Topic: Archimedes Principle.
Why do we weigh less in water?
Hey.
Looks like you have gained a lot of weight.
Come on.
Let us check your weight.
You weigh 132 pounds.
Alright.
Don't get stressed.
Check your weight in water.
Look.
It is much lesser.
Do you know why you weighed less in water?
The answer to this is based on Archimedes Principle.
Not that principal.
Archimedes Principle is a fundamental truth which states that an object fully or partly immersed in a fluid.
Experiences an upward force called buoyant force which is equal to the weight of the fluid displaced by it.
It went over your head, right?
No worries.
Let me explain.
Gravity always pulls us downwards.
Hence, the weight of our body always acts downwards whether we are on ground or in water.
However, according to Archimedes Principle.
When we are in water, we experience an upward force.
Buoyant force which is equal to the weight of water displaced by us.
This upward force cancels a portion of our weight causing us to weigh less in water.
Topic: Newton's Third Law of Motion.
Why does a swimmer push the water backward?
No.
There is a hole in the boat.
Now, how will you reach the shore?
Don't worry.
Do as I say.
Jump in the water and push the water backward with your hands.
This will help you to move forward towards the shore.
No.
You are not pushing the water backward.
See.
You are moving forward towards the shore.
Do you know why you were able to move forward by pushing the water backward?
To understand this, you need to learn about Newton's third law of motion.
Newton's third law of motion states that, for every action, there is an equal and opposite reaction.
No.
Not that action.
Wait, let me explain.
When we swim, we apply force and push the water backward with the help of our hands.
This is action.
In response to this action, the water pushes us forward with an equal force.
This is reaction.
Thus, in order to move forward and swim, the swimmer pushes the water backward.
Topic: Convex mirror.
Why is a convex mirror used as a rear view mirror?
Wow.
What an amazing car.
But your rear view mirror is missing.
No.
Don't use a concave mirror.
It will usually show you magnified images.
Why don't you listen to me?
See, I was correct.
Now put the convex mirror and see the result.
Look.
The image is much smaller, right?
Do you know what the difference between the two mirrors was?
If we use a concave mirror for our car, we will not be able to see the vehicles behind us properly.
This is because the concave mirror will magnify the object and we will see a very enlarged image.
Thus, we require a mirror which gives us a wider view of the vehicles behind us.
In this case, a convex mirror proves to be the right choice.
This is because a convex mirror forms a highly diminished image.
Thus making the traffic look much smaller.
As a result, we can see a large number of vehicles in a very small mirror.
Topic: Heat.
Why do woolen clothes keep us warm?
The night is quite cold and you have worn cotton clothes.
Why don't you wear some woolen clothes?
They will keep you warm.
Please listen to me.
Look.
You are shivering.
At least now wear some woolen clothes.
Now you are feeling warm, right?
Do you know why?
Wait, I will tell you.
Both wool and cotton are bad conductors of heat, that is, they do not conduct or transmit heat to the surrounding.
Then why do we feel warm only in woolen clothes?
This is because, in woolen clothes, there are more tiny spaces as compared to that in cotton clothes.
Due to these extra tiny spaces, woolen clothes trap more air as compared to cotton clothes.
Now, since air is a bad conductor of heat, woolen clothes containing more air.
Do not allow much of our body heat to escape into the atmosphere, thus keeping us warm.
Topic: Atmospheric Refraction.
Why do stars seem higher than they actually are?
Hey.
That is not the actual position of the star.
Please listen.
Fine.
Then bear the consequences.
See.
I told you.
Do you know why this happens?
This happens because of atmospheric refraction.
Atmospheric refraction is the refraction or bending of light caused by the earth's atmosphere.
It went over your head, right?
No worries.
Let me explain it to you in detail.
In the earth's atmosphere, the air closer to the earth's surface is denser due to gravity.
But as we move higher, the air density goes on decreasing.
This forms different media of air.
The layer of air closer to the earth's surface forms the denser medium.
While the layer of air higher up in the sky forms the rarer medium.
Now, when the light of a star enters our atmosphere and passes from one medium to another.
Atmospheric refraction takes place and the light bends.
When this light reaches our eyes, our eyes trace it backward as a straight line.
Due to this, the light appears to come from a higher point.
Hence, the star seems to be higher than it actually is.
Topic: Gravitational Force.
Why do you weigh less on the moon?
Hey.
What are you doing?
It is not possible.
You cannot fly on earth.
Please listen.
Look.
I had warned you.
Now, why don't you try the same activity on the moon?
