Evan's Space

Wonders of Physics


Leave a comment

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)


Leave a comment

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.


Leave a comment

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.


Leave a comment

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.


Leave a comment

Fiery Re-entry into Earth’s Atmosphere

SPACE CAPSULE simple

SPACE CAPSULE

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


Leave a comment

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!

 


Leave a comment

2017PurePhyP2Q5 Two points on the rope wave with displacement-time graphs given

In this this question, the displacement-time graphs are given, which are different from displacement-distance graphs.

In the displacement-time graphs of A and B, they show the displacement of that particular point at different timing. E,g, at t = 0s, the A is at the rest position (0 displacement) and at time 0.2 s it is at the maximum displacement. This means A is going up from t = 0 s to 0.2 s.

Solutions:
(a) Amplitude: 1.5 cm
(b)(i) Frequency is the number of complete waves produced in 1 second.
(ii) period T = 0.8s, f = 1/T = 1/0.8 = 1.25 Hz
(c) Closest possible positions of A and B, (refer to the video), is when the
time taken for the wave to move from A to B is T/4 = 0.8/4 = 0.2 s.
speed = distance/time = 38/0.2 = 190 cm/s approx. 200 cm/s
(ii) There are various possibilities in which B can be 38 cm to the right of A. Besides T/4, it can be 1.25T or 2.25 T etc. Hence the speed can be other values.

Refer to the video explanation below


Leave a comment

Marshmallow Hulk in Vacuum Jar

When the pump is switched on and the air in the jar is gradually removed, the pressure in the jar decreases. There will be fewer air molecules per unit volume in the far. Hence rate of collision of the air molecules with one another and with the wall and hulk will be reduced. As pressure P = F/A, the force acting per unit area decreases, the pressure decreases.

In the marshmallow, there are pockets of air at normal atmospheric pressure initially. As the pressure in the jar decreases, the pockets of air in the marshmallow expands due to this pressure difference. Hence the hulk expands and its volume increases.


Leave a comment

Impressive Bugatti Chiron

This impressive Bugatti Chiron can accelerate from rest to 400 km/h and decelerate to a complete stop in merely 42 seconds! Our normal cars on the expressway travel about 90 km/h and the F1 race yesterday night is about 300 km/h. This Bugatti Chiron is faster than most bullet trains and comparable to the speed of a magnetic levitation train!

Capturehttps://www.bugatti.com/chiron

Before we look at the video, let’s do some calculations:

Capture2

Let’s find the acceleration of the car to reach 400 km/h in 32.6 sec:

Converting 400 km/h to m/s:    400km/1h = 400 000m/3600s =111 m/s

acceleration, a = (v – u)/t = (111 – 0) / 32.6 = 3.4 m/s2

hmmm…. this acceleration doesn’t seem impressive… it is way below free fall acceleration!

But we are not being fair here. To achieve the max speed of 400 km/h is not easy due to the resistive force (air resistance and friction) as speed increases. We should compare fairly the acceleration to reach 100 km/h instead like how we typically compare sports car like Ferrari etc.

Let’s find the acceleration of the car to reach 100 km/h (27.8 m/s) in 2.4 sec:

acceleration, a = (v – u)/t = (27.8 – 0) / 2.4 = 11.6 m/s2

This is greater than acceleration due to gravity (free fall) and much faster than most sports cars in the market like Ferrari or Lamborghini!

Now, let’s find the deceleration of the car when it slows down from 400 km/h to a complete stop in 41.9 – 32.6 = 9.36 s

acceleration, a = (v – u)/t = (0 – 111) / 9.36 = -11.9 m/s2

Take note of the spoiler being activated when it decelerates. This increases the drag (air resistance) to slow down the car, in addition to using the normal brakes. It is the same principle as the aeroplane when it lands and slows down on the runway.

 

 

 


Leave a comment

Which is better to cool the food?

ice in cooling food items

Other examples in our daily lives:

20170621_105522

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)

Capture1Capture2

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.

Related posts about ice:

Density of ice – Why ice floats on water?

Will whole lake be frozen during winter?

 


Leave a comment

Gas expands and contracts the most

The 3 states of matter –  solid, liquid and gas.

In general, when a body is heated, it expands and volume increases. The mass remains the same. Since density = mass/volume, its density decreases (less dense). For instance, warm air rises as it is less dense. In terms of kinetic theory, the particles will increase in kinetic energy. The average spacing between the particles increases (assuming not in a closed container).

Likewise, when a body is cooled, the opposite occurs. The body contracts and volume decreases. It becomes denser.

Due to the differences in particles arrangement of solid, liquid and gas, each expands by different amount when heated and vice versa. Which expands the most when heated and contracts the most when cooled?

The following demonstration of the ‘Pee Boy’ is a good video to show the concepts.

Explanation:

The tiny hole at the penis is too small for any water to enter on its own. So using thermal transfer in the different states, the following steps are taken:

  1. Put the hollow empty boy into the hot water. [air inside the boy expands more than the solid ceramic, hence bubbles are seen coming out of the hole]
  2. Put the hollow empty boy now into the cold water. [The air inside contracts and volume decreases. This creates a low pressure and water is then sucked into the boy through the tiny hole]
  3. Place the boy on a platform. [The boy is only partially filled with water. The head portion is filled air while the bottom portion is filled with water]
  4. Pour hot water over the head. [As the whole boy is heated by the running hot water, the air in the head portion expands much more than the water at the bottom and the solid ceramic of the boy. Hence the air pressure increases and it pushes the water out of the boy]
  5. And he pees!!! Quite powerful indeed!


Leave a comment

How a fuse works?

A fuse is a safety device that is added to an electrical circuit to prevent excessive current flow. It has the same function as a circuit breaker. However , a fuse must be replaced once it melts (blows). A circuit breaker can be reset after it trips.

Electric symbol for fuse
Capture

Fuse is connected to the live wire. It consists of a short piece of thin wire. In the event of an electrical fault, when a current that exceeds its fuse rating flows through, it heats up and melts (fuse blows). When the fuse blows, the electrical appliance is disconnected from the high potential of the live wire (usually 240 V). Hence it protects the appliance and the user.

The choice of fuse is always slightly higher than the actual current flowing through the appliance, and it has to be a whole number. Typical household fuses off the shelves are rated at 1 A, 2 A, 3 A, 5 A, 10 A and 13 A. But in theory, we just state a fuse which is slightly higher and is a whole number.