Think of gradient as the steepness of a slope as you climb to the right (as time increases). 1) If it is getting steeper > increasing gradient 2) If it is getting less steep > decrease gradient 3) If it is flat > no/zero gradient For distance-time graph , gradient represents the speed . For speed-time graph , gradient represents the acceleration .

Scalar quantity is a quantity with magnitude but no direction. Vector quantity is a quantity with both magnitude and direction .

Transverse waves are waves in which travel perpendicular to the direction of vibration of particles.

In a vacuum room, where is no air resistance, the bowling ball will have the same acceleration as the feather, which is 10 m/s2 on earth. Hence both will have the same speed at any time and reach the floor at the same time. In vacuum, the speed-time graph for both objects will be the same, i.e. straight line, start from origin and with a constant gradient (constant acceleration) of 10 ms-2 due to free-fall acceleration due to gravity.

Physical Quantity is a quantity that can be measured and it consists of a numerical magnitude (size) and a unit . (e.g. 5m where 5 is the numerical magnitude and m is the unit) 1) The seven base quantities and their SI units Out of the seven, for O level Physics, you are required to learn the first 5 (length, mass, time, current and temperature) . 2) Some common derived quantities and units You can think of base quantities as the smallest building blocks (like Lego bricks). When you combined any base quantities to ‘build’ into something else, they becomes a derived quantity.The derived quantity is ‘derived’ or ‘gotten from’ when 2 or more quantities are combined together. 3) Prefixes for SI units Prefixes are used to represent very large or small quantities.For example, instead of writing 1000000 m, it will be more convenient to write 1Mm, which means the same thing!

Prefixes are used to simplify the writing of very big or very small numbers. For instance, instead of having to write 12 500 000 m every time, you can simplify it by using the prefix mega (M), hence it can also be written as 12.5 Mm. Similarly, 0.00054 s can be written as 0.54 ms. If you are unsure or confused with the unit conversions, you can refer to the following videos. 1) Prefixes can be used for different physical quantities units. 2) Unit conversion with examples 3) Unit conversion for units of area and volume 4) Unit conversion for units of speed and density

Answer: Option C Focal length f is the distance between the focal point and the centre of the lens (optical centre) . Note that only when parallel rays of light enter a converging lens, the rays will converge to a point. That point is considered to be focal point F (principal focus). The distance between focal point F and the optical centre is the focal length f. Refer to below. As none of the options is similar to the above definition, you have to consider that the rays from a distant (far away) object are considered parallel . Hence the sharp image formed on the screen is considered the forcal point F of the lens and the distance between the image and the optical centre is the focal length f. If it is a close object, the rays entering are not considered parallel. Hence even if the rays converged to a point, that point is NOT focal point F and the distance between this converged point and the optical centre is NOT focal length f.

When gas in piston and heated but pressure remains constant. It is easier to explain in terms of the average force acting on a unit area. 2008PPp1q15 Answer: B 2017PPp1q15 Answer: A In short:  Temperature increases , Kinetic Energy increases , Rate of Collision decreases , Collision force on wall increases (due to higher speed), Average force per unit area remains constant , Pressure constant . As piston is free to move, it will move to the right such that the pressure remains constant (equal to atmospheric pressure outside). As the piston moves to the right, the volume inside the piston increases .  Surface area of piston in which the air molecules collide increases . The rate of collision decreases as the number of molecules remains constant. With higher KE of molecules, the molecules will collide the wall with greater force . Though rate of collision decreases, with each collision having greater impact force, this makes average force per unit area acting on the wall of piston remains the same. Hence pressure remains constant. Misconception : Many think that the rate of collision remains the same, which is wrong . Considering per unit area, if rate of collision remains the same, with the speed of the molecules increases, there will be greater collision force with the wall. This will result in the average force per unit area being higher, hence the pressure would increase.  If the rate of collision increased , with the speed of the molecules increases, there will be greater collision force with the wall. This will result in the average force per unit area being much higher, hence the pressure would increase even more.