1) Newton’s first law states that an object will remain at rest or in uniform motion (constant speed) in a straight line unless an external force acts on the body. In other words, when a body is at rest or moving at constant speed in a straight line (constant velocity) , straight away you should know it is Newton’s first law . Next you must know these 3 basics concepts about 1st law: – forces acting on the body are balanced – net force / resultant force acting on the body is zero – there is no acceleration. 2) Newton’s second law states that when a net force (resultant force) acts on a body , it will cause an acceleration on the body (accelerating or decelerating). Basically F = ma where F is the net or resultant force in N,m is the mass in kga is the acceleration in ms-2 In other words, when a body is moving faster or slower (or going round a bend), you should know its Newton’s second law . Next you must know these 3 basic concepts about 2nd law: – forces acting on the body are not balanced – there is a net force / resultant force acting on the body – there is an acceleration (accelerating of net force is in the direction of motion, or decelerating if the net force is opposite to the direction of motion)

Instead of the usual finding of potential difference across a component, in this question, you are required to find the p.d. across two points on the two separate branches. If you can relate the river system to the DC circuit, it will help in solving the last part. View the video tutorial

Do you understand why there is a potential difference between P and Q? What if you are to find the potential difference between P and Q? There is a shortcut to getting the answer for this MCQ, but watch the whole video to understand the concepts fully and find the value of the potential difference. Answer: Option D Video the explanation Another question with similar concept: 2012PPp2q07 https://www.sgphysicstuition.com/post/2012ppp2q07-find-the-p-d-across-a-and-c

Prefixes are used to denote very large or small physical quantities . It is important that you know all these prefixes well.

Light and sound are both waves. So both carry energy from one place to another. Light , which is part of the electromagnetic spectrum, is a transverse wave , It can travel through a vacuum at speed 3.0 x 108 m/s. As the light travels from an optically less dense medium (air) to an optically denser medium (liquid or glass) , the light undergoes refraction and bends towards the normal due to a decrease in speed . Light: Optically less dense medium to denser medium:  – speed decreases – wavelength shorter – frequency remains constant Sound is a longitudinal wave . It requires a medium to pass through and it cannot pass through a vacuum . Opposite to light, as the sound travels from a less dense medium (air) into a denser medium (water or solid) , the speed increases . Sound: Less dense medium to denser medium: – speed increases – wavelength longer – frequency remains constant Refers to the image below to understand how the waves behave in different mediums.

To do similar questions, it is important to know the 3 rays which have rules to follow in order to locate the image. Look through the video to learn the concepts and hopefully you can do similar questions in future.

The 3 states of matter –  solid, liquid and gas. In general, when a body is heated, it expands and volume increases. The mass remains the same. Since density = mass/volume, its density decreases (less dense). For instance, warm air rises as it is less dense. In terms of kinetic theory, the particles will increase in kinetic energy. The average spacing between the particles increases (assuming not in a closed container). Likewise, when a body is cooled, the opposite occurs. The body contracts and volume decreases. It becomes denser. Due to the differences in particles arrangement of solid, liquid and gas, each expands by different amount when heated and vice versa. Which expands the most when heated and contracts the most when cooled? The following demonstration of the ‘Pee Boy’ is a good video to show the concepts. Explanation: The tiny hole at the penis is too small for any water to enter on its own. So using thermal transfer in the different states, the following steps are taken: Put the hollow empty boy into the hot water. [air inside the boy expands more than the solid ceramic, hence bubbles are seen coming out of the hole] Put the hollow empty boy now into the cold water. [The air inside contracts and volume decreases. This creates a low pressure and water is then sucked into the boy through the tiny hole] Place the boy on a platform. [The boy is only partially filled with water. The head portion is filled air while the bottom portion is filled with water] Pour hot water over the head. [As the whole boy is heated by the running hot water, the air in the head portion expands much more than the water at the bottom and the solid ceramic of the boy. Hence the air pressure increases and it pushes the water out of the boy] And he pees!!! Quite powerful indeed!

In the three states of matter, in general, solid is the best conductor as the particles are closely packed in an orderly manner, hence thermal energy can be passed down by the collision of the particles in the solid faster. On the other hand, liquid and gas are relatively considered poor conductor . Gas is the worst conductor as the particles are far apart. The following experiment demonstrates that water is indeed a poor conductor of thermal energy. An ice cube is kept at the bottom of the boiling tube by the net. The water at the top of the boiling tube is heated and started to boil. But the ice is not fully melted. This shows that thermal energy transferred through the water from heated water at the top to the ice is weak, hence indicating that water is a poor conductor . How about thermal transfer through convection current? In this experiment, the heated water at the top expands, the volume of the heated water increases, becomes less dense and remains at the top. The cooler water, which is denser, remains at the bottom. Hence there is no convection current formed throughout the entire water in the tube . So thermal energy transfer to the ice cube through convection is not present here.

Convection is a process in which thermal energy is transferred within a fluid (liquid or gas) due to the difference in density which creates a current. The video below shows the convection current in a heated tube filled with water. Colour dye is added to enable us to see the convection current through our naked eyes.

adiometer is a device for measuring the amount of infrared radiation. It consists of freely pivoted rotor with four vanes perpendicular to one another. The vane surface is painted alternate black and silver. The rotor is enclosed in a partial vacuum glass bulb. In general, when exposed to infrared radiation , the rotor will spin . The greater the amount of radiation, the faster the rotor spins . The actual working principle is actually much more complicated. Refer to the videos below for detailed explanation. For our context in O-level, we can briefly explained based on what we learned. As the vanes of the rotor are exposed to infrared radiation, the black side of the vane absorbs more radiation as it is a good absorber of radiation and hence its at higher temperature. The silver side reflects the radiation. The air molecules at the black side will get heated up and gain more kinetic energy . Hence rate of collision is higher and the air molecules collide on the black side with more force than the silver side. This results in a net force on the black surface and the rotor spins in a specific direction as shown in the video. The more complicated theory how radiometer works

he paper over metal or over plastic/wood will get burned faster? Everyone knows that metal is a good conductor of thermal energy. But for experiments like this, many would have guessed it wrongly. A paper is wrapped over metal and insulator (plastic, wood etc) and is exposed to the flame, the paper over the insulator becomes charred faster and burned faster . The paper over the metal takes a longer time to be charred and burned. This is because metal is a good conductor of thermal energy. When the spot (paper over metal) is exposed to the flame, the metal conducts the thermal energy away from that spot to other parts of the metal . So the temperature increase at that spot is slower , hence the ignition temperature of the paper (approx. 230oC) will be reached much slower , compared to the spot where the paper is over an insulator.