Evan's Space

Wonders of Physics

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Fiery Re-entry into Earth’s Atmosphere (updated)

This post was updated following the first astronauts launched by SpaceX returned home safely on 3 Aug 2020.

SpaceX’s Crew Dragon heat shield shown off after first orbital-velocity reentry

How do spacecraft re-enter the Earth? | HowStuffWorks

Why Is It So Difficult For A Returning Spacecraft To Re-Enter Our Atmosphere?

Returning from Space: Re-entry – PDF format

SpaceX In-Flight Abort Test

SpaceX Falcon Heavy- Elon Musk’s Engineering Masterpiece

Shuttle Atlantis STS-132 – Amazing Shuttle Launch Experience

How to Land the Space Shuttle… from Space

First astronauts launched by SpaceX return to earth (3 Aug 2020)

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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.


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


Displacement-time and Velocity-time Graph of Ball Thrown Up / Ball Dropped

1) When a ball is thrown up and it comes down

When the ball leaves the hand, there is no upward force acting on the ball. The only force acting on the ball is its weight. This net force is opposite to the motion of the ball, hence causing the ball to decelerates. Refer to this post for the explanation (comics)


2) When the ball is dropped and it re-bounces back (assume no energy lost)

When the ball is released, the only force acting on the ball is its own weight. This net force on the ball is in the direction of the motion, hence causing the ball to accelerate as it falls. Note that the acceleration due to gravity is assumed to be 10 ms-2, where air resistance is negligible.

3) When the ball is dropped and it re-bounces back (in reality with energy lost)

In reality, when the ball hits the ground, there will be some energy converted to heat and sound. So the ball will never return to its original height that it was released. So how will the graphs look like?

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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


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Distance-time and Speed-time graph of 2 objects in motion

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.

motion of 2 objects on distance or speed time 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.

Understanding Free-Fall (acceleration due to gravity)

On earth, the gravitational field strength is 10 N/kg and the acceleration due to gravity is 10 ms-2.

That means when you release an object from you hand, the object will fall with increasing speed. The acceleration is a constant 10 ms-2 (acceleration due to gravity). Simply put, it means in 1 second, the speed of the object will increase by 10 m/s.

free fall

In general, if there is no air resistance or air resistance is negligible, the speed-time graph is a straight line with constant gradient and passes through origin, i.e. speed is directly proportional to the time.

If there is air resistance, there is a maximum constant speed if the object continues to fall. Hence the graph is different.

More importantly, remember that any object that you released on earth, whether there is air resistance or no air resistance, the initial acceleration when it is released is always a constant 10 ms-2!

Refer to the video for the explanation and some examples.


Kinematics Graphs of ball thrown up and come down

When a ball is thrown up vertically from a height and the falls to the ground, the various kinematic graphs below shows the same motion of the ball.

Fixing up as positive, if the ball is going up, it’s displacement and velocity will be in the positive region. When it falls, the direction is opposite, hence it’s in the negative portion.

Recall that the gradient of a velocity-time graph represents the acceleration. Using this concept, it will be easier to understand that the acceleration is a negative constant acceleration, thus – 10 ms-2.

(Thanks Maria for suggesting this post and the addition of acceleration-time graph)

Click here to refer to the post related to this motion.

Velocity-time and Displacement-time Graph for a ball being thrown up

Velocity-time and Displacement-time graph for Ball thrown up and comes down

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Double-Insulated Appliance – Air purifier in the classroom

Double-insulated appliances are common around us. The air purifiers in our classroom are one good example.

In these appliances, only two wires are needed, namely Live and Neutral wires. Earth wire is not required as the casing is made of insulator like plastic for this case.

The classic air purifier in our classroom
The “double squares” symbol means that this appliance is double-insulated.
As you see, the earth wire is not required.

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How to read ammeter and voltmeter

In general, the ammeter and voltmeter can be read to 2 decimal place. Hence all recordings must be in 2 d.p.

Ammeter is a device to measure the size of the current. It is connected in series and it has very low resistance.

How to read an ammeter

Voltmeter is a device to measure the potential difference between two points on the circuit. It is connected in parallel and it has very high resistance (no current should flow through them).

How to read a voltmeter


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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.


