Answer: Option D Note that the reading shown on the detector (536 counts per second) includes the sum of count rate from the radioactivity material and also the background radiation of 44 counts per second. So to find the count rate purely from the radioactive material, you need to first minus 44 from the 536 = 492 counts per second. Given that the half-life is 34 hrs, so in the duration of 68 hrs, there are 68/34 = 2 half-lives. So 492 c/s after 1 half-life will becomes 492/2 =246 c/s Another half-life later, it will becomes 246/2 =123 c/s Note that 123 c/s is purely from the radioactive material, not the reading on the detector. Hence to find the reading on the detector, you need to add back the background count of 44 c/s = 123 + 44 = 167 c/s

Answer: Option B Using conservation of energy (COE), the total energy of the car at X = total energy at Y. At X, the car only has gravitational potential energy. If there is no friction, the magnitude of the kinetic energy must be equal to the GPE at X. But since there is frictional force, there will be some energy wasted as work done against friction, usually in the form of internal (thermal) energy and sound energy. Hence, GPE at X = KE at Y + WD against friction Refer to the video explanation

Answer: Option B We usually encounter decay graph of the unstable radiaoactive substance. But this graph is the mass-time graph of the stable isotope. So the mass increases as time goes by.

Answer: Option D We always use FLHR to find the force on the wire. But magnetic forces is a action-reaction pair. So when the force acting on the wire is upwards, there is also an equal and opposite force acting downward on the magnet.

Answer: Option B LDR (light dependent resistor) is a type of input transducer. When light intensity decreases, the resistance of LDR increases and vice versa. You can redraw the circuit so that it looks like the usual potential divider that you are familiar with. Refer to the video below.

Answer: Option C This is a heating curve. So the specific latent heat of fusion here refers to the melting process, which occurs from time 100s to 250s.

Answer: Option A The direction of the magnetic field is downwards in the diagram. You can think of a North Pole of a magnet that is at the top and/or a South Pole at the bottom. Since the top is N, and the pole of the bar magnet nearer is also N, as like poles repel, the N of bar magnet will move away and rotate clockwise. The S of pole of the bar magnet will be attracted to the N at the top, hence likewise it will cause the bar magnet to rotate clockwise. Alternatively you can image the bar magnet to be like a compass needle, and the needle will follow the direction of the magnetic field with N of the needle pointing in the same direction of the magnetic field direction.

Answer: Option D Refer to the video for the explanation

Answer: Option A For such questions, always locate accurately the position of the image of the O. Then draw the ray from the image to the observer. To determine the maximum scope of view using the mirror, always use the two extreme edges of the mirror as point of incidences.

Answer: Option D The most common mistake is to equate total energy at X equals to total energy at Y. At Y, there is no way to know the speed as the speed varies which depends on the speed at X. Since the question wants to find the minimum speed at X to reach Y, you should focus on the highest point of the hill. If you can just clear the hill, you will sure reach Y. So you should consider the total energy at the highest point of the hill and assume the ball just barely reaches the top and stops there. Hence at the highest point, the energy of the ball is just GPE and you can find the minimum speed at point X. Any speed higher, you can clear the hill and reach Y.

Answer: Option B Refer to the video explanation

Answer: Option B To solve this question, you need to apply your FLHR. From the image, you will be able to know the orientation of the force F (thumb) and the magnetic field M (first finger). For the radiation path that goes straight, it indicates that it is not affected by the magnetic field. That means the radiation has no charge, hence it is gamma rays (a type of electromagnetic wave) For the radiation path that curves upwards, that means there is a force acting to the top of the image. So orientate the FLHR according and the 2nd finger (convention current) points to the right. Note the the direction of the convention current is the path in which a positive charge will take, so the radiation must be positive charge, i.e helium nucleus (alpha particles).