Q1 (c) (ii)
According to Doppler effect, red-shift suggests that the distant galaxies are moving away from us. Galaxies that are further away from us show a bigger red-shift. This means they are moving away faster, suggesting that the universe is expanding.
Q3 (c)
Since Force = Change in momentum/Time taken, another factor that affects the size of the force on the car is the change in momentum of the car. The higher the change in momentum of the car the greater the force on the car.
Q4(b)(i)
From the tumour, when an isotope emits a positron, the positron would meet an electron very close to the location from which it is emitted. When they meet, the positron and electron annihilate each other, emitting two gamma rays in opposite directions. These gamma rays are detected by two opposite detectors. The time difference between these detections is used to determine the location of the tumour.
Q4(b)(ii)
Typical half-life for radioactive tracers is about 6 hours. If the isotope is produced at a far away location and brought into the hospital, by the time it is injected into the patient, its activity would have already reduced to a very low level, making it difficult to make observations.
Q5(b)
Some of the electrons in the lower energy levels in the atoms absorb the energy of UV rays. As a result these electrons move to higher energy levels. These electrons then fall back to different low energy levels, emitting their energy as visible light.