# Evan's Space

## Wonders of Physics ## Merry Christmas 2019 ## SISTIC 2019 on 20th Nov 2019

Thankful to have a chance to share my work at SISTIC 2019 and learn from others.     ## Unsual Behaviour of Less Dense Balloon in Air or Water Of course if a heavy ball is suspended from the car we all know the ball will move in opposite direction of a decelerating car due to inertia. We are all familiar to this where inertia is applied to a body which is denser than the surrounding medium (air or liquid) which is less dense. That’s why this balloon’s behaviour surprises us!

Let’s assume the water molecules are initially moving at constant speed with the tank before the deceleration.  Due to inertia, when the tank decelerates, the water molecules continue its state of motion forward. Hence the water molecules gush to the right side of the tank, displacing (pushing) the balloon to the left. Hence the balloon moves to the left!

Refer to this Youtube video by Smarter Everyday and you can see the similar experiment of helium balloon (less denser) in the air (denser) of a car.

## Force required to pull the block up the inclined plane

There are 3 scenarios with slight variations. Calculate the force F needed to pull the block up the inclined plane.

View the video below to understand how to solve these types of question.

You can also view the solutions below. Click here to view another post related to this concept: work done to bring an object up

## Using Slinky Coil to demonstrate Transverse and Longitudinal Waves

Though slinky coil is commonly used to demonstrate transverse and longitudinal waves, you must not quote it as an example for either of the waves.

• Transverse waves are waves in which the direction of the wave is perpendicular to the direction of the vibration of the particles. Examples are light wave, water wave or all the waves in the electromagnetic spectrum (which light is one of the waves.
• Longitudinal waves are waves in which the direction of the wave is parallel to the direction of the vibration of the particles.  Example is sound wave.

## Converging Lens Overview

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