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


3 pin-plug


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Kinematics Videos

1) Speed and Average Speed

Usain Bolt World Record 100 m in 9.58s in Berlin, Germany!
What is his average speed?
Is there portion of the race where his speed is faster than the average speed?

How about his 200 m record?

How about his 4 x 100 m relay?

2) Acceleration

Experience the thrilling acceleration of a launching roller coaster. Calculate the initial acceleration!

Bugatti Chiron 0 – 400 – 0 km/h in 42 seconds.

Ferrari World Abu Dhabi – World’s Fastest Roller-coaster which can reach top speed of 240 km/h in 4.9 s! What is the acceleration in m/s2.

Hyperloop train – next generation of super fast train

3) Free-fall and Weightlessness

Weightlessness in outer space

Zero-gravity plane: You can experience weightlessness on earth!

Jumping From Space! – Red Bull Space Dive

Brian Cox visits the world’s biggest vacuum | Human Universe

Experience G-force in a Centrifuge

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The Uncle (aka Physics Guru) in coffeeshop

This uncle is able to hang the bags on the side of the table while enjoying his coffee in peace.

How did he do that?!

So as long as the wooden chopstick is strong enough, he can hang a few bags like this.

Another type of gadget to hang your bags by the table is this. As long as the centre of gravity (CG) is directly below the support area (area of the hanger in contact with the table), the bag will not fall.

It is the same working principle as most balancing toys.

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Black is a good emitter of infrared radiation

In theory, we have learned that

Dull (matt/rough) and black surface is a good emitter and good absorber of infrared radiation.

In this simple demonstration, the container sides are painted white matt, silver smooth and black matt.

Hot water is poured into the container and the thermal energy is conducted to the whole container. The temperature of the container is more or less uniform.

Using a infrared thermometer, we measure the temperature of the different surfaces.

From this simple demonstration, we can conclude that dull black surface is the best emitter of infrared radiation and smooth silver surface is the worst emitter of infrared radiation.


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Radiometer – Thermal Transfer

Radiometer is a device for measuring the amount of infrared radiation. It consists of freely pivoted rotor with four vanes perpendicular to one another. The vane surface is painted alternate black and silver. The rotor is enclosed in a partial vacuum glass bulb.

In general, when exposed to infrared radiation, the rotor will spin. The greater the amount of radiation, the faster the rotor spins.

The actual working principle is actually much more complicated. Refer to the videos below for detailed explanation.

For our context in O-level, we can briefly explained based on what we learned. As the vanes of the rotor are exposed to infrared radiation, the black side of the vane absorbs more radiation as it is a good absorber of radiation and hence its at higher temperature. The silver side reflects the radiation. The air molecules at the black side will get heated up and gain more kinetic energy. Hence rate of collision is higher and the air molecules collide on the black side with more force than the silver side. This results in a net force on the black surface and the rotor spins in a specific direction as shown in the video.

The more complicated theory how radiometer works


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Convection Toy – Angels Carousel

This simple toy is made possible using convection current in the air.

The ‘fan’ of the carousel is similar to the windmill we are familiar with.  Just that the kinetic energy of the wind is created by the stream of hot air rising up from the bottom. It is the opposite of an electric fan.

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Fiery Re-entry into Earth’s Atmosphere



SpaceX’s Crew Dragon heat shield shown off after first orbital-velocity reentry

How do spacecraft re-enter the Earth? | HowStuffWorks

Why Is It So Difficult For A Returning Spacecraft To Re-Enter Our Atmosphere?

Returning from Space: Re-entry – PDF format

SpaceX In-Flight Abort Test

SpaceX Falcon Heavy- Elon Musk’s Engineering Masterpiece

Shuttle Atlantis STS-132 – Amazing Shuttle Launch Experience

How to Land the Space Shuttle… from Space

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Metal is a good conductor – paper over metal gets burned slower

The paper over metal or over plastic/wood will get burned faster?

Everyone knows that metal is a good conductor of thermal energy. But for experiments like this, many would have guessed it wrongly.

A paper is wrapped over metal and insulator (plastic, wood etc) and is exposed to the flame, the paper over the insulator becomes charred faster and burned faster.

