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.

When a system is in equilibrium , 2 conditions are met. 1) Principle of Moment (POM): Sum of anticlockwise moment = Sum of clockwise moment 2) Net force = 0N; all forces are balanced: Total downward forces = Total upward forces Whenever a force passed through the pivot point, that force will not create any moment as there is no perpendicular distance from the force to the pivot. Hence this force will not be included in the calculation of moments.

Solutions: Option B You can view the video tutorial here or the written solutions below.

A short-cut is to think about a couple who are in love or quarrel. When in love, both are walking together in one direction holding hand (i.e. current in both wires are in same direction ). Hence there is ATTRACTION between them. When the couple quarrel, both are walking in opposite direction (i.e. c urrent in opposite direction ). Hence there is REPULSION between them.

The distance-time (displacement-time) graph on the left is not related to the speed-time (velocity-time) graph. Both are of difference scenarios. But both shows the motion of 2 objects and both graph has an interception point. Do the interceptions point mean the same time in each graph? The image below shows the basic interpretation of the respective graphs. To have examples of the graphs with values and the step-by-step how to find the time it overtakes, refer to the videos below. Finding the exact time where the overtaking takes place.

01 The various physical quantities in kinematic Video tutorial 02: Acceleration Video tutorial 03: Graphical Analysis of Motion (Displacement (distance)-Time Graph and Velocity (speed)-Time Graph) Video tutorial 04: Describing the motion of the car – practice Video tutorial

Simply put, electric appliance can be very simple with just a live wire bring in the current and a neutral wire to bring the current out. The appliance can just work like this forever (without the hassle of having earth wire, fuse etc), provided there is no electric fault developed. Both the earth wire and the fuse are safety features to protect the user and the appliance respectively . Take a look at the video below to understand the rationale. Recall these notes given: