# Evan's Space

## 2016 Pure Physics P1 Q40 – Voltage and Power of Bulb Y when X is switched off

Solutions: Option C
[There is another similar question in 2013 Nov Pure Physics P1Q40. The answer is Option C too. Refer to the last section of this post]

Refer to the 2 videos below. The 1st one is faster if you know that the bigger the resistance, by proportion, the bigger the potential difference of the component as it will take a larger portion of the emf wrt to the other components in series. This method is especially useful for MCQ.

If you are still unsure, you may put in values to find the I, V and P across the components. This will be more tedious and time-consuming.

Another similar question is 2013 Nov Pure Physics P1 Q40.

Solutions: Option C

## resultant force-time graph link to speed-time graph

Refer to the video tutorial below for explanation.

## Identifying what lens, focal length and image from 2 rays – PP2010P1Q23 and SP2014P1Q11

These 2 questions are actually the same. Q23 is from 2010 Pure Physics P1 while Q11 is from 2014 Sci Physics P1. Take a look at these 2 questions. If you are not sure, view the video below for the explanation.

If you do not know how to answer these 2 questions, view this video and also refer to the lens summary below.

## Which is better to cool the food?

Other examples in our daily lives:

In some supermarket, the seafood are placed outside of air-conditioned place. The seafood is kept cold by putting crushed ice covering the seafood to keep the them cold and fresh.

Refer to this Sci Physics question N2008P2Q6(b)

Solutions:
For the solid that does not melt, when thermal energy is absorbed from the surrounding  food, its temperature starts to rise. So it is not so effective at keeping the food cool.
For ice-pack, when thermal energy is absorbed from the surrounding food, it starts to melt. During melting process, a much larger quantity of thermal energy is absorbed from the food to melt per unit mass of ice, the temperature remains constant at 1oC, and the melting process is long. Hence ice-pack is more effective at keeping the food cool.

Density of ice – Why ice floats on water?

Will whole lake be frozen during winter?

## Time interval between two sounds from a gun – direct and reflected sound

Solutions: Option B

You can view the video tutorial here or the written solutions below.

## 2013SPp1q17 What is the new P when V across resistor is doubled?

Solutions: C

Alternatively, you may put values to work out using the 2 basic formulae of P = IV and V = IR. Refer the video below.

## What is the speed-time graph of the object when pulling force is removed?

A constant horizontal force F, of magnitude 16 N, is applied to an object at rest on a rough surface. The constant force of friction is 8 N. At X, the force F is removed.

Which speed-time graphs correctly shows the motion of the object?

Solutions: Option B

Either B or D is correctly. But you need to find the magnitude of the deceleration after X in order to determine if the magnitude (gradient) is greater or smaller than the magnitude of the acceleration before X.

## Ratio of masses of same substance when heated

Two blocks X and Y, which are made of the same metal, are heated by heaters of the same power rating. The variation of temperature with time of both blocks is shown on the same graph below.

What is the ratio of the mass of X to that of Y?

A   1 : 3          B   1 : 4         C   3 : 1        D   4 : 1

Option: B