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Wonders of Physics


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Understanding Newton’s 1st and 2nd Laws of Motion

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 kg
a 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)

Click here to know more about Newton’s 3rd Law


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Practical – Lens Experiment

The video below shows a typical lens experiment. (reference to O-Level SciPhy 2015). I will briefly go through the set-up, main steps and how to get the 1st set of readings.

In the next video, it highlights the various types of lens practical which you might have in the school lab. e.g. different kind of crossed-wire, a beaker of water as a converging lens and different kind of images formed.

Key points:
1) Make sure the object (illuminated crossed-wire), lens and screen are aligned properly.
2) Source of Error: identifying the sharpest image
Improvement: (i) Repeat the experiment a few times for the same
independent variable to identify the sharpest image.
(ii) Move the lens (or screen) forward and backward about
the sharp image, until the sharpest image is determined.
3) In general, the focal lens of the lens used in the lab is usually 10 cm or 15 cm. Most lens experiment requires you to find the focal length. There is also a easy way to quickly determine the focal length before starting the experiment.

Refer to this post for another lens experiment which is different and more challenging.


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Parachute Jump – Speed-time graph

In a typical parachute jump, there are various distinct stages/sections as you can see from the graph in the video below.

AB = constant acceleration, free fall, a = 10 ms-2
BC = decreasing acceleration
CD = constant speed, zero acceleration
at D = the time where the parachute is fully opened
DE = constant deceleration
EF = lower constant speed, zero acceleration
FG = constant deceleration

After you have learned Dynamics, you should be able to explain each stage using forces acting on the skydiver, namely the weight and air resistance.

Refer to the video below to understand the motion at various stages and how to explain in terms of forces, esp the part on why the acceleration is decreasing during BC.

You can refer to the detailed explanation in words in the comics below. Hope this post helps you to understand better!

velocity-time graph of parachute jump


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Ticker Tape Timer

Ticker tape timer is a device which punches dots on a tape at specific time interval. The tape can be attached to a moving object. With information of the frequency of the ticker timer and the spacing of the dots, we are able to analyse the motion of the moving object.

View the following videos to understand more about the ticker tape timer and also some typical questions on ticker tape timer.

Explanation of Ticker Tape Timer

Questions on ticker tape timer

Refer to this post for another example of finding acceleration

https://evantoh23.wordpress.com/2010/10/26/20101026ticker-tape-timer-finding-acceleration/


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Why the fuse and switch must be on the live wire?

A common mistake is to state that the reason for switch to be on the live wire is “so that the switch can turn on/off the appliance”. This explanation is wrong.

Refer to the view for the reason why switch and fuse must be on the live wire.

IMG-0582.JPG

In general, in the event where the fuse is blown or the switch is open, it disconnect the appliance from the high potential of the live wire. So the appliance will no become ‘live’ and it is save to touch even though there is a fault.

 


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Why do we need earth wire and fuse?

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:

Function of earth wire.JPG

 

Function of fuse.JPG

Click here for other related posts

double-insulation

3 pin-plug

 


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Units conversion involving prefixes

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.

prefixes

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


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Processes for Transfer of Thermal Energy

Three processes of thermal transfer: conduction, convection and radiation.

Conduction vs Convection:

Similarity:

  • both requires a medium for thermal transfer to take place.

Differences:

  • convection cannot take place in solid but conduction can.
  • conduction is due to vibration and collision of molecules while convection is due to fluid density changes.

Radiation vs Conduction & Convection

  • Radiation can take place in vacuum but both conduction and convection require a medium.


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Light and sound wave diagram in different mediums with different density

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.
Click here to revise on the calculation of refractive index for light

light and sound


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Converging Lens Overview

1. Converging lens (convex lens)
Converging lens, also known as convex lens,  is thicker at the centre. Below shows some examples.

types of converging lens

In O-level, we learned about symmetrical converging lens. i.e. the curvature of the lens are the same on both sides. As light rays pass through the converging lens, the rays come closer together.

refraction of light at the boundaries of the lens

Take note that the bending of light, refraction, takes place on the air-glass boundaries on both sides of the lens (as shown above). But for easy drawing, we draw the bending at the imaginary centre vertical which passes through the optical centre as shown below.