There you will weigh much less and thus, you can fly short distances.
See.
I was correct.
Do you know why you weighed less on the moon?
To understand this, we need to first learn about weight.
No.
Not those weights.
Weight is the force with which an object is pulled towards the center of the earth.
Any other celestial body.
This force is called the gravitational force.
Greater the gravitational force exerted by the celestial body, more will be the weight of an object present on it.
So, does this mean that the moon exerts less gravitational force than the earth?
You are right.
But do you know why?
This is because gravitational force is directly proportional to the mass of an object.
Greater the mass, greater will be the gravitational force exerted by it and thus, more will be our weight.
As the moon has lesser mass than the earth, it has a weaker gravitational pull.
Hence, we weigh less on the moon.
Topic: Pressure.
Why is it difficult to cut with a blunt knife?
Seems like you are hungry.
Why don't you have an apple?
No.
Don't use the blunt knife.
Use the sharp knife.
Looks like you want to go ahead with the blunt knife only.
See, you are finding it difficult to cut the apple now.
Now, try cutting with the sharp knife.
Look.
The apple gets cut very easily.
Confused, right?
Let me explain.
All this happens due to a concept called pressure.
What is Pressure?
It is the force applied per unit area.
That means the pressure will be directly proportional to force and inversely proportional to area.
Now, we applied the same amount of force on both the knives.
Then, why was the result different?
This is because the area on which the force acted was different.
In the case of a blunt knife, the cutting edge is thicker.
That means, the area is larger.
As a result, the force of our hand falls over a larger area of the knife, producing lesser pressure.
Hence, it is difficult to cut with a blunt knife.
Topic: Density.
Why do firemen crawl in smoke filled rooms?
Hey, remember, the upper part of the room will be filled with smoke.
So, crawl while you are going inside.
I won't.
My legs will become dirty.
Why don't you listen?
Look.
You are not able to breathe properly as you have inhaled a lot of smoke.
This happened because you did not care about the density of air.
Well.
I know all about cavity, but what's this new thing called density.
Don't worry.
I will explain.
Density is the measure of mass present per unit volume.
Lesser the density, lighter will be the object and greater the density, heavier will be the object.
So in this case, what is lighter, the air or the smoke?
Wait.
Let me think.
I guess the answer is smoke.
Yes, you are absolutely correct.
Smoke is lighter than air.
As it is lighter, it rises up in the room and occupies the space at the top.
Hey, where are you going?
It is not over yet.
The air being heavier than smoke tends to remain below.
Hence, if we crawl, we will get sufficient oxygen to breathe.
We can safely come out of the room without being suffocated.
Topic: Ultrasound.
Why is ultrasound used in sonar?
Hey.
Looks like you are searching for a treasure hidden in a sunken ship.
Why don't you use a sonar?
It will emit ultrasounds and help you locate the ship.
You know what.
I have a better idea.
No.
A music system will make ordinary sound.
So, it is of no use.
Don't fool me.
I know that the music system is the right choice.
See.
You are not able to find the ship.
Now, will you use a sonar?
Look.
You easily found the ship.
Do you know how a sonar could locate the sunken ship?
It was because of ultrasound.
Ultrasounds are sounds having very high frequencies which start from 20,000 Hertz.
So, is ultrasound used in sonar because of its high frequency?
You are absolutely correct.
Due to its high frequency, an ultrasound can penetrate to a greater depth.
Thus helping us to locate the depth of the sea, sunken ships, etc.
But, I am not able to hear the ultrasound?
It is because human beings can hear sound frequencies from 20 Hertz to 20,000 Hertz.
As ultrasounds have frequencies higher than 20,000 Hertz, we cannot hear them.
Topic: Refraction of Light.
Why does a pencil look bent in water?
Wow.
That was a very cool magic trick.
Now check this out.
Hey.
Stop crying.
Your magic wand is not bent or broken.
Look.
It is safe.
But do you know why your magic wand appeared bent in water?
It is because of refraction of light.
Refraction of light is the change in direction of light.
When it passes from one medium to another obliquely.
So, what do you think are the two media here?
You don't know.
No worries.
I will tell you.
They are air and water.
When the light rays coming from the tip of the magic wand pass from water to air.
They change their direction, that is, they get refracted.
When these refracted rays reach our eyes, our eyes trace them backward as straight lines.
Due to this, the rays appear to come from a point slightly above the original position.
Hence, the magic wand appears to be bent in water.
Similarly, like the magic wand, a pencil appears to be bent in water.