The paper over the metal takes a longer time to be charred and burned. This is because metal is a good conductor of thermal energy. When the spot (paper over metal) is exposed to the flame, the metal conducts the thermal energy away from that spot to other parts of the metal. So the temperature increase at that spot is slower, hence the ignition temperature of the paper (approx. 230oC) will be reached much slower, compared to the spot where the paper is over an insulator.

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Water is a poor conductor

In the three states of matter, in general, solid is the best conductor as the particles are closely packed in an orderly manner, hence thermal energy can be passed down by the collision of the particles in the solid faster.

On the other hand, liquid and gas are relatively considered poor conductor. Gas is the worst conductor as the particles are far apart.

The following experiment demonstrates that water is indeed a poor conductor of thermal energy.

An ice cube is kept at the bottom of the boiling tube by the net. The water at the top of the boiling tube is heated and started to boil. But the ice is not fully melted.

This shows that thermal energy transferred through the water from heated water at the top to the ice is weak, hence indicating that water is a poor conductor.

How about thermal transfer through convection current? In this experiment, the heated water at the top expands, the volume of the heated water increases, becomes less dense and remains at the top. The cooler water, which is denser, remains at the bottom. Hence there is no convection current formed throughout the entire water in the tube. So thermal energy transfer to the ice cube through convection is not present here.

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Balloon at reduced pressure

A partially inflated balloon is placed inside a sealed container as air is pumped out of the by syringe. As the pressure of the container is reduced, the volume of the balloon increases. The pressure inside the balloon decreases.

Note that the pressure inside the balloon is not equal to the pressure of the container. The number of air molecules in the balloon is fixed. As the pressure of the container decreases, this creates a pressure difference inside and outside the balloon. This causes the balloon to expand and its volume to increase. The number of air molecules per unit volume inside the balloon decreases, hence pressure inside the balloon decreases.

But as the balloon is elastic, the wall of the balloon is stretched. Hence the pressure of the balloon will be greater than the pressure of the container. Refer to the video below.


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Boiling water at reduced pressure – Water boils at 75 oC

At sea level (where most of us are), the standard atmospheric pressure is about 101325 Pa. The boiling point of water is at 100 oC which we are familiar with.

But as you climbed up e.g. Mount Everest at 8,848 m, the pressure is low and the boiling point of the water is about 71oC. So that’s the hottest cofe you can have on top of the cold mountain!

Hence as the pressure decreases, the boiling point of the water decreases. As with lower pressure, the water molecules requires lesser energy to break the intermolecular forces to escape into the atmosphere, hence boiling point is lower.

This video shows the same effect. Using the syringe, the air is pumped out of the container to reduce the pressure. The water at 75 oC , (below the usual boiling point of 100 oC) will start to boil and you can observe the bubbles forming!


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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.


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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Work Done Energy and Power Videos

1) Good examples of energy conversion.

Trampoline + Dodgeball

Bungi Jump

Sky Jump – Dubai

Felix Baumgartner Space Jump World Record 2012

2) The World’s Largest Hydroelectric Dam – The Three Gorges Dam

The world’s largest ship elevator and ship locks

3) Roller Coaster

Chain Lift

Formula Rossa – World’s Fastest Coaster

Launch Coaster

4) Wind and Solar Energy

5) Space Shuttle returning (or meteoroid) into Earth’s atmosphere – Lost in GPE converted to thermal energy + sound energy (assume terminal velocity)

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Physical Quantities Videos

The smallest to the biggest thing in the universe. 
This video shows the various prefixes used to denote very small or big numbers.

Is Usain Bolt faster than a typical car on the road with a constant speed of 60 km/h?
Calculate the average speed of Usain Bolt for the 100 m world record of 9.58 s in m/s. Then convert it to km/h to do a comparison.

Wanna watch his 200 m race?

How to use a vernier caliper?
The precision of vernier caliper is 0.01 cm (0.001 m).

How to use a micrometer screw gauge?
The precision of the micrometer is 0.01 mm

Factors that affect the period of the pendulum bob
Only length and gravitational field strength (g) affect the period.
– Length of pendulum increases, period (T) is longer
– Gravitational field strength (g) decreases, period (T) is longer (i.e. on the moon)

Mass of pendulum bob and angle of release (5 to 10 degree) will not affect the period (T).