03_physics

2. The 3 Rays

The following 3 rays are important for us to construct the ray diagram and locate the image. We always draw these 3 rays as they have rules to follow, hence guiding us in our drawing.

the 3 raysRefer to the video below for better understanding of the 3 rays.

3. The 4 Key Scenarios
Depending on the distance of the object to the centre of the lens (object distance u), the kind of image you get varies.
IMG_1375

Refer to the video below for the better understanding of how the various images are formed.

3. The Pattern
Besides knowing the 4 key scenarios, it is important to know how the image behaves as the object is moved towards the lens.

In general, as the object (starting from a distance of >2f) moves closer to the lens, the image will move further away from the lens and the size of the image becomes bigger.

But when the object is within a focal length, as it moves closer to the lens, the virtual image moves closer to the lens and it becomes smaller compared to the image previously. But the virtual image is always bigger than the object.

Refer to the video for better visualisation and understanding.

 

4) Other posts on converting lens:

What is focal length and how to identify

Finding focal length f of the lens (using a mirror and pin practical experiment)

Different converging lens ray diagram questions (must know)

Different ways to have a sharp image formed on the screen

Which distance is the focal lens of the converging lens? Olevel question

 

 

 

 

 

 


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When to use the concept PV = constant and P1V1 = P2V2 to solve?

When temperature is constant (for o-level), when a fixed mass of gas (fixed number of air molecules) is compressed in a closed system (e.g. piston), the volume V decreases and pressure P increases, and vice versa.

But when you multiply pressure and volume, PV, it is always a constant.

PV = constant

Hence we can always equate the PV of the first scenario = to the PV of the second scenario, provided there is no addition or removal of air molecules from the system.

Hence, you have P1V1 = P2V2

The followings are 4 different questions which require this concept to solve. Do revise them.

Solutions: Option D (refer to the worked solutions below)

Solutions: Option D

Solutions: A

4)

Solutions:

 

5) img_0196

Solution: Option C

img_0194img_0195


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Different Lens Ray Diagram questions

Drawing ray diagrams for converging lens come in many forms. But the basic concepts needed are the 3 rays (which have rules to follow) to locate the image. The following ray diagrams for the 4 scenarios must be learned well, together with the respective image characteristics and applications.

IMG_1375

 

With the basic concepts learned, when questions are asked in different ways, you should be able to draw the ray diagrams. Refer to some different ray diagram questions below and their video tutorials.

A

b.jpg

C

D

E

F

G

H

I

J

Click the followings for other lens ray diagram questions. https://evantoh23.wordpress.com/2010/11/09/20101109converging-lens-important-concepts/

https://evantoh23.wordpress.com/2015/08/27/olevel-sp-p1-focal-length-of-lens-using-distant-object/


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Vector Diagram – Parallelogram Method for 4 different kinds of questions

For Combined Science Physics Olevel, when constructing vector diagram, a method called Parallelogram Method is commonly used.

Parallelogram Method is used when 2 forces are known, and the resultant (net) force R, can be found.

Below show 5 different variations of questions when can be solved by using Parallelogram Method:

(A) Body has acceleration, forces are unbalanced, there is a resultant force R (Newton’s 2nd law)
(B) Body at rest, all forces are balanced, there is no resultant force R (Newton’s 1st law)
(C) Body moving at constant velocity, all forces are balanced, there is no resultant force (Newton’s 1st law)
(D) Finding unknown force (with resultant force R  + another known force are given)
(F) Vector diagram is applicable to all vector quantities (with magnitude and direction). Not just for forces.

Click here to view video tutorials on how to construct parallelogram step-by-step

Vector Diagram Parallelogram 01Vector Diagram Parallelogram 02Vector Diagram Parallelogram 03Vector Diagram Parallelogram 04

For Pure Physics, another method is the closed-loop triangle, where only 1 known force with 2 other unknown force. Click here to view the video tutorials.


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Waves Summary

 

waves 01

Click here to view the simulations transverse and longitudinal waves

Students are confused when they need to visualise what is the direction of the particles the next moment of the wave. There is a simpler way solve such question.

To know what is the motion of the particles as a transverse wave passes through, click to view the question and the video tutorial.

waves 02

Click here to view of a question asked on rope wave – how should you move your hand to create different waves.

waves 03 graphs

Click here to view another example on graphs

waves 04 wavefronts

Click here to know more about wavefronts created in a ripple tank.