Gravity (Weight)

Gravity এর বাংলা অর্থ হল মহাকর্ষ। মহাবিশ্বে সকল বস্তুর মধ্যে যে আকর্ষণ বল ক্রিয়া করে তাকে মহাকর্ষ বল বলে। 1687 সালে Sir Issac Newton যখন তার *Phisophiae Naturalis Principia Mathematica *বইটি প্রকাশ করেন,তখন মানবজাতি মহাকর্ষ সম্পর্কে একটি পূর্ণাঙ্গ ধারণা লাভ করে। চল মহাকর্ষ নিয়ে জানা যাক।

ক্লাস ৮ এ উঠলেই NCTB Curriculum এ মহাকর্ষ অন্তর্ভুক্ত হয়ে যায়।তা ই Physics এর সাথে Official পরিচয় হওয়ার আগেই আমাদের মাথায় আরও কিছু বস্তুর মত মহাকর্ষ ও ঢুকিয়ে দেওয়া হয়। তবে,এক্ষেত্রে সবচেয়ে মজার বিষয় হল,আমাদের ক্লাস এ যে ঘটনা দিয়ে মহাকর্ষ বুঝানো হয় তা কিন্তু ঠিক না! আসলে আংশিক ঠিক। সাধারত বলা হয় যে, Newton গাছের নিচে বসে ছিল এবং তার মাথায় আপেল পড়ার ফলে তার মাথা খুলে যায় এবং সে মহাকর্ষীয় সূত্র আবিষ্কার করে। আপেল এর জায়গায় কেন কাঠাল বা তরমুজ পড়ল না এটা নিয়ে পোলাপাইন এর আক্ষেপের শেষ নেই। আসলে ঘটনা হল,Newton ভাবুক মেজাজ এ বাগানে বসে ছিলেন এবং গাছ হতে একটি আপেল পড়তে দেখে তার মনে এই সূত্রের idea জন্ম নেয়। এখন পোলাপাইন মনে করতে পারে আমি কেমনে আমা্দের শ্রদ্ধেইয় স্যারদের কে ভুল বলতে পারি। এক্ষেত্রে reference হিসেবে A Brief History of Time-Bantam Edition এর ৫ নং পৃষ্ঠা দেখতে পার।

**মহাকর্ষ **বল বলতে সেই বলকে বুঝায় যা মহাবিশ্বের যে কোন দুইটি বস্তুর মধ্যে ক্রিয়া করে ।

Gravity

Let's watch some physics in action:

When the giant pumpkin hits the ground, it definitely feels the force of the ground stopping it.

It may seem a bit strange that gravity can act without touching an object. The force of gravity acts at a distance. Near the surface of a planet, it is ubiquitous, felt by every single object.

Gravity is one of the fundamental forces in the universe. We'll talk more about its nature when we learn about these fundamental forces. But for now, let's focus on gravity as we experience it every day, near a planet's surface. We'll start with one of the most famous observations in the development of physics, made by Galileo way back in the 16th century. Galileo postulated that all objects, regardless of how heavy they are, fall at the same rate. In fact, he supposedly dropped two balls of different masses off the Leaning Tower of Pisa to prove it.

Now, pretend for a moment you haven't heard this idea before, and consider whether it makes sense. Actually, from our everyday experience such a premise seems completely counter-intuitive. Imagine dropping a feather and a hammer together, right where you are.

Of course we expect the hammer to hit first, but the reason has little to do with gravity, and everything to do with air resistance, a form of drag force which we'll also discuss later. This same experiment when carried out in a vacuum will uphold Galileo's postulate. Here's a beautiful demonstration:

By dropping objects off different heights, you can show that a freely falling object has constant acceleration over time (near the surface of a planet).

How do we figure out this acceleration. Well, we can get a rough estimate easily enough from this handy recreation of Galileo's Leaning Tower of Pisa experiment.

The Leaning Tower of Pisa is 56m tall.

The more precise value for the acceleration due to gravity near the earth's surface is $g=9.8m/s^2$.

So saying that all objects accelerate at the same rate regardless of mass is equivalent to saying that the force of gravity is proportional to the mass of the object. Specifically, \begin{equation} F=mg \end{equation} where $g$ is the acceleration we found above.

The force of gravity on an object is also called its weight. And now we see why weight and mass are so often confused in casual conversation. So long as we stay on earth, they're directly related to each other with the proportionality constant $g$.

And now for one final question. The force of gravity on the surface of the moon is about $\frac{1}{6}$ that of earth.

Check your answer against this demonstration courtesy of the Apollo 15 mission:

So now you know where to direct any remaining conspiracy theorists that think the moon landing was faked...

Have you ever done sky-diving? Or dropped something from your hands? why do you/the object keep falling towards the ground? Is it a supernatural entity..... or a force of nature? Lets find out.

** The Discovery of Gravity**

As you see in the above video, while he was sitting under an apple tree, Sir Isaac Newton discovered that a force of nature acted on all objects which pulled them towards the earth as a result of which the apple dropped on his head. With this rose a question in his mind, "Why does everything fall towards the Earth when nothing is touching them?" After further research, he rose to a conclusion that the force being exerted by Earth pulled everything towards it, and he called that force Gravity. This was one of the biggest discoveries in the field of Physics.

**Earth's Gravity **

Let us take the example of Earth and a ping pong ball.

Gravity depends on two factors mass and distance. -

1. Mass - Earth has a mass much larger than that of the ping pong ball, thus the Earth exerts a large gravitational pull on the ball. But the mass of the ping pong ball is really less, so its gravitational pull is almost negligible.

2. Distance - If the distance between the Earth and the ball is less, the Earth's pull would be more, and if the distance is more, the pull is considerably less.

Gravity in The Solar System

The planets in the Solar System are held together by the Sun's gravitational pull. This is why, as the Sun rotates, the planets also orbit around it. The moons also orbit the planets due to the same reason.

Lets do some Maths

The Earth's gravitational pull causes an acceleration of $9.8 \frac{\rm m}{{\rm s}^2}$. This is same for objects of all mass. In terms of time, if it takes an object $x$ seconds to reach the ground, then its velocity (at the moment of touching of ground) would equal $x\, {\rm s} * 9.8 \frac{\rm m}{{\rm s}^2} = 9.8 x \frac{\rm m}{\rm s}$.

Have you ever put on your thinking caps and thought about why your football whenever is kicked in the air high up doesn't stays there but comes just on the ground. This is because of something called as: Gravity.

But WHAT is Gravity? (Hint: It is not matter, a type of force)

What is 'Gravity' ?

In simple words, 'Gravity' is the force exerted by the planets which pull other objects (sometimes even planets!) towards them.

Discovery of Gravity:

Here, it is a video for the simpler explanation:-

So as you can see in the above video, Sir Isaac Newton discovered gravity. He was a very active child in his childhood. He played with his friends everyday. One day, he was too tired and thus decided to sit under an apple tree. There suddenly, an apple fell on his head. He then started to test other things and found that anything that he threw in the air, eventually came down to earth. Then he tied a string to a ball and then imagined that there was an invisible string tied to all the objects to the earth. He was right. This string is called as 'Gravity'.

What is Gravitational pull?

It is the force created due to Gravity. The pull depends upon the mass of the object.

The gravitational pull of Earth is 9.8G (9.807 m/s)

The gravitational pull of Moon is 1.6G (1.622 m/s)

The gravitational pull of Mars is 3.7G (3.711m/s)

Now some interesting 'Gravity' questions before proceeding further

Question 1

Question 2

There are some very interesting videos related to these questions-

Some maths...

Of course, in each physics topic there IS an interesting part and there IS ALSO some boring maths parts. Because physics is a heterogenous mixture of SCIENCE+MATHS.

First of all, what is gravity? Hint: I'm not talking about the George Clooney film.

Gravity, or rather gravitational force, is one of the four fundamental forces of the universe, which basically causes everything that has mass to attract everything else. Generally, the greater an object's mass, the greater gravitational attraction it has for other objects.

For example, a great way to experience gravity (other than just, you know, living on Earth) is to pick up a ball in your hand and letting it go. Naturally, it falls down, as you would expect it to. Why does it fall down (or, in other words, towards the Earth) and not in any other directions, even though all objects have gravitational attraction for each other? This is simply because the Earth's large mass greatly overshadows the masses of anything ON Earth, thus exerting the most force on the ball and causing it to "fall" towards it.

Now, the actual gravitational force exerted on objects is sometimes called a familiar phrase, called "weight." Weight, unlike mass, is a force, using Newtons ($kilograms*meters/second^2$) as a standard unit of measure instead of just kilograms. The formula for it is

$W=mg$

where "$W$" is weight (in Newtons), $m$ is mass (in kilograms), and $g$ is a constant (9.81 $m/s^2$ on Earth).

The force of gravity on the moon is considerably less than the force of gravity on the earth. This is because the moon has a smaller size and mass when compared to the earth. Watch the video below of Neil Armstrong in 1969 during the first moon landing. He appears to be bouncing around rather than simply walking. That is the result of a smaller force of gravity pulling the astronauts down towards the surface of the moon. In fact, his feet aren't falling to the surface of the moon as fast as they fall towards the surface of the earth. The force that the moon applies on the astronauts on the moon is less than the force applied by the earth. The force is gravity, what we know of as weight. Thus, on the moon, things seems less heavy and fall more slowly.

Newton's equation to determine the force gravity is:

If we consider the moon:

The constant g shown in the equation above is also referred to as the acceleration due to gravity [measured in $m/s^2$]. That is the rate at which an object will fall in the absence of air resistance. In other words, when an object has no other force acting on it other than the force of gravity. Look at this using Newton's 2nd law of motion:

Determine the force of gravity, or weight of Neil Armstrong on both the moon and the earth using Newton's equation for gravity. Convert your answer from Newtons to Pounds.

Also determine the value of the acceleration due to gravity, $g$ on both the earth and the moon.

[round your answers to the nearest tenth]