft leaves the Earth and travels at a constant speed of 0.95c to the star. The time taken to travel to the star according to a clock on the spacecraft is 70 years.
- A train is moving to the east. A ball is rolled across the smooth floor of the train initially perpendicular to the south side of the train at 1 ms
^{-1}.A person in the train sees the ball move in a parabolic arc towards the east of focal length 1 m. The acceleration of the train is 0.5 ms^{-2}to the east. - A white hot metal rod is cooled to room temperature.Its mass does not change.
- In Michelson and Morley's interferometer the light rays interfere destructively. This is called a null result.
- In the aether theory the time taken by light to travel along each of the equal arms of the interferometer is the same.
- Interference fringes are not caused by the reflections from the half silvered mirror in the Michelson-Morley experiment.
The result of the Michelson-Morley experiment is that the time taken by light to travel a given path depends on the direction of the light ray.

Simultaneous events are seen to happen at the same time instant in the same reference frame.

- Michelson and Morley measured a change in fringe
*spacing*when they rotated their apparatus through 90° - Michelson and Morley expected to measure a change in fringe
*spacing*when they rotated their apparatus through